Kleihauer–Betke Fetal Bleed Calculator
Kleihauer–Betke Fetal Bleed Calculator
Convert a Kleihauer acid-elution percentage into the volume of a fetomaternal haemorrhage — reported as fetal whole blood, with the fetal red cell figure that anti-D dosing actually needs.
Kleihauer–Betke Fetal Bleed
% fetal cells × blood volume0.3% fetal cells, maternal blood volume 5,000 mL
Formula
Fetal red cells (mL) = fetal whole blood ÷ 2, taking the fetal haematocrit as about 0.5
- whole blood, not red cells
- this formula returns fetal WHOLE BLOOD. The two differ roughly twofold and anti-D dosing depends on which you have — the UK convention is written per mL of red cells, the US convention per mL of whole blood
- maternal blood volume
- 5,000 mL by convention at term. It is an assumption, not a measurement, and the answer moves with it — a 3,500 mL volume gives 30% less bleed for the same film
- the UK shortcut
- Mollison's estimate multiplies the fetal-to-adult cell ratio by 2,400 and returns fetal RED CELLS directly. For a 0.3% film that is 7.2 mL, against the 7.5 mL this page's halved figure gives — close agreement, once you know which quantity each is reporting
- fetal haematocrit
- taken as 0.5, which is where the twofold factor comes from and why one 300 µg vial is quoted both as 30 mL of whole blood and as 15 mL of red cells
Worked example
0.3% fetal cells, maternal blood volume 5,000 mL
0.3 ÷ 100 = 0.003
0.003 × 5,000 = 15.0 mL of fetal whole blood
Fetal red cells = 15.0 ÷ 2 = 7.5 mL, at a fetal haematocrit of 0.5
The UK Mollison shortcut on the same film gives 0.003 × 2,400 = 7.2 mL of fetal red cells — the same bleed, reached another way
Above 4 mL of fetal red cells, so extra anti-D and a follow-up sample are both needed
The same Kleihauer result, four ways
| Fetal cells on the film | Fetal whole blood (mL) | Fetal red cells (mL) | UK Mollison estimate (mL fetal red cells) |
|---|---|---|---|
| 0.1% | 5.0 | 2.5 | 2.4 |
| 0.2% | 10.0 | 5.0 | 4.8 |
| 0.3% | 15.0 | 7.5 | 7.2 |
| 0.5% | 25.0 | 12.5 | 12.0 |
| 1.0% | 50.0 | 25.0 | 24.0 |
| 2.0% | 100.0 | 50.0 | 48.0 |
What makes a Kleihauer wrong
| Problem | Effect | What to do |
|---|---|---|
| Sample taken before the fetal cells have mixed | Underestimate | Allow time for mixing after the event, and take the sample within two hours of delivery or the sensitising event |
| Too few cells counted | Wide confidence limits; at low percentages the estimate is barely better than a guess | Count at least 6,000 cells — the BSH minimum for reasonable precision |
| Hereditary persistence of fetal haemoglobin, β thalassaemia, sickle cell disease | Maternal F cells resist acid elution and are counted as fetal — a false positive | Confirm by flow cytometry, which can identify cells by RhD status as well as by haemoglobin F |
| Estimated bleed above 4 mL of fetal red cells | The anti-D dose now depends on a number the method cannot deliver precisely | Confirm by flow cytometry wherever possible, then repeat to confirm clearance |
Whole blood or red cells: the question the number does not answer by itself
The Kleihauer–Betke test exploits the fact that fetal haemoglobin resists acid elution while adult haemoglobin does not. A maternal film is bathed in acid buffer, adult cells are left as pale ghosts, fetal cells stay darkly stained, and the fetal cells are counted as a percentage of the maternal red cells on the slide. Multiplying that fraction by an assumed maternal blood volume of 5,000 mL converts it into a volume of bleed — the figure this page returns.
What that figure is a volume of is the part that gets lost. This formula returns fetal WHOLE BLOOD. The quantity the United Kingdom anti-D convention is written against is fetal RED CELLS, and the two differ about twofold at a fetal haematocrit of 0.5. A 0.3% film is 15 mL of fetal whole blood and 7.5 mL of fetal red cells; feeding 15 into a formula expecting red cells doubles the dose, and feeding 7.5 into one expecting whole blood halves it. Mollison’s shortcut, used in UK laboratories, multiplies the fetal-to-adult ratio by 2,400 and returns red cells directly — 7.2 mL on the same film, which is reassuring agreement between two conventions that look nothing alike.
Timing and precision are the method’s real weaknesses. Fetal cells need time to mix through the maternal circulation, so a sample taken immediately after the event underestimates the bleed, while the BSH asks for it within two hours of delivery or the sensitising event. Precision depends on how many cells were counted: 6,000 is the stated minimum, and below that the confidence limits around a small percentage are wide enough to change the anti-D dose. Flow cytometry is more accurate and more reproducible, and is the confirmatory test whenever the estimate exceeds 4 mL of fetal red cells.
False positives come from the mother rather than the baby. Any condition that raises maternal haemoglobin F populates the film with cells that resist elution and are counted as fetal: hereditary persistence of fetal haemoglobin, the beta thalassaemias, and sickle cell disease. The pattern is a scattering of weakly stained cells rather than the sharply stained clusters of true fetal cells, but that distinction is a morphological judgement. Flow cytometry, which can gate on RhD as well as on haemoglobin F, settles it.
Frequently asked questions
Does the Kleihauer give fetal whole blood or fetal red cells?
The percentage multiplied by a 5,000 mL maternal blood volume gives fetal WHOLE BLOOD. Halve it for fetal red cells, taking the fetal haematocrit as about 0.5. Which one you need depends on the anti-D convention: the UK dose is per mL of red cells, the US dose per mL of whole blood.
How much fetal blood does a 0.3% Kleihauer represent?
About 15 mL of fetal whole blood, which is 7.5 mL of fetal red cells, assuming a maternal blood volume of 5,000 mL. The UK Mollison shortcut gives 7.2 mL of fetal red cells for the same film, so the two conventions agree closely.
When should the maternal sample be taken?
After the fetal cells have had time to mix into the maternal circulation, but the BSH asks for it within two hours of delivery or the sensitising event. Too early and the bleed is underestimated; too late and fetal cells are already being cleared.
What causes a false positive Kleihauer?
Anything that raises maternal haemoglobin F, because those cells also resist acid elution. Hereditary persistence of fetal haemoglobin, the beta thalassaemias and sickle cell disease are the usual causes. Flow cytometry, which can gate on RhD as well as haemoglobin F, resolves them.
Is flow cytometry better than the Kleihauer?
Yes, for accuracy and reproducibility. Acid elution is a manual count down a microscope and its precision is poor at the low percentages that matter most. UK guidance asks for flow cytometric confirmation whenever the estimate exceeds 4 mL of fetal red cells.
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References
- British Committee for Standards in Haematology. The estimation of fetomaternal haemorrhage. Transfus Med. 1999;9(1):87–92.
- Kleihauer E, Braun H, Betke K. Demonstration von fetalem Hämoglobin in den Erythrozyten eines Blutausstrichs. Klin Wochenschr. 1957;35(12):637–638.
- Mollison PL, Engelfriet CP, Contreras M. Blood Transfusion in Clinical Medicine. 10th ed. Blackwell Science; 1997.