The Kleihauer-Betke (KB) test quantifies fetal cells in maternal blood by exploiting which principle, and what dose of anti-D RhIg is given per mL of fetal blood detected?
- A Fetal cells are detected by anti-HbF antibody fluorescence; 50 mcg covers 15 mL fetal whole blood
- B Fetal RBCs have higher osmotic fragility and are selectively lysed; 600 mcg needed per 30 mL fetal blood
- C Fetal cells are larger (macrocytes); any KB test positive mandates 1500 mcg anti-D
- D Fetal hemoglobin (HbF) resists elution by acid; 300 mcg anti-D covers 15 mL fetal RBCs (or 30 mL fetal whole blood) ✓
Explanation
The KB test exploits acid elution: adult HbA is eluted from RBCs after acid treatment (pH 3.2), leaving ghost cells, while fetal HbF resists elution and stains pink with eosin. Counting pink cells (fetal) vs ghost cells (adult) estimates the percentage of fetal cells and volume of fetomaternal hemorrhage (FMH). One 300 mcg vial of anti-D RhIg covers 15 mL of fetal RBCs (or 30 mL whole fetal blood). For large FMH detected by KB, additional doses are calculated: total anti-D = (KB% × maternal blood volume in mL / 30) × 300 mcg. Rosette test is a qualitative screening test; KB is quantitative. Flow cytometry using anti-HbF antibody is more accurate than KB for quantifying large FMH.
Reference: Williams Obstetrics, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.