American Board of Internal Medicine (ABIM) Certification Practice Exam

Which laboratory finding helps to distinguish iron deficiency anemia (IDA) from α-thalassemia trait?

1. Hemoglobin electrophoresis

2. Red cell distribution width (RDW)

3. Mean corpuscular volume (MCV)

4. Platelet count

The correct answer is: Red cell distribution width (RDW)

The laboratory finding that is most useful in distinguishing iron deficiency anemia (IDA) from α-thalassemia trait is the red cell distribution width (RDW). In iron deficiency anemia, the RDW is typically increased. This increase occurs because iron deficiency leads to the production of smaller red blood cells (microcytosis) as well as larger ones as the body attempts to compensate for anemia, resulting in a heterogeneous population of red blood cell sizes. Therefore, the RDW reflects a greater variability in red blood cell size. In α-thalassemia trait, on the other hand, the RDW is usually normal. The red blood cells produced are more uniform in size because the underlying issue is a deficiency in the number of alpha-globin chains rather than a problem with iron availability. As a result, the sizes of red blood cells are consistently small and do not vary significantly. While hemoglobin electrophoresis can provide important information about different hemoglobin types, it is primarily used to diagnose hemoglobinopathies and may not effectively differentiate between IDA and α-thalassemia trait. The mean corpuscular volume (MCV) would typically be low in both conditions. Platelet count changes do not effectively distinguish the two disorders either,