A Rh-negative woman delivers an Rh-positive baby. Kleihauer-Betke (KB) test reveals 1.5% fetal cells in maternal circulation. How is the total volume of feto-maternal haemorrhage (FMH) and the anti-D dose calculated using this test?
- A KB% × maternal blood volume (5000 mL) = FMH volume; anti-D 250 IU per mL of fetal whole blood ✓
- B KB% × 50 = mL of fetal red cells; anti-D 125 IU per mL of fetal red blood cells
- C Volume of FMH (mL) = KB% × maternal blood volume; anti-D 20 µg (100 IU) per mL of fetal whole blood
- D KB test results are unreliable; flow cytometry always required for dose calculation
Correct answer: A. KB% × maternal blood volume (5000 mL) = FMH volume; anti-D 250 IU per mL of fetal whole blood
Explanation
Kleihauer-Betke calculates FMH as follows: Volume of FMH (fetal whole blood) = (KB% / 100) × maternal blood volume (approximately 5000 mL for a term pregnant woman). The number of anti-D vials required is then calculated at 250 IU (50 µg) of anti-D per mL of fetal whole blood. For a KB of 1.5%: FMH = 0.015 × 5000 = 75 mL; anti-D required = 75 × 250 = 18,750 IU = ~18 vials of 1000 IU anti-D. This formula (NICE, RCOG Green-top Guideline 22) must be known precisely for NEET PG clinical scenarios.
Reference: Williams Obstetrics, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.