INTRODUCTION — Sickle cell disease (SCD) is an inherited group of disorders characterized by the presence of hemoglobin S (HbS), either from homozygosity for the sickle mutation in the beta globin chain of hemoglobin (HbSS) or from compound heterozygosity of a sickle beta globin mutation with another beta globin mutation (eg, sickle-beta thalassemia). The hallmarks of SCD are vaso-occlusive phenomena and hemolytic anemia. Sickle cell trait is a benign carrier condition characterized by heterozygosity for the sickle hemoglobin mutation.

Screening and diagnosis of sickle cell disorders are discussed here.

Discussions of the clinical manifestations and management of sickle cell disorders are presented separately:

●(See "Sickle cell trait".)

●(See "Overview of compound sickle cell syndromes".)

●(See "Overview of the clinical manifestations of sickle cell disease".)

●(See "Overview of the management and prognosis of sickle cell disease".)

●(See "Sickle cell disease in infancy and childhood: Routine health care maintenance and anticipatory guidance".)

●(See "Evaluation and management of fever in children and adults with sickle cell disease".)

BACKGROUND — Hemoglobin normally is soluble in the erythrocyte and does not polymerize. Hemoglobin is a tetramer of two alpha globins and two beta globins. Hemoglobin S (HbS) is an abnormal hemoglobin that results from a point mutation in the beta globin gene that causes the substitution of a valine for glutamic acid as the sixth amino acid of the beta globin chain. The resulting hemoglobin tetramer (alpha2/beta S2) becomes poorly soluble when deoxygenated [1].

A 2018 study that used whole-genome sequencing data from a number of sources (2500 individuals in the 1000 genomes project, 320 individuals in the African Genome Variation project, and 108 individuals from Qatar) to perform a linkage disequilibrium analysis was able to construct a phylogeny that placed the age of the sickle mutation at 295 generations, equivalent to approximately 7300 years, during the Holocene Wet Phase [2]. The investigation suggested a single origin of the mutation in West or Central Africa, perhaps in the Green Sahara, which was wet and rainy at the time, or in the equatorial rainforest. Malaria was endemic in both of these areas during the Holocene period. The nearly exclusive presence of the original haplotype in certain regions of Africa suggest a single origin. This likely preceded a population split, possibly in the area of Cameroon, which led to the Bantu expansions approximately 2400 years later (approximately 5000 years ago).

The pathological polymerization of deoxygenated HbS is essential to vaso-occlusive phenomena [1]. The polymer assumes the form of an elongated rope-like fiber, which usually aligns with other fibers, resulting in distortion of erythrocytes into the classic crescent or sickle shape and a marked decrease in red cell deformability.

HbS polymerization is a primary determinant of the severity of sickle cell disease (SCD). Polymerization is affected by the presence or absence of other Hb mutations co-occurring with HbS, as well as the concentration of fetal Hb within the red cell. (See "Mechanisms of vaso-occlusion in sickle cell disease".)

However, polymerization alone does not account for the pathophysiology of SCD. Subsequent changes in red cell membrane structure and function, disordered cell volume control, and increased adherence to vascular endothelium also play an important role [1,3]. Hemolysis generates reactive oxygen species and free plasma hemoglobin (Hb). This free plasma Hb is a powerful scavenger of nitric oxide, whose deficiency plays an important role in the vascular pathology of SCD [4]. (See "Pulmonary hypertension associated with sickle cell disease", section on 'Introduction'.)

TERMINOLOGY — We use "sickle cell disorders" to refer to all conditions in which an individual carries the sickle hemoglobin S gene mutation on at least one beta globin gene.

●If the other beta globin gene is normal, the individual has sickle cell trait, a benign carrier condition that is not a disease. (See "Sickle cell trait".)

●The combination of one sickle mutation and one alpha globin gene mutation results in two benign carrier conditions, sickle cell trait and alpha thalassemia trait (also called alpha thalassemia minima), and no clinical disease. (See "Pathophysiology of thalassemia", section on 'Terminology and disease classification'.)

●"Sickle cell disease (SCD)" is an umbrella term that includes all patients who have the sickle mutation plus a second beta globin gene mutation, the combination of which causes clinical sickling. The other beta globin mutation could be the sickle mutation or a different mutation in the beta globin gene (eg, one associated with beta thalassemia or hemoglobin C disease, or others). Patients with a sickle cell disease exhibit a clinical phenotype, anemia, and laboratory evidence of sickling. (See "Overview of compound sickle cell syndromes".)

