Once the MCV has placed the patient in a category, two more indices sharpen the differential. MCHC is the mean corpuscular hemoglobin concentration: the average hemoglobin per unit volume of red cells, normally around 33 to 36 g/dL. When MCHC falls below roughly 32 g/dL, the cells are hypochromic, meaning they carry less hemoglobin than normal for their size. Hypochromia is a strong clue toward iron deficiency or thalassemia, both of which produce small, pale cells. A high MCHC is unusual and should raise suspicion of spherocytosis or a spurious result.
RDW is the red cell distribution width: a measure of how much red cell sizes vary, expressed as a coefficient of variation. A normal RDW means the red cells are relatively uniform in size. A high RDW means anisocytosis, a mixture of large and small cells. This distinction matters because it separates uniform from mixed populations. Iron deficiency typically raises the RDW early, before the MCV has fallen, because the first microcytes appear alongside normal cells. Thalassemia trait, by contrast, usually has a normal RDW because all the cells are uniformly small. A high RDW in a patient with a normal MCV suggests a mixed deficiency or an early stage of a single deficiency. The RDW is therefore not just a number; it tells you whether you are looking at one process or two.