The Kleihauer-Betke (acid elution) test is performed to quantify fetomaternal hemorrhage. The test exploits the fact that fetal hemoglobin (HbF) is resistant to acid elution while adult hemoglobin (HbA) is eluted. A woman has 1 fetal cell per 3000 maternal cells seen on KB smear. Approximately how many fetal cells in 300,000 maternal cells would be seen per 300,000 RBCs, if this is conceptually extended: and what approximate anti-D dose is required (300 mcg anti-D protects against 30 mL fetal whole blood)?

• A 50 fetal cells; 150 mcg anti-D
• B 100 fetal cells; 600 mcg anti-D needed
• C 100 fetal cells; 300 mcg anti-D covers 30 mL fetal blood and is sufficient ✓
• D 25 fetal cells; 75 mcg anti-D

Explanation

In Kleihauer-Betke interpretation: the proportion of fetal cells (1 per 3000) is multiplied by maternal blood volume (~5000 mL) to estimate the fetal bleed volume. At 1 fetal cell per 3000 maternal RBCs, this represents approximately 1/3000 × 5000 mL = 1.67 mL fetal whole blood equivalent, which is well within the coverage of a standard 300 mcg anti-D dose (which covers 30 mL of fetal whole blood). Thus one vial of 300 mcg anti-D is sufficient in this scenario.

Reference: Williams Obstetrics, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.