OVERVIEW OF DIAGNOSTIC TESTING — The goals and methods of diagnosis of sickle cell disease (SCD) vary with the age of the patient. DNA-based testing is used for prenatal diagnosis. The diagnostic methods used after birth are those that separate hemoglobin species according to amino acid composition (hemoglobin electrophoresis or thin layer isoelectric focusing), solubility testing, and examination of the peripheral blood smear. (See "Fetal blood sampling".)

Characterization of adult hemoglobins in the fetal and newborn periods can be difficult because of the predominance of hemoglobin F (HbF), which confounds detection of hemoglobin S (HbS) by solubility testing.

Clinical manifestations of SCD are not present at birth, and usually begin to become apparent after the first few months of life as the concentration of HbS rises and HbF declines. Sickled cells can be seen in the peripheral blood of children with SCD at three months of age, and moderately severe hemolytic anemia is apparent by four months of age.

REPRODUCTIVE TESTING AND COUNSELING — Individuals with sickle cell disorders should be offered preconception counseling to determine the risk of having a child with a sickle cell disorder. Individuals at risk should be offered hemoglobinopathy testing early in pregnancy and the opportunity for prenatal diagnosis where appropriate. (See "Prenatal screening and testing for hemoglobinopathy".)

Prenatal testing may include obtaining fetal DNA samples by chorionic villus sampling at 8 to 10 weeks gestation [5]. Other diagnostic approaches such as in vitro fertilization (IVF) with preimplantation genetic diagnosis or testing fetal cells isolated from maternal blood have been reported in sickle cell disorders but are not routinely used [6-8]. Reproductive centers in the United States and Europe are developing protocols for preimplantation diagnosis of sickle cell disorders [9,10].

NEWBORN SCREENING — Infants with sickle cell disease (SCD) generally are healthy at birth and develop symptoms only when fetal hemoglobin levels decline later in infancy or early childhood.

The goals of newborn screening (NBS) include [11]:

●Early recognition of affected infants

●Early medical intervention to reduce morbidity and mortality, particularly from bacterial infections

●Institution of regular and ongoing comprehensive care through a multidisciplinary sickle cell clinic, in collaboration with the primary care physician, whenever feasible

●Access for families/caregivers of children with SCD to accurate information about the diagnosis, clinical manifestations, treatment options, and age-appropriate anticipatory guidance toward the management of these emerging issues

As an example of the effectiveness of these programs, the use of prophylactic penicillin and the provision of comprehensive medical care have reduced the mortality of SCD during the first five years of life from approximately 25 percent to less than 3 percent [12-14].

The magnitude of the problem in the United States is demonstrated by the frequency of carrier conditions for hemoglobinopathies:

●Data obtained in California from 1998 to 2006 found that the genetic trait for sickle cell and/or thalassemia occurred in 1 in 75 California births [15,16].

●The gene frequency is much higher in the African-American population: 4 percent for HbS, 1.5 percent for HbC, and 4 percent for beta thalassemia [17]. The gene frequency is also high in individuals from other racial and ethnic groups. In a study from California, 12.5 percent of infants with HbS/beta thalassemia births were born to parents who self-identified as having Hispanic ethnicity on the newborn screening form [16].

These frequencies predict the occurrence of 4000 to 5000 pregnancies per year at risk for SCD in the United States [17].

Although there have been advances in the diagnosis and management of SCD in high-income countries, much remains to be learned about the optimum implementation of NBS programs and medical management of patients in low-income countries [18]. For example, a strategy that utilized a team of specially trained midwives at two large maternity services in Cotonou, Republic of Benin, resulted in almost 80 percent of women informed of the risk for SCD agreeing to testing of their offspring. Eighty-five percent of offspring testing positive enrolled in a sickle cell program, and more than 80 percent of these infants were still followed by the program at five years. The mortality rate for this cohort of children under five years of age with SCD was 10 times lower than the general rate recorded in the Republic of Benin [19].

Types of screening programs — Historically, two programs have been used for newborn screening (NBS): selective screening of infants of high-risk parents, and universal testing of newborns.

Universal testing seems preferable due to its economy and superiority of detection [20-22]. This approach has been endorsed by a consensus panel convened by the National Institutes of Health [23]. A cost-effectiveness analysis found that targeted screening of African-American newborns for SCD is cost-effective [24]. However, universal screening identifies more infants with disease and prevents more deaths. The advantages of this approach can be illustrated by the following:

●Observational studies have demonstrated that targeted screening does not identify all individuals. As examples:

•Targeted screening missed 20 percent of African-American newborns with SCD born in the state of Georgia [25].

•The number of SCD diagnoses doubled when screening was changed from targeted to universal in the state of Connecticut [26].

●Self-identification of race is a fluid definition that may change based on local social dynamics.

