In a case of low MCV and low RDW but high RBC, what diagnosis might be indicated?

The scenario of low mean corpuscular volume (MCV), low red cell distribution width (RDW), and high red blood cell (RBC) count is suggestive of thalassemia.

In thalassemia, the body produces an abnormal form of hemoglobin, leading to the production of smaller-than-normal red blood cells, resulting in the low MCV. The low RDW indicates that the red blood cells are generally uniform in size, which is characteristic of thalassemia since the disorder typically causes a consistent microcytic anemia with little variation in cell size. The elevated RBC count is also a typical manifestation in thalassemia as the marrow attempts to compensate for the ineffective erythropoiesis and hemolysis, leading to an increased production of red blood cells.

This combination of lab findings—particularly the low RDW and high RBC count—helps to differentiate thalassemia from other causes of microcytic anemia, such as iron deficiency anemia, where RDW is usually elevated. In iron deficiency anemia, the variability in red blood cell sizes becomes greater as the body depletes its iron stores leading to both small cells and a mixture of sizes, reflecting the body's attempt to respond to the deficiency.