Red cell indices (MCV, MCH, MCHC)

Principle

• Indices describe average red cell size and haemoglobin content, calculated from RBC count, Hb and Hct (or measured MCV on analysers).
• MCV (fL) = Hct (L/L) ÷ RBC (× 10^12/L) × 1000, or Hct (%) × 10 ÷ RBC.
• MCH (pg) = Hb (g/dL) × 10 ÷ RBC (× 10^12/L).
• MCHC (g/dL) = Hb (g/dL) ÷ Hct (%) × 100, or Hb (g/dL) ÷ Hct (L/L).
• Example: Hb 15 g/dL, Hct 0.45, RBC 5.0 × 10^12/L gives MCV 90 fL, MCH 30 pg, MCHC 33.3 g/dL.

Interpretation

• Microcytic hypochromic (low MCV and MCH): iron deficiency, thalassaemia trait, anaemia of chronic disease, sideroblastic anaemia.
• Macrocytic (high MCV): B12 or folate deficiency (megaloblastic), alcohol, liver disease, hypothyroidism, reticulocytosis, MDS, drugs such as hydroxyurea.
• Normocytic: acute blood loss, haemolysis, renal disease, marrow failure.
• High MCHC (above 36): hereditary spherocytosis, or artefact.
• RDW helps: often high in iron deficiency, often normal in thalassaemia trait.

Normal values

• Typical adults: MCV about 80–100 fL, MCH about 27–32 pg, MCHC about 32–36 g/dL; RDW-CV about 11.5–14.5%. Local ranges apply; children have lower MCV.

Quality control and pitfalls

• MCHC is a useful analyser check; a value well above 37 g/dL usually indicates an error.
• Cold agglutinins: low RBC count, falsely high MCV, MCH and MCHC; warm to 37 °C and re-run.
• Lipaemia, icterus or high WBC: falsely high Hb, so high MCH and MCHC.
• Hyperglycaemia or hypernatraemia can falsely raise MCV on impedance analysers.
• Storage of EDTA blood causes red cell swelling and rising MCV.

Clinical use

• Morphological classification of anaemia and choice of further tests.