As of 2008, screening for SCD in newborns is mandated in all 50 states of the United States and the District of Columbia, regardless of birth setting [27]. In fact, the importance of NBS programs is being increasingly recognized worldwide. Due to changes in immigrant migration patterns, several European and African countries are initiating NBS programs. These programs face logistical and economic challenges for comprehensive implementation and patient outreach [28].

In the United States, any high-risk infant should have documentation of newborn screen results by one to two months of age so that confirmatory testing, if necessary, can be performed, and parental education, penicillin prophylaxis, and referral for comprehensive care can be implemented [29]. (See 'Approach to a positive result from newborn/infant screening' below.)

The methods for implementation of NBS programs vary somewhat from state to state. In Texas, as an example, "heel stick" filter paper screen is performed within 72 hours of life, with a second screen required at one to two weeks. Testing is by isoelectric focusing, and abnormal specimens are confirmed by repeat testing or by DNA analysis. Tandem mass spectrometry (MS/MS) is an alternative method for newborn screening as it is able to detect hemoglobin (Hb) peptides following digestion of blood spots with trypsin [30]. (See 'Hemoglobin patterns' below.)

Abnormal results are reported to the NBS program, data are maintained on a computer registry, and case management services are initiated to ensure medical follow-up. In a review of newborn screen outcomes from 1992 to 1998, 2,292,698 live births were recorded in Texas and 94 percent had specimens collected. The overall prevalence of SCD by ethnic group per 10,000 live births was: 29.91 African American, 0.11 White American, 0.29 Hispanic American, and 2.47 other/unknown [31]. Despite these measures, other states have reported gaps in compliance with early medical intervention, parental education, and provision of comprehensive health services [32].

In California, a two-tier approach to universal NBS is used. First, NBS is performed on dried blood spots utilizing high performance liquid chromatography (HPLC). Abnormal hemoglobin findings are then referred to the Hemoglobin Reference Laboratory (HRL), where additional testing, including DNA sequencing, may then be performed for final confirmation. With this approach, of approximately 530,000 annual NBS, approximately 2118 samples were referred for HRL over an eight-year period. Hemoglobin (Hb) genotypes included: sickle hemoglobinopathies (32 percent), alpha thalassemia conditions (24 percent), beta thalassemia conditions (4 percent), and other Hb variants, including traits (41 percent) [16,33].

Despite high percentages of NBS completions by states and high rates of follow-up testing where abnormal findings were detected, there are ongoing problems with timely follow-up and implementation of comprehensive care. One study suggests that there is a wide variation in stakeholder notification (physician, hospital, families/caregivers, hematologists), which can lead to alteration in early intervention [34]. Other states have reported gaps in compliance with early medical intervention, parental education, and provision of comprehensive health services [32].

Additional information and resources related to newborn screening results are discussed separately. (See "Newborn screening".)

Methodology and diagnostic errors — The recommended approach to neonatal testing is to obtain blood samples by heel stick or cord blood and spot the sample onto filter paper for stable transport and subsequent electrophoresis, thin-layer isoelectric focusing, or HPLC [35,36]. Additional information about these methods is discussed separately. (See "Methods for hemoglobin analysis and hemoglobinopathy testing".)

A requirement for tests used in NBS is the capability to distinguish among HbF, S, A, and C.

Diagnostic errors that can occur during neonatal testing include the following:

●Solubility testing may not be valid, because of the large amount of HbF present in fetal blood.

●Hb electrophoresis testing after transfusion of red cells can result in an incorrect diagnosis. In such cases, SCD diagnosis should utilize DNA testing or be postponed for at least four months after transfusion.

●In very premature babies, HbA may not be detected, resulting in misdiagnosis. Very premature babies with sickle cell trait may be found to have HbS levels greater than HbA, resulting in the incorrect diagnosis of hemoglobin S/beta⁺ thalassemia [37].

Hemoglobin patterns — According to convention, the patterns of Hb present are described in descending order according to the quantities detected (table 1):

FS pattern — Newborns with homozygous sickle cell anemia (HbSS) have predominantly HbF with a small amount of HbS and no HbA (ie, the FS pattern). An FS pattern is also found in newborns who have sickle cell-beta⁰-thalassemia, sickle cell-hereditary persistence of fetal hemoglobin, and, because HbD and HbG have the same electrophoretic mobility as HbS, sickle cell-hemoglobin D disease and sickle cell-hemoglobin G disease. Family studies are confirmatory; in the newborn with sickle cell-beta⁰-thalassemia, for example, one parent has sickle cell trait and the other beta thalassemia minor. When family members are not available, the diagnosis is established by DNA-based testing or repeat hemoglobin analysis at three to four months.

