An Rh-negative unsensitised woman at 30 weeks sustains abdominal trauma in a road traffic accident. After administration of the standard anti-D dose, which laboratory test is used to determine whether an additional larger dose of anti-D immunoglobulin is required?

• A Kleihauer-Betke acid elution test on maternal blood ✓
• B Indirect Coombs test on maternal serum
• C Direct Coombs test on cord blood
• D Rosetting test on paternal blood

Explanation

The Kleihauer-Betke test uses acid elution to stain fetal haemoglobin-containing cells dark against pale adult red cells in a maternal peripheral smear, allowing quantification of the volume of fetomaternal haemorrhage. The calculated volume determines how much extra anti-D must be given beyond the standard 300 microgram dose, which covers bleeds up to approximately 30 ml of fetal whole blood. The indirect Coombs detects established maternal antibody and cannot guide dosing.

Reference: Ian Donald's Practical Obstetric Problems, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.