Does an elevated RBC count with low MCV and MCH indicate microcytic erythrocytosis?

An elevated RBC count with low MCV and MCH is a recognized laboratory pattern—microcytic erythrocytosis—not a diagnosis by itself. In a 77-year-old man, it merits confirmation and etiologic evaluation rather than attribution to hydration status alone.

Clinical interpretation

• Low MCV and MCH indicate that individual red cells are small and contain less hemoglobin (microcytosis and hypochromia), while the elevated RBC count indicates erythrocytosis.
• This combination is particularly suggestive of thalassemia trait or another hemoglobinopathy. In a 137-patient cohort with microcytic erythrocytosis, 67.9% had a thalassemia or hemoglobinopathy; beta-thalassemia trait was most common.
• Other clinically important possibilities include iron deficiency coexisting with thalassemia, secondary erythrocytosis, or polycythemia vera (PV), in which iron deficiency can obscure the expected hemoglobin/hematocrit elevation.

Why hemoconcentration alone is insufficient

• Plasma-volume contraction from volume loss, gastrointestinal losses, or diuretics can cause relative erythrocytosis: higher measured RBC concentration, hemoglobin, and hematocrit despite normal total red-cell mass.
• It does not intrinsically cause persistently low MCV and MCH, which reflect red-cell properties. Osmotic dehydration can variably affect MCV in experimental settings, but does not reliably explain hypochromia or the full pattern.

Practical implications

• Repeat the CBC after correcting plausible volume depletion and review prior blood counts.
• Initial evaluation commonly includes peripheral smear and ferritin/iron studies. If iron deficiency is not explanatory, hemoglobin analysis can assess beta-thalassemia; normal electrophoresis does not exclude alpha-thalassemia.
• Persistent erythrocytosis—especially with leukocytosis, thrombocytosis, splenomegaly, or iron deficiency—supports evaluation with erythropoietin and JAK2-directed testing for PV.

Bottom line

• The claim is supported: elevated RBC count plus low MCV/MCH indicates microcytic erythrocytosis and generally requires an additional hematologic or iron-related explanation beyond simple hemoconcentration.