How is the bleed volume calculated?

The estimated feto-maternal haemorrhage equals the percentage of fetal cells on the Kleihauer film multiplied by the maternal blood volume, then by a correction factor of 1.22 that accounts for the larger size and lower staining density of fetal cells. The result is reported as millilitres of fetal whole blood.

How much anti-D covers a given bleed?

One unit of 125 IU (about 25 micrograms) of anti-D immunoglobulin neutralises roughly 1 mL of fetal red cells, which is about 2 mL of fetal whole blood. The calculator divides the estimated whole-blood bleed by 2 to get fetal red cell volume, then works out how many 500 IU or 1500 IU doses are needed.

When should a Kleihauer test be done?

In RhD-negative women a Kleihauer or flow-cytometry test is performed after any potentially sensitising event beyond 20 weeks and routinely after delivery of an RhD-positive baby, to detect bleeds larger than a standard prophylactic dose can cover.

What if the test percentage is very low?

Small bleeds under about 4 mL of fetal red cells are fully covered by a standard 1500 IU postnatal dose in most protocols, so no extra anti-D is needed. The calculator flags when the estimated bleed exceeds the cover of the dose you select.

Is this a substitute for laboratory dosing?

No. It reproduces the standard BCSH-style estimation for education and cross-checking. Actual anti-D dosing must follow local transfusion guidelines, the laboratory's quantified result, and confirmatory flow cytometry where the Kleihauer is high.

Kleihauer-Betke Feto-Maternal Haemorrhage Estimator

The Kleihauer-Betke acid-elution test stains fetal haemoglobin-containing cells in a maternal blood film, allowing the size of a feto-maternal haemorrhage to be estimated. In RhD-negative mothers this determines whether the standard prophylactic dose of anti-D immunoglobulin is enough or whether more is required to prevent sensitisation.

How it works

The bleed volume comes directly from the stained cell fraction:

FMH (mL whole blood) = (fetal cells % / 100) x maternal blood volume x 1.22
fetal RBC volume      = FMH / 2
anti-D units needed   = fetal RBC volume / cover-per-dose

The factor 1.22 corrects for the larger volume and weaker staining of fetal cells relative to adult cells. Maternal blood volume defaults to about 5000 mL at term. Because roughly 125 IU of anti-D covers 1 mL of fetal red cells, a 500 IU dose covers about 4 mL and a 1500 IU dose about 12 mL of fetal red cells.

Worked example and notes

If 0.3% fetal cells are reported in a mother with a 5000 mL blood volume, the estimated bleed is 0.003 x 5000 x 1.22, about 18.3 mL of fetal whole blood, equivalent to roughly 9 mL of fetal red cells. A single 1500 IU dose covers about 12 mL of fetal red cells, so this bleed is fully covered and no top-up is needed. Larger bleeds need additional anti-D and follow-up testing to confirm clearance. This estimator is for education and bedside cross-checking; final dosing must follow the laboratory’s quantified result and local transfusion guidance, ideally confirmed by flow cytometry when the Kleihauer is high.