FAS and FSA patterns — The newborn with sickle cell trait will have HbF, HbA, and HbS (ie, the FAS pattern). The quantity of HbA is greater than that of HbS. If the quantity of HbS exceeds that of HbA (the FSA pattern), the presumptive diagnosis is sickle cell-beta⁺-thalassemia. Alternatively, the FAS pattern may be seen in a newborn with HbSS who has received a red blood cell transfusion prior to the hemoglobin analysis; this is often the case for premature infants admitted to the Neonatal Intensive Care Unit. Based on the ambiguity of the diagnosis, we treat all newborns with FAS with a prior red blood cell transfusion as having SCD until further evaluations can be made. Obtaining the hemoglobin analysis on the parents, DNA-based testing on the child, or repeat hemoglobin testing at age three to six months may be helpful.

In the future, PCR-based diagnosis from blood spotted onto a filter paper may be used to detect sickle cell genes directly [38].

Approach to a positive result from newborn/infant screening — Informing the parents of the results of newborn screening has value for the child and potentially other family members (table 2). It is important that the clinician fully understand the findings and their implications in preparation for discussing them with the parents.

●FS pattern (suggests sickle cell disease) – An FS pattern suggests homozygous sickle cell anemia or sickle-beta⁺ thalassemia, both of which are also referred to as sickle cell disease (SCD). Both of these are associated with risks of many acute and chronic complications and require comprehensive management. (See 'FS pattern' above.)

Upon diagnosis of SCD, there is an immediate obligation to implement a program of comprehensive care for the affected child and family [29,39,40]. This requires medical professionals with special expertise in SCD and access to multidisciplinary teams, including social workers, psychologists, nurses, genetic counselors, and nutritionists. The patient should be referred to a hematologist with expertise in managing SCD or a hemoglobinopathy center.

The following is appropriate while awaiting specialist referral:

•Inform the family of the diagnosis of an SCD

•Initiate prophylactic penicillin (125 mg orally twice daily)

•Inform the family that they should seek immediate medical attention if the infant has a fever of 101°F (38.3°C) and inform the treating clinician of the SCD diagnosis

•Teach the family how to examine the spleen and document whether the size is normal. Provide instructions on how to determine if splenic sequestration is occurring, and, if so, emphasize the need for prompt evaluation in an emergency department for management

•Provide information regarding the genetic inheritance of the disorder

Additional details of management are presented separately. (See "Sickle cell disease in infancy and childhood: Routine health care maintenance and anticipatory guidance" and "Overview of the management and prognosis of sickle cell disease".)

●FSA pattern (suggests a compound state) – An FSA pattern suggests a compound state involving a sickle cell mutation and another mutation such as beta⁺-thalassemia. (See 'FAS and FSA patterns' above.)

As with the FS pattern, the patient requires comprehensive care from a hematologist or hemoglobinopathy center. The following is appropriate while awaiting specialist referral:

•Inform the family of the diagnosis of a sickle disorder.

•Provide information regarding the genetic inheritance of the disorder.

•Repeat hemoglobin electrophoresis in three to six months to further characterize the hemoglobinopathy.

●FAS pattern (suggests sickle cell trait) – For infants with an FAS pattern, the family should be informed of the likely diagnosis of sickle cell trait. For those with an FAS pattern after receiving a blood transfusion, additional evaluations are needed to determine the diagnosis.

The following is also appropriate:

•Provide education regarding the inheritance of sickle cell diseases, and reassurance that the infant does not have a chronic blood disorder.

•Explain that under settings of significant hypoxia (eg, high altitude unpressurized aircraft, very strenuous exercise with dehydration), sickling can occur and there is a risk of hematuria, worsening of traumatic hyphema, and a very rare type of renal cancer. (See "Sickle cell trait", section on 'Clinical findings'.)

•Counsel patients on the reproductive consequences of sickle cell carrier status (eg, potential for SCD if the other parent is also a carrier). (See "Sickle cell trait", section on 'Reproductive issues'.)

•Offer hemoglobinopathy screening to other family members who may carry a sickle cell mutation and are unaware of their carrier status and referral for genetic counseling or hematologic consultation if further information is desired.

Additional information regarding the sickle cell trait (carrier status) is provided by the Centers for Disease Control and Prevention.

OLDER CHILDREN AND ADULTS — Despite newborn screening, many patients with sickle cell disease (SCD) or sickle cell trait may be undiagnosed, in part due to immigration of young, unscreened patients from other countries [41].

Laboratory methods — Diagnosis of the sickle cell disorders can be made with several methods, which are discussed in more detail separately. (See "Methods for hemoglobin analysis and hemoglobinopathy testing".)

