Carrier Screening and Prenatal Diagnosis of Haemoglobinopathies
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Direct answer
Carrier screening for haemoglobinopathies identifies women and their partners who carry beta thalassaemia, alpha thalassaemia or sickle cell trait, so that an affected fetus can be detected early. Screening starts with a complete blood count and haemoglobin analysis by HPLC, and the partner is tested whenever the woman is a carrier. Prenatal diagnosis by chorionic villus sampling or amniocentesis is offered when both partners carry a variant.
What you must remember
- Screening test: complete blood count with MCV and MCH, followed by HPLC; an HbA2 above about 3.5 per cent suggests beta thalassaemia trait.
- Iron deficiency mimic: iron deficiency lowers HbA2 and can mask beta thalassaemia trait, so iron status is corrected and rechecked before the result is interpreted.
- Alpha thalassaemia: silent carriers often have normal haemoglobin studies, so DNA analysis is needed; HbH disease and Hb Bart's hydrops fetalis are the severe forms.
- Partner testing: if the woman is a carrier, the partner is tested, because carrier status is a couple-level risk.
- Risk in two beta carriers: each pregnancy carries a 25 per cent risk of beta thalassaemia major, and each child has a 50 per cent chance of being a carrier.
- Prenatal timing: chorionic villus sampling is usually done at 10 to 13 weeks, and amniocentesis from about 15 weeks.
- Counselling: non-directive; the decision about continuing the pregnancy rests with the couple within the legal framework.
A partner-linked algorithm from first screen to diagnosis
A couple attending antenatal clinic with a family history of anaemia needs an answer in a clear sequence. The partner is tested only once the woman's result is known, and each step narrows the choice of prenatal test.
- Screen the woman: complete blood count, MCV, MCH and HPLC; iron deficiency is excluded before HbA2 is interpreted.
- Test the partner if she is a carrier: the same panel is run, with DNA analysis for alpha thalassaemia where indicated.
- Count the risk: when both partners are beta carriers, each pregnancy carries a 25 per cent risk of an affected child.
- Offer prenatal diagnosis: chorionic villus sampling at 10 to 13 weeks or amniocentesis after about 15 weeks, with DNA-based testing on fetal material.
- Counsel the couple: the results, options and the meaning of an affected diagnosis are explained in a non-directive way.
- Plan newborn care: if an affected child is expected, the neonatal and paediatric team is involved early.
Where the exam tries to catch you
Vignettes exploit iron deficiency and silent alpha carriers, and they reward the student who tests the partner rather than stopping at the woman.
- Trap: Calling a microcytic woman a thalassaemia carrier without checking iron. Correct: Iron deficiency lowers MCV and HbA2 and can hide the trait, so iron replacement and repeat testing come first.
- Trap: Testing only the woman when the partner's result would change management. Correct: Carrier status is a couple-level risk, so the partner's HPLC or DNA test is essential.
- Trap: Assuming a normal HbA2 excludes alpha thalassaemia. Correct: Silent alpha carriers can have normal haemoglobin studies, so DNA testing is the confirmatory step.
Frequently asked questions
What HbA2 level suggests beta thalassaemia trait?
An HbA2 above about 3.5 per cent on HPLC suggests beta thalassaemia trait. Iron deficiency can lower HbA2 and mask the diagnosis, so iron status is checked first. Confirmation is by DNA analysis where needed.
Why is alpha thalassaemia harder to detect by routine screening?
Silent alpha thalassaemia carriers often have normal haemoglobin and HbA2 levels, so routine HPLC misses them. DNA analysis detects the deletions and variants responsible. Hb Bart's on newborn screening may be an early clue.
When is chorionic villus sampling performed for haemoglobinopathy diagnosis?
Chorionic villus sampling is usually performed between 10 and 13 weeks of gestation. It gives an early diagnosis, with a small risk of miscarriage and the need for fetal DNA testing. Amniocentesis is the alternative after about 15 weeks.
What is the risk to a child of two beta thalassaemia carriers?
Two beta thalassaemia carriers have a 25 per cent risk in each pregnancy of a child with beta thalassaemia major. Each child also has a 50 per cent chance of being a carrier. Prenatal diagnosis is offered to couples who wish it.