The Kleihauer-Betke (KB) test is performed post-delivery in an Rh-negative mother who delivered an Rh-positive infant. The KB result is 1.5%. Assuming maternal blood volume is 5,000 mL and fetal cell volume is twice the maternal, the approximate volume of feto-maternal hemorrhage (FMH) is:
- A 7.5 mL
- B 75 mL ✓
- C 150 mL
- D 15 mL
Correct answer: B. 75 mL
Explanation
The Kleihauer-Betke calculation for FMH volume uses: FMH (mL) = (% fetal cells in maternal blood / 100) × maternal blood volume × correction factor. With 1.5% fetal cells, maternal blood volume 5,000 mL, the uncorrected estimate = 0.015 × 5,000 = 75 mL fetal RBCs. Anti-D prophylaxis dose is calculated based on this: standard 300 mcg anti-D covers 30 mL fetal RBCs (15 mL packed cells); for 75 mL, additional doses are required. This question tests the formula and its clinical implication for anti-D dosing.
Reference: Williams Obstetrics, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.