●High performance liquid chromatography (HPLC) is the preferred method of diagnosis. HPLC is a highly precise technique for identification and quantification of hemoglobins and can be fully automated. In California and the majority of the United States, newborn samples are screened with HPLC. This technique detects most hemoglobin variants by their different retention times. It is highly sensitive and specific and provides both quantitative and qualitative interpretation [16]. Major disadvantages are the initial cost of the apparatus and reagents.

●Improvement in high voltage capillary electrophoresis makes it a comparable technique to HPLC.

●Thin-layer isoelectric focusing is a highly accurate and cost effective tool for the diagnosis of sickle or other hemoglobin variants [22]. The bands on isoelectric focusing are sharper than those on electrophoresis and can distinguish some hemoglobins not seen on standard electrophoresis. Isoelectric focusing is more complicated because it is also sensitive to the presence of methemoglobin and glycosylated hemoglobins.

●Cellulose acetate electrophoresis at pH 8.4 is a standard method of separating hemoglobin S (HbS) from other Hb variants. However, HbS, G, and D have the same electrophoretic mobility with this method (figure 1).

●Citrate agar electrophoresis at pH 6.2 separates HbS from HbD and G, which co-migrate with HbA in this system.

●There is no clinical situation in which a sodium metabisulfite test (Sickledex) is clinically indicated in the screening or management of sickle cell disorders (trait or disease). The test characteristics of the Sickledex confer a high likelihood of diagnostic error [42].

Thin-layer isoelectric focusing will separate HbS, D, and G. Alternatively, the combination of cellulose acetate electrophoresis with either citrate agar electrophoresis or a solubility test allows a definitive diagnosis of a sickle cell disorder. Even with thin-layer isoelectric focusing, it is still necessary to use a confirmatory solubility test for HbS.

In certain regions of the world, high-throughput molecular protocols (DNA testing) are being developed for hemoglobin disorders that may eventually replace the HPLC. DNA testing may be especially useful in cases such as those in which high concentrations of HbF raise the possibility of hereditary persistence of fetal hemoglobin [43]. (See "Fetal hemoglobin (hemoglobin F) in health and disease", section on 'HbF in the thalassemias and hereditary persistence of fetal hemoglobin'.)

Available DNA-based methods are discussed in detail separately. (See "Methods for hemoglobin analysis and hemoglobinopathy testing", section on 'Molecular genetic (DNA-based) methods'.)

Of note, the "sickle cell prep" using metabisulfite or dithionite is no longer in routine clinical use and is largely of historical interest. Rare exceptions are presented separately.

Point of care (POC) diagnostics — Point of care (POC) diagnostics are of potential use in low-resource countries with an at-risk population. Unlike the standard laboratory-based methods listed above, POC testing does not require expensive laboratory equipment and infrastructure.

Several promising techniques for simple, rapid, inexpensive diagnosis of SCD are being developed, including paper-based tests that quantify sickle hemoglobin, a density-based rapid test using aqueous multiphase systems, and lateral flow immunoassays [44,45]. Another approach to POC testing involves development of monoclonal antibodies directed against common hemoglobin variants [46]. These techniques appear to have excellent sensitivity and specificity to diagnose sickle cell anemia using dry blood and/or liquid samples. Pilot data indicate these tests can be performed in a clinical setting by minimally trained medical staff [46]. They can be used in remote areas of the world where hemoglobinopathies are common but trained clinicians, technical staff, and laboratory equipment are lacking [47,48].

These techniques are undergoing field testing and validation in newborn screening programs in resource-poor, high-risk areas [49,50]. Additional details about POC testing is presented separately. (See "Methods for hemoglobin analysis and hemoglobinopathy testing", section on 'Point-of-care assays'.)

Hemoglobin A2 measurement — Measurement and interpretation of levels of the minor hemoglobin, HbA2, has traditionally been considered valuable for the diagnosis of concomitant beta thalassemia. However, interpretation requires some expertise.

Beta thalassemia heterozygotes generally have an increased HbA2 level (table 1). However, a cross-sectional study demonstrated that most clinical diagnoses of sickle cell-beta⁰-thalassemia in African Americans with SCD did not match the associated genotype [51]. Most of the individuals considered to have a sickle cell-beta⁰-thalassemia phenotype based on hemoglobin analysis had elevated HbA2, high MCV, and high red blood cell count; on genetic analysis, these individuals actually had HbSS and concurrent alpha chain deletions. As an example, 13 of 18 (72 percent) of individuals with a genotype of HbSS and with one alpha chain deletion were misclassified as having sickle cell-beta⁰-thalassemia. Another 11 of 12 (92 percent) of those with HbSS and two alpha chain deletions were misclassified as having sickle cell-beta⁰-thalassemia.

