The Kleihauer-Betke (KB) test detects fetal erythrocytes in maternal circulation after a potentially sensitizing event. The calculation to determine the volume of fetomaternal hemorrhage (FMH) and additional anti-D dose needed uses the formula: FMH volume (mL) = % fetal cells × maternal blood volume × correction factor. The standard correction factor used is:
- A 50 (standard WHO correction)
- B 300 (for standard 300 mcg anti-D dose calculation)
- C 25 (adjusting for maternal-fetal HbF differences) ✓
- D No correction factor; % fetal cells × 5000 mL maternal blood volume
Explanation
The Kleihauer-Betke formula: FMH (mL) = (% fetal cells / 100) × maternal blood volume (5000 mL) × 50 (hematocrit correction). The correction factor accounts for the dilution of fetal cells in maternal blood relative to the difference in cell sizes and hematocrit. Some sources use the simplified formula: FMH (mL) = % fetal cells × 50. After calculation, additional anti-D immunoglobulin is given: one extra 300 mcg (1500 IU) dose covers approximately 30 mL fetal whole blood (15 mL fetal RBCs). The standard postpartum anti-D dose of 300 mcg covers up to 30 mL fetal blood; KB test is done if massive FMH is suspected (abruption, cord accident, external cephalic version, manual removal of placenta).
Reference: Williams Obstetrics, 26th ed.
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Written and medically reviewed by the StethoPrep medical team.