Hemoglobinopathies are the most common monogenic disorders worldwide, affecting millions of people. They are broadly classified into two groups: qualitative defects — abnormal hemoglobin structure (sickle cell disease, HbC, HbE, HbD) — and quantitative defects — reduced synthesis of normal globin chains (α- and β-thalassemia). Laboratory diagnosis relies on CBC parameters, RBC indices, peripheral blood smear examination, and hemoglobin separation techniques.
Normal Hemoglobin Types
Normal adult hemoglobin (HbA, α₂β₂) comprises 95–98% of total hemoglobin. HbA2 (α₂δ₂) accounts for 2–3.5%, and HbF (fetal hemoglobin, α₂γ₂) accounts for < 1% in adults. At birth, HbF is the predominant hemoglobin (70–90%), with HbA increasing during the first year of life as γ-chain synthesis is replaced by β-chain synthesis. Each globin chain contains a heme group with an iron atom that reversibly binds oxygen. The α-globin genes are located on chromosome 16 (two copies, HBA1 and HBA2), and the β-globin gene cluster is on chromosome 11.
Sickle Cell Disease
Sickle cell disease (SCD) is caused by a point mutation (GAG → GTG, Glu6Val) in the β-globin gene, producing hemoglobin S (HbS). Deoxygenated HbS polymerizes into long fibers, distorting RBCs into sickle shapes. Sickled cells cause vaso-occlusion (painful crises, acute chest syndrome, stroke, priapism), chronic hemolytic anemia, and progressive organ damage. Laboratory findings: Hb 6–10 g/dL, elevated reticulocyte count (RPI > 2), and variable platelet count (increased in chronic disease). The blood smear in SCD shows sickle cells (drepanocytes), boat cells, polychromasia (reticulocytes), nucleated RBCs, Howell-Jolly bodies (functional asplenia), and target cells.
Diagnosis of Sickle Cell Disease
The HbS solubility test (dithionite reduction test) detects HbS polymerization in a concentrated hemoglobin solution, producing turbidity. It is positive for HbS but does not distinguish sickle cell trait (HbAS) from disease (HbSS). Hemoglobin electrophoresis at alkaline pH (cellulose acetate or agarose gel) separates HbA, HbF, HbS, and HbC by charge: HbA migrates fastest, followed by HbF, then HbS, with HbC slowest. High-performance liquid chromatography (HPLC) and capillary electrophoresis are the standard methods for quantifying hemoglobin fractions. Capillary zone electrophoresis (CZE) at alkaline pH resolves HbA, HbF, HbS, and HbC by their electrophoretic mobility. In homozygous SCD (HbSS), HbS is 80–95%, HbF 2–20%, and HbA2 normal. In sickle cell trait (HbAS), HbA is 55–65% and HbS 35–45%. Isoelectric focusing (IEF) — including its capillary format, capillary isoelectric focusing (CIEF) — provides high resolution for variant identification. DNA-based testing (PCR, allele-specific hybridization, or sequencing) confirms the genotype.
Sickle Cell Trait
Sickle cell trait (HbAS) affects 1–3 million people in the United States and 25–30% of the population in parts of West Africa. It is usually asymptomatic with normal hemoglobin, normal RBC indices, and no anemia. The blood smear is normal, and the HbS solubility test is positive. Hemoglobin electrophoresis shows HbA 55–65%, HbS 35–45%, and HbA2 normal. Complications are rare but include exercise-related rhabdomyolysis, renal medullary carcinoma, and hematuria.
β-Thalassemia
β-thalassemia is caused by reduced (β⁺) or absent (β⁰) synthesis of β-globin chains, leading to an excess of α-globin chains that precipitate and cause ineffective erythropoiesis and hemolysis. β-thalassemia major (Cooley anemia) requires lifelong transfusion from infancy, presents with severe microcytic hypochromic anemia (Hb < 7 g/dL), marked hepatosplenomegaly, and skeletal changes from marrow expansion. β-thalassemia intermedia has variable severity, with Hb 7–10 g/dL, and may not require regular transfusion. β-thalassemia trait (minor) is asymptomatic with mild microcytic anemia, Hb rarely < 10–11 g/dL, and RBC count often elevated. Diagnosis: HbA2 is elevated (> 3.5%, typically 4–8%) in β-thalassemia trait, while HbF may be mildly increased. In β-thalassemia major, HbF is markedly elevated (up to 90%), HbA2 is variable, and HbA is decreased or absent.
α-Thalassemia
α-thalassemia results from deletion or mutation of one or more of the four α-globin genes. One α gene deletion (α-thalassemia minima, silent carrier) is clinically silent with normal CBC. Two α gene deletions (α-thalassemia trait) cause mild microcytic hypochromic anemia similar to β-thalassemia trait, but HbA2 is normal. Three α gene deletions (HbH disease) produce moderate hemolytic anemia with microcytic indices, and HbH (β₄ tetramers) is detectable on electrophoresis as a fast-moving band. Hemoglobin electrophoresis shows HbH (5–40%) and variable Hb Bart’s (γ₄) at birth. Four α gene deletions (Hb Bart’s hydrops fetalis) is incompatible with life, causing severe fetal anemia, hydrops, and intrauterine death.
Distinguishing Thalassemia Trait from Iron Deficiency
Differentiating β-thalassemia trait from iron deficiency anemia is a common laboratory challenge. In thalassemia trait: RBC count is normal or elevated (uncompensated microcytosis), RDW is normal, ferritin and iron studies are normal, and HbA2 is elevated. In iron deficiency: RBC count is low, RDW is elevated, ferritin and TSAT are low, and HbA2 is decreased. The Mentzer index (MCV / RBC count) is < 13 in thalassemia trait and > 13 in iron deficiency. The blood smear in thalassemia trait shows microcytosis, hypochromia, and target cells.
Hemoglobin E and Other Variants
HbE (Glu26Lys) is the most common hemoglobin variant in Southeast Asia, where it can reach allele frequencies of 50% in some populations. HbE is mildly unstable and produces mild microcytosis and hypochromia in heterozygotes. HbE-β-thalassemia is a clinically important compound heterozygote causing severe anemia. HbC (Glu6Lys) is common in West Africa. HbC disease (homozygous) causes mild hemolytic anemia with abundant target cells and HbC crystals in RBCs. HbD-Punjab (Glu121Gln) is common in India and can co-inherit with sickle cell disease. HbO-Arab, HbG-Philadelphia, and Hb Lepore are other clinically important variants. High-performance liquid chromatography and capillary electrophoresis have greatly improved variant detection and quantification.
