Gera Tools

Kleihauer-Betke Feto-Maternal Haemorrhage Estimator

Estimate feto-maternal bleed volume and anti-D dose

Converts the Kleihauer-Betke percentage of fetal cells into an estimated feto-maternal haemorrhage volume and the extra anti-D immunoglobulin required for RhD-negative mothers, following BCSH-style dosing. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Last updated Source: Gera Tools

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.

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

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 correction factor 1.22 accounts for fetal cells being roughly 22% larger than adult cells and staining less densely — without this adjustment the bleed would be underestimated. Maternal blood volume defaults to approximately 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 covers about 12 mL of fetal red cells.

Worked examples

Example 1 — Small bleed, single standard dose sufficient. Lab reports 0.3% fetal cells; maternal blood volume 5000 mL.

  • FMH = (0.3 / 100) × 5000 × 1.22 = 18.3 mL whole blood
  • Fetal RBC volume = 18.3 / 2 = 9.15 mL
  • A 1500 IU dose covers ~12 mL fetal RBC, so the standard postnatal dose is sufficient with no top-up needed.

Example 2 — Larger bleed requiring additional anti-D. Lab reports 1.2% fetal cells; maternal blood volume 5000 mL.

  • FMH = (1.2 / 100) × 5000 × 1.22 = 73.2 mL whole blood
  • Fetal RBC volume = 73.2 / 2 = 36.6 mL
  • A single 1500 IU dose covers only ~12 mL, so approximately 3 × 1500 IU doses would be needed, confirmed by follow-up testing.

Clinical context

The Kleihauer-Betke test is indicated after potentially sensitising events in RhD-negative women, including external cephalic version, antepartum haemorrhage, trauma, invasive procedures, and routinely after delivery of an RhD-positive baby. Flow cytometry is more sensitive and quantitatively accurate for larger bleeds, and many centres use it to confirm a high Kleihauer result before giving additional anti-D