These findings illustrate why HbA2 measurement above or below a certain threshold is not an accurate approach to distinguish HbSS from sickle cell-beta⁰-thalassemia phenotype. In the rare event that distinguishing the two SCD genotypes may be important for genetic counseling, gene analysis is required to have an accurate diagnosis.

Heterozygotes with delta-beta (δβ) thalassemia have a normal or decreased HbA2 level with a high HbF level. With many laboratory techniques, HbA2 levels are influenced by closely migrating hemoglobin variants such as HbE. Of importance, using HPLC and electrophoresis, HbA2 is overestimated in the presence of HbS, while severe iron deficiency reduces the HbA2 level; this is a particular problem in pregnant women who may be thalassemia carriers. When HbS is not present, microcolumn chromatography or HPLC accurately quantifies HbA2. Electrophoresis with scanning densitometry does not accurately quantitate HbA2 levels [52].

Findings in sickle cell anemia — The chronic hemolysis in those with sickle cell anemia (ie, HbSS) is usually associated with a mild to moderate anemia (hematocrit 20 to 30 percent), reticulocytosis of 3 to 15 percent (accounting for high or high-normal mean corpuscular volume [MCV]), unconjugated hyperbilirubinemia, and elevated serum lactate dehydrogenase (LDH) and low serum haptoglobin. The peripheral blood smear reveals sickled red cells (picture 1), polychromasia indicative of reticulocytosis, and Howell-Jolly bodies reflecting hyposplenia (picture 2). The red cells are normochromic unless there is coexistent thalassemia or iron deficiency. If the age-adjusted MCV is not elevated, the possibility of sickle cell-beta-thalassemia, coincident alpha thalassemia, or iron deficiency should be considered. (See "Diagnostic approach to anemia in adults".)

The HbF level is usually slightly to moderately elevated and HbA is absent (figure 1). The amount of HbF is a function of the number of reticulocytes that contain HbF, the extent of selective survival of HbF-containing reticulocytes to become mature HbF-containing erythrocytes, and the amount of HbF per red cell. Each variable is separately regulated and the expression of each shows interpatient variability [53]. In some patients with sickle cell anemia alone, values are as high (1 to 4 percent) as the modest elevations seen in heterocellular hereditary persistence of fetal hemoglobin [53]. (See "Overview of compound sickle cell syndromes".)

In addition, certain beta globin haplotypes appear to be related to factors that regulate production of HbF. As examples, the Arab-Indian and Senegal haplotypes are associated with higher levels of HbF (over 20 percent in some cases), probably due to linkage with important gamma globin regulatory sequences in the locus control region [54-56]. In one study of Senegalese patients, the mean HbF was 8.2 percent, and approximately one-half of patients had a benign form of sickle cell anemia [56]. (See "Fetal hemoglobin (hemoglobin F) in health and disease", section on 'Sickle cell disease'.)

Diagnostic patterns in other sickle cell disorders — Several of the sickle cell disorders may have similar results with electrophoresis or isoelectric focusing. Additional information from examination of the peripheral smear often helps to separate the sickle cell diseases. (See "Overview of compound sickle cell syndromes".)

●The diagnosis of HbSC disease is made by HPLC, isoelectric focusing, or hemoglobin electrophoresis, which demonstrates HbS and HbC in approximately equal amounts (or slightly more HbS than HbC), with no HbA present (figure 1). Two independent Hb analysis techniques are necessary to distinguish HbSC from HbSC Harlem and other compound heterozygotes. The peripheral blood smear shows a predominance of target cells, with rare sickled cells that may be canoe-shaped (picture 3).

●Results from electrophoresis or thin-layer isoelectric focusing (IEF) are similar in sickle cell anemia and sickle cell-beta⁰-thalassemia, as nearly all the hemoglobin consists of HbS, with no HbA evident. Differences in the levels of HbF and HbA2 and in the peripheral blood smear may be useful in distinguishing these disorders (table 1). In those with sickle cell anemia, both sickled and target cells are seen; red cell indices are generally normal. In sickle cell-beta⁰-thalassemia, sickled cells, target cells, and hypochromic microcytic discocytes are prominent. If one parent does not have sickle cell trait, this is a useful indicator of the presence of sickle cell-beta⁰-thalassemia in the child, rather than sickle cell anemia.

