Anaemia: pattern-based investigation — KFP-style written assessment

KFP 1 (10 marks)

A 34-year-old woman presents with fatigue. Hb 118 g/L, MCV 74 fL, ferritin 12 µg/L, CRP 2 mg/L. She is a vegetarian, menstruates heavily (8 days, changing hourly on day 2), and her father has beta-thalassaemia trait. RBC count 6.1 x10^12/L. ^{[6] [13]}

1. Calculate the Mentzer index and interpret with its limits. (3) ^[6]
2. Which single test best resolves the iron-deficiency versus beta-trait question here, and why is timing critical? (4) ^[13]
3. Outline the treatment plan including dosing strategy. (3) ^[5]

Model answers

1. Mentzer index = MCV/RBC = 74/6.1 = 12.1. Below 13 suggests beta-thalassaemia trait; above 13 suggests iron deficiency. Her index sits in the trait range — but the index is a screen with imperfect specificity, and her heavy periods plus vegetarian diet are strong IDA drivers, so both conditions may coexist. ^[6]
2. Serum ferritin first (already done: 12 µg/L — confirms iron deficiency), then HPLC or haemoglobin electrophoresis for HbA2 quantification. Critical timing point: iron deficiency suppresses HbA2 elevation, so electrophoresis must be interpreted with iron status in mind — ideally after iron repletion if trait remains suspected. Ferritin below 15 ng/mL characterised IDA while 95% of trait cases exceeded it in a pregnancy screening study. ^[13]
3. Alternate-day morning oral iron (e.g. 60-100 mg elemental iron on alternate days): at equal total doses it matched daily ferritin gains with fewer GI side effects in randomised trials. Address menstrual blood loss as the driver — gynaecological review alongside iron replacement. Recheck Hb and ferritin at 8–12 weeks to confirm response, then continue 3 months after normalisation to replete stores. ^[5]