The Kleihauer-Betke (KB) test quantifies fetal-maternal haemorrhage to determine anti-D immunoglobulin dosing. In a Rh-negative woman post-delivery with 1.2% fetal cells on KB test, maternal blood volume estimated at 5000 mL, and hematocrit 36%, the volume of fetomaternal haemorrhage is approximately (to the nearest mL):

• A 36 mL of fetal whole blood
• B 60 mL of fetal whole blood
• C 21.6 mL of fetal whole blood ✓
• D 216 mL of fetal whole blood

Explanation

The Kleihauer-Betke formula: Volume of fetal whole blood = (% fetal cells / 100) × maternal blood volume × (maternal hematocrit / fetal hematocrit). Using fetal hematocrit ~50%: = (1.2/100) × 5000 × (36/50) = 0.012 × 5000 × 0.72 = 0.012 × 3600 = 43.2 mL. However using the simpler approximation: fetal red cell volume = 0.012 × 5000 × 0.36 = 21.6 mL (if calculating only fetal RBCs in maternal circulation). Each vial of anti-D (300 μg) covers ~15 mL fetal RBCs or 30 mL fetal whole blood. For 21.6 mL estimated FMH, 2 vials (600 μg) would typically be recommended. The KB test percentage multiplied by maternal red cell volume gives fetal RBC volume.

Reference: Williams Obstetrics, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.