●Sickle cell-beta⁺-thalassemia and sickle cell trait both have substantial amounts of both HbA and HbS. Sickle cell trait is not associated with anemia or microcytosis and has a HbA fraction that exceeds 50 percent, along with 35 to 45 percent HbS (figure 1) [57]. Sickle cell-beta⁺-thalassemia is associated with anemia, microcytosis, and a HbA fraction that ranges between 5 and 30 percent [58]. Sickle cell trait in combination with alpha thalassemia can be suspected when there is less than 35 percent HbS (table 3). (See "Overview of compound sickle cell syndromes", section on 'Sickle-beta thalassemia' and "Overview of compound sickle cell syndromes", section on 'Sickle-alpha thalassemia'.)

●For over 20 years, solubility tests (Sickledex and others) have been recognized as inadequate tests to determine the presence of SCD or sickle cell trait [42,59]. Further, the American College of Obstetricians and Gynecologists (ACOG) recommends that only hemoglobin analysis should be done prenatally in pregnant women to determine sickle cell trait status or other hemoglobinopathy trait status. Despite the acknowledged limitations of the solubility test, obstetricians continue to use the solubility test to screen for sickle cell trait and SCD in prenatal clinics [60].

There are some SCD variants that are uncommon yet important [61]. (See "Overview of compound sickle cell syndromes".)

●Sickle cell-hemoglobin D-Punjab (D-Los Angeles) and sickle cell-hemoglobin O-Arab are moderate to severe diseases characterized by anemia, reticulocytosis, and often, macrocytosis.

●Sickle cell-hemoglobin C-Harlem is slightly milder than sickle cell anemia with a similar blood film.

●Sickle cell-hemoglobin Lepore, also a moderately severe disease, is more characterized by microcytosis and a blood smear comparable to sickle cell-beta-thalassemia.

●Sickle cell-hereditary persistence of fetal hemoglobin is usually asymptomatic or extremely mild. Mild anemia and reticulocytosis may be noted. The electrophoresis may be misread as sickle cell anemia with an elevated HbF. Definitive diagnosis requires family studies or DNA analysis. (See "Fetal hemoglobin (hemoglobin F) in health and disease", section on 'Sickle cell disease'.)

●Sickle cell-hemoglobin E is a clinically mild disease. The blood film shows targeting and variable microcytosis.

Approach to a positive result in older children or adults

●Sickle cell trait – Older children or adults may be diagnosed with sickle cell trait as part of screening of asymptomatic family members or prenatal screening. Individuals with sickle cell trait should be reassured that they do not have a chronic blood disease. They should understand the risks of complications of significant hypoxia, the effect on the accuracy of HbA1c in monitoring diabetes, and the reproductive implications of sickle cell trait status [62].

All individuals with a child with SCD should be offered the opportunity to receive individualized genetic counseling regarding the risk of having another child with SCD. Cultural sensitivity and clinical competency are required regarding individual genetic counseling after the SCD status of both the child and the parent is acknowledged. Both cultural sensitivity and clinical competence are required because the hemoglobin analysis of the father may raise the possibility of non-paternity. Understanding the complex genetics of maternal uniparental disomy and SCD is a critical component of assessment of genetic counseling of the paternity status [63,64]. Additional information regarding the sickle cell trait (carrier status) is provided by the Centers for Disease Control and Prevention. Additional details regarding management are discussed in detail separately. (See "Sickle cell trait".)

●Sickle cell disease – Older children or adults diagnosed with sickle cell disease (SCD) may be at risk for many acute and chronic complications. They require comprehensive evaluation and management by a hematologist with experience in SCD or a hemoglobinopathy center. (See "Sickle cell disease in infancy and childhood: Routine health care maintenance and anticipatory guidance", section on 'Age five years to adolescence' and "Sickle cell disease in infancy and childhood: Routine health care maintenance and anticipatory guidance", section on 'Older adolescents' and "Overview of the management and prognosis of sickle cell disease".)

Once a diagnosis of SCD has been made, there are a number of additional evaluations that are helpful in predicting disease course and identifying early markers of disease complications. This may include testing to identify common coinheritance or genetic polymorphisms that influence the severity of SCD:

One example is alpha thalassemia trait (one alpha globin gene deletion), which occurs in approximately 35 percent of individuals with HbSS, or two alpha globin gene deletions, occurring in approximately 5 percent of the individuals with HbSS [65].

Molecular (DNA-based) techniques are required to detect alpha thalassemia trait. The presence of alpha thalassemia trait can decrease the imbalance between alpha and beta globin chains, which in turn decreases abnormal hemoglobin polymerization and reduces anemia and clinical complications.

In the newborn period, hemoglobin Barts can be quantitated as an indication of the presence of alpha thalassemia trait by protein electrophoresis or HPLC. Quantitative Barts is often performed as part of the newborn screening program and reported to providers. (See "Diagnosis of thalassemia (adults and children)", section on 'Overview of subtypes and disease severity'.)

Other polymorphisms alter the level of fetal hemoglobin expression. (See "Fetal hemoglobin (hemoglobin F) in health and disease", section on 'Sickle cell disease'.)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Sickle cell disease and thalassemias".)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5^th to 6^th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10^th to 12^th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

●Basics topics (see "Patient education: Sickle cell trait (The Basics)" and "Patient education: Sickle cell disease (The Basics)" and "Patient education: When your child has sickle cell disease (The Basics)")

SUMMARY AND RECOMMENDATIONS

●The diagnosis of sickle cell disorders can take place in several settings: prenatal testing, newborn screening, diagnosis of symptomatic individuals, and counseling of those with sickle cell trait. (See 'Reproductive testing and counseling' above and 'Newborn screening' above and 'Older children and adults' above.)

●Diagnosis of one of the sickle cell disorders is generally made via high performance liquid chromatography (HPLC), isoelectric focusing (IEF), or gel electrophoresis techniques. In prenatal diagnosis and in areas with high frequency of non-sickle-cell disorders, polymerase chain reaction (PCR) techniques, or direct DNA testing may be used. These methods are described in more detail separately. (See 'Methodology and diagnostic errors' above and "Methods for hemoglobin analysis and hemoglobinopathy testing".)

●Only general patterns of hemoglobin production are available during the newborn period because beta globin production (which includes production of hemoglobins A and S [HbA and HbS]) is not fully developed (table 1). (See 'Hemoglobin patterns' above.)

If questions arise as to interpretation, the tests should be repeated at age three to six months, when beta globin production is complete. DNA testing can be performed if clinically indicated. (See 'FAS and FSA patterns' above.)

●Hemoglobin electrophoresis testing after transfusion of red cells can result in an incorrect diagnosis. In such cases, sickle cell diagnosis should utilize DNA testing or be postponed for at least four months after transfusion. In very premature babies, HbA may not be detected, resulting in misdiagnosis. In addition, very premature babies with sickle cell trait may be found to have HbS levels greater than HbA, resulting in the incorrect diagnosis of hemoglobin S/beta⁺ thalassemia. (See 'Methodology and diagnostic errors' above.)

●For children and adults, the combination of HPLC and IEF allows for a definitive diagnosis of one of the sickle cell disorders. (See 'Older children and adults' above.)

●Sickledex or solubility testing is not adequate to test the presence of sickle cell disease or sickle cell trait. (See 'Laboratory methods' above.)

●The most common sickle cell disease disorders and their HPLC, IEF, and gel electrophoresis patterns are as follows (table 1) (see 'Findings in sickle cell anemia' above and 'Diagnostic patterns in other sickle cell disorders' above):

•Sickle cell trait – In sickle cell trait, the usual pattern is to find >50 percent HbA, 35 to 45 percent HbS, and <2 percent HbF. The presence of <35 percent HbS suggests the presence of alpha thalassemia.

•Sickle cell anemia – In sickle cell anemia (ie, HbSS), there is 0 percent HbA, <2 percent HbF, normal amounts of HbA2, and the remainder HbS.

•Hemoglobin SC disease – In hemoglobin SC disease, HbS and HbC are both present.

•Sickle cell-beta⁺-thalassemia – In sickle cell-beta⁺-thalassemia, there is 5 to 30 percent HbA, increased HbA2, with the remainder HbS. Target cells and hypochromic red cells are also present.

•Sickle cell-beta⁰-thalassemia – In sickle cell-beta⁰-thalassemia, there is 0 percent HbA, along with variable amounts of HbF, increased amounts of HbA2, with the remainder HbS. Target cells and hypochromic microcytic red cells are also present.

●Informing the parents of the results of testing (newborn screen (table 2) or testing of an older child) has value for the child and potentially other family members, as does informing adults with a new diagnosis of SCD or sickle cell trait. (See 'Approach to a positive result from newborn/infant screening' above and 'Approach to a positive result in older children or adults' above.)

●Management of SCD, which includes a number of post-diagnostic evaluations, is discussed in detail separately. (See "Overview of the management and prognosis of sickle cell disease", section on 'Routine evaluations and treatments' and "Sickle cell disease in infancy and childhood: Routine health care maintenance and anticipatory guidance".)

ACKNOWLEDGMENT — We are saddened by the death of Stanley L Schrier, MD, who passed away in August 2019. The editors at UpToDate gratefully acknowledge Dr. Schrier's role as Section Editor on this topic, his tenure as the founding Editor-in-Chief for UpToDate in Hematology, and his dedicated and longstanding involvement with the UpToDate program.

The UpToDate editorial staff also acknowledges extensive contributions of Donald H Mahoney, Jr, MD to earlier versions of this topic review.