A 27-year-old woman at 20 weeks has haemoglobin 10.2 g/dL, MCV 66 fL, serum ferritin 45 ng/mL, and RDW within normal limits. Her RBC count is 5.8 million/mm3. Which calculation best distinguishes the two most likely diagnoses?

• A Mean corpuscular haemoglobin concentration divided by RBC count
• B MCV divided by RBC count (Mentzer index) ✓
• C Transferrin saturation multiplied by MCH
• D RDW multiplied by serum ferritin

Explanation

The Mentzer index, calculated as MCV divided by RBC count, separates beta thalassaemia trait from iron deficiency in microcytosis. An index below 13 suggests thalassaemia trait, because the RBC count remains disproportionately high; an index above 13 favours iron deficiency. Here the normal ferritin and low RDW already point toward thalassaemia trait. Confirmatory testing is HbA2 estimation, which is raised above 3.5 percent in beta thalassaemia trait.

Reference: Williams Obstetrics, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.