Hemoglobin vs hematocrit: what each measures and why they move together

Every red cell is a bag of hemoglobin. Hemoglobin measures how much of that protein is in a volume of blood; hematocrit measures how much of that volume the cells themselves take up. Because a normal red cell holds a fixed concentration of hemoglobin (the MCHC, about 32–36 g/dL), the two numbers track each other almost perfectly: multiply the hemoglobin by three and you have the hematocrit, within a point or two. A hemoglobin of 14 g/dL goes with a hematocrit of about 42%.

Anything that changes the plasma volume moves both the same way. Dehydration concentrates them; the extra plasma of pregnancy or intravenous fluids dilutes them; altitude, smoking, sleep apnea and testosterone raise both by driving red-cell production; bleeding and iron, B12 or folate deficiency lower both.

The ratio breaks when the cells are abnormal. In hereditary spherocytosis the cells are dense, so the hemoglobin is high for the hematocrit (a high MCHC); in a lipemic sample or with very high white counts the analyzer over-reads the hemoglobin; with abnormal cell shapes or clumping the calculated hematocrit drifts. Most analyzers measure hemoglobin directly and calculate hematocrit, so when the two tell different stories the hemoglobin is usually the one to trust, and a blood film explains the difference.

Anemia is defined on hemoglobin — the WHO thresholds are under 13 g/dL in men, under 12 in non-pregnant women and under 11 in pregnancy — because hemoglobin is what carries oxygen and what transfusion and treatment decisions are based on. At the other end, hematocrit is the traditional number: the WHO criteria for polycythemia vera use a hematocrit above 49% in men or 48% in women (or hemoglobin above 16.5/16), and the Endocrine Society's guidance on testosterone therapy acts on a hematocrit of 54% because it is the hematocrit that tracks blood viscosity and clot risk.

Low values are investigated with the red-cell size (MCV) and ferritin: small cells point to iron deficiency, large cells to B12, folate, alcohol or thyroid, normal-sized cells to blood loss, kidney disease or chronic illness. High values are first repeated hydrated and, if persistent, explained by smoking, sleep apnea, altitude, testosterone or lung disease before the uncommon bone-marrow cause (polycythemia vera, checked with a JAK2 test) is considered.