A Rh-negative woman is being evaluated for Rh isoimmunization risk. The Kleihauer-Betke (KB) test is performed after delivery. What is the principle of the KB test, and what does a result of 1.8% fetal cells indicate in terms of required anti-D dose?

• A KB test detects hemoglobin S (sickle hemoglobin) in maternal blood; 1.8% implies massive fetomaternal hemorrhage requiring surgical intervention
• B KB test uses acid elution: adult hemoglobin (HbA) is eluted from RBCs leaving ghost cells, while fetal hemoglobin (HbF)-containing cells remain darkly stained; 1.8% fetal cells implies ~90 mL fetomaternal hemorrhage requiring 900 mcg (4500 IU) of anti-D ✓
• C KB test is a flow cytometry assay detecting fetal D-antigen-positive cells; 1.8% fetal cells requires standard 300 mcg anti-D
• D KB test detects maternal Rh antibody titer; 1.8% indicates critical titer requiring plasmapheresis

Explanation

The Kleihauer-Betke (KB) test exploits the resistance of fetal hemoglobin (HbF) to acid elution. Maternal blood smear is treated with acid buffer: adult hemoglobin (HbA) is denatured and eluted from RBCs, leaving ghost (pale) cells, while fetal cells containing HbF resist acid elution and stain darkly with eosin. The percentage of fetal cells (dark cells) is counted microscopically. To calculate fetomaternal hemorrhage (FMH): FMH volume = (% fetal cells × maternal blood volume [~5000 mL] × 0.92 correction factor). For 1.8% fetal cells: FMH ≈ 1.8/100 × 5000 × 0.92 ≈ 82.8 mL whole blood ≈ 41 mL RBCs. Each 300 mcg (1500 IU) anti-D covers ~15 mL fetal RBCs; so ~82 mL requires ~5 vials of 300 mcg ≈ 1500 mcg total. The exact calculation varies by protocol.

Reference: Williams Obstetrics, 26th ed.

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