Haemoglobinopathies in pregnancy — structured oral station (12 minutes)

Opening prompt — "Tell me your interpretation of these results."

Model response — say it in this order: ^[5][6]

• "This is a woman of South-East Asian background with microcytosis (MCV 74 fL) and a normal ferritin (75 micrograms per L). Iron deficiency is excluded by the ferritin. The most likely diagnosis is thalassaemia trait — alpha-thalassaemia trait is particularly common in Vietnamese and other South-East Asian populations."^[5][6]
• "I would send haemoglobin electrophoresis or HPLC to look for beta-thalassaemia trait or haemoglobin variants. For alpha-thalassaemia, electrophoresis is uninformative in single-gene deletion carriers; DNA testing is required if she is at risk."^[2]
• "The key implication is reproductive — I need to know her partner's status. If both are carriers of alpha-thalassaemia, each pregnancy carries a 1 in 4 risk of Hb Barts hydrops fetalis, which is lethal."^[3]

Examiner is listening for: the ferritin-based exclusion of iron deficiency, the ethnicity-based pre-test probability, and the reproductive implications. ^[5][6]

Probe 1 — "Why is the ferritin so important here?"

• "Ferritin below 30 micrograms per L is diagnostic of iron deficiency; a ferritin above 30 — and especially above 60 — rules out iron deficiency. The temptation is to treat any microcytosis with iron, but in a thalassaemia carrier that risks iron overload — the carrier already absorbs iron efficiently and does not need exogenous iron."^[5][6]
• "The Mentzer index (MCV divided by red cell count in millions per microlitre) above 13 favours iron deficiency, below 13 favours thalassaemia trait. This woman has a high red cell count, consistent with thalassaemia trait."

Probe 2 — "Her partner is also a carrier. What is the risk to the fetus and what do you offer?"

• "If both parents are alpha-thalassaemia carriers, each pregnancy carries a 1 in 4 risk of Hb Barts hydrops fetalis — gamma-4 tetramers that cannot release oxygen — which is lethal in utero or shortly after birth. Maternal complications include pre-eclampsia (mirror syndrome) and obstructed labour."^[3]
• "I would refer to a genetic counsellor and offer prenatal diagnosis by chorionic villus sampling at 11 to 13 weeks or amniocentesis at 15 to 16 weeks, with DNA analysis of the alpha-globin genes."^[3]
• "If the fetus is affected, the options are termination of pregnancy or, in expert centres, intrauterine transfusion — but the long-term outcome is poor even with intrauterine transfusion."^[3]

Probe 3 — "What if her partner is beta-thalassaemia carrier and she is also beta-thalassaemia carrier?"

• "Beta-thalassaemia major (Cooley anaemia) is the most severe outcome — transfusion-dependent from infancy with iron overload and its cardiac, endocrine and hepatic consequences. Beta-thalassaemia intermedia is variable."^[2]
• "Both beta-thalassaemia carrier parents carry a 1 in 4 risk of beta-thalassaemia major in each pregnancy. Prenatal diagnosis is offered by CVS or amniocentesis. Some couples choose preimplantation genetic diagnosis with IVF."^[2][4]

Probe 4 — "She asks whether being a carrier affects her pregnancy. How do you answer?"

This is a scored domain, not a courtesy. Demonstrate it out loud: ^[5]

• Move to her eye level, use her name, acknowledge the worry.
• "Being a carrier does not usually cause problems in your own pregnancy. You may have mild anaemia, but it does not require iron — in fact, taking iron could be harmful because carriers absorb iron efficiently and can develop iron overload."
• Be specific about the implications: "The key issue is whether your partner is also a carrier, because that determines whether your baby could have a more serious form. I would like to test him today if he can come in, or arrange for him to be tested soon."

Probe 5 — "She goes on to have a healthy pregnancy. What about future pregnancies?"

• "Counsel on carrier status, partner testing, and the availability of prenatal diagnosis. If both partners are carriers, options include prenatal diagnosis in each pregnancy or preimplantation genetic diagnosis with IVF."^[3][4]
• "Encourage genetic counselling before the next conception. Family screening is important — siblings and other relatives may also be carriers."^[4]