CASE HISTORY • Part 1
As a part of a routine physical examination for college sports, an 18-year-old college freshman with a completely negative past medical history has a routine CBC. The results are as follows:
CBC: Hematocrit/hemoglobin - 39%/13 g/dL
MCV - 74 fL MCH - 29 pg MCHC - 30 g/dL
RDW-CV - 18%
WBC count - 7,500/μL
Platelet count - 180,000/μL
Reticulocyte count/index - 2.0%/1.7 Questions
In addition to iron deficiency, an inherited defect in globin chain synthesis is the other leading cause of microcytic anemia worldwide in children and adults. The frequency and severity of the several types of thalassemia depend on the racial or ethnic background of the population. For example, β-thalassemia is commonly seen in individuals from Africa and the Mediterranean area, whereas α-thalassemia and hemoglobin E disease are common in Southeast Asian populations. For certain subpopulations, the incidence of a microcytic, hypochromic anemia secondary to thalassemia can exceed that due to iron-deficiency anemia.
The diagnosis of thalassemia requires an understanding of normal globin chain synthesis, familiarity with the laboratory tests used to identify chain deficiencies, and a careful clinical evaluation of the patient. Because it is easy to confuse the microcytosis of iron deficiency with that of thalassemia, iron studies need to be performed on all microcytic patients. Moreover, both conditions can coexist. Accurate diagnosis of the chain synthesis abnormality is also very important for genetic counselling.
NORMAL GLOBIN CHAIN SYNTHESIS
Globin, the protein portion of the hemoglobin molecule, is a tetramer of 2 α and 2 non–α-globin chains (Figure 6-1). The α-globin chains are encoded by 2 closely linked genes (α2 and α1) on chromosome 16. The non-α genes, β, γ, and δ, are encoded by a cluster of genes on chromosome 11. The pairing of these several globin chains produces 3 types of hemoglobin: hemoglobin F (α2γ2), hemoglobin A (α2β2), and hemoglobin A2 (α2δ2). A failure in α-globin chain synthesis can result in hemoglobin H, a tetramer of β-globin chains.
Globin genes. Specific genes on chromosomes 16 and 11 control the production of the α-, β-, γ-, and δ-globin chains.
Quantity of Hemoglobin and Diagnosis
Globin gene expression varies with the patient's age. Throughout fetal life, red blood cells contain only hemoglobin F (Figure 6-2). During the first several months of life, γ-globin gene expression is gradually suppressed and both β-globin and δ-globin syntheses are activated. In adults, red blood cells contain primarily hemoglobin A (96%–97% of ...