Case 4: A Kleihauer-Betke Result You Should Not Trust
Ms. K is a 29-year-old G2P1, blood group O, D-negative, who delivered a 3,410 g infant at 39 weeks after manual removal of a retained placenta. She received 300 µg of RhIG at 28 weeks; her antibody screen at that time was negative. Cord blood types D-positive with a weakly positive DAT (1+ IgG). A maternal sample drawn 1 hour after delivery shows 3.2% fetal cells by Kleihauer-Betke, and the anti-D titer is 1:4. The technologist notices that the maternal smear contains numerous target cells and some sickled forms, and the chart notes that the mother has sickle cell trait. Maternal weight is 92 kg. The blood bank is asked how many vials of RhIG to give and whether the Kleihauer-Betke result can be trusted.
1. Why might this Kleihauer-Betke result be wrong, and what should replace it?
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The Kleihauer-Betke test is an acid-elution stain: fetal cells containing hemoglobin F resist elution and stain darkly, while adult cells appear as ghosts. It is a manual, subjective count and it fails whenever the mother has a population of cells that also resist elution.
This mother has sickle cell trait, and her film shows target cells and sickled forms. Hemoglobin S-containing maternal cells can resist acid elution and be scored as fetal cells, inflating the percentage. Pregnancy itself, hereditary persistence of fetal hemoglobin, and thalassemia do the same thing. A result of 3.2% in this patient is therefore not trustworthy.
The replacement is flow cytometry using an anti-HbF antibody combined with a marker that identifies fetal cells - fetal red cells are carbonic-anhydrase-negative while maternal cells are positive. This distinguishes true fetal cells from maternal F-cells and gives a quantitative result within hours. A repeat Kleihauer-Betke with a maternal negative-control slide is only a stopgap, and it does not solve the underlying interference.
2. Using a corrected fetal-cell percentage of 0.6% from flow cytometry, calculate the RhIG dose and state the timing rules.
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Step 1 - maternal blood volume: 92 kg × 65 mL/kg ≈ 6,000 mL.
Step 2 - volume of the fetal bleed: 0.6% × 6,000 mL = 36 mL of fetal whole blood.
Step 3 - coverage: one 300 µg vial neutralizes about 15 mL of fetal red cells, which is roughly 30 mL of fetal whole blood. 36 ÷ 30 = 1.2 → round up → 2 vials.
For contrast, the uncorrected Kleihauer-Betke would have given 3.2% × 6,000 = 192 mL → 7 vials, a seven-fold over-treatment that also wastes a scarce product.
Timing rules: give the dose as soon as possible and within 72 hours of delivery; if that window has already passed, give it anyway. For multi-vial doses, split the injection sites or use an intravenous RhIG product. Confirm the infant is D-positive before treating, and perform the weak D test on cord cells - a weak D-positive infant should be managed as D-positive and the mother should receive RhIG.
3. What else should be done for this mother and this infant before discharge?
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Mother
- Confirm the fetal-cell percentage by flow cytometry before the dose is finalized, and record the weight, percentage, calculation, and number of vials in her chart.
- Interpret the anti-D titer of 1:4 in context: it is consistent with passive anti-D from the 28-week dose. Repeat the antibody screen at about 3 and 6 months. If anti-D persists or the titer rises at 6 months, she may have been immunized rather than passively coated - a large fetomaternal hemorrhage is a classic cause of RhIG failure.
- Document the size of the hemorrhage and the dose in the prenatal record, because detectable anti-D at a future delivery can be mistaken for immune anti-D.
Infant
- The infant is D-positive with a weakly positive DAT from passively transferred anti-D. Monitor bilirubin and hemoglobin over the first days and follow the hemolytic disease of the newborn protocol if the bilirubin rises.
- Document the cord blood type, DAT, and the mother's RhIG dose together so that the next pregnancy is managed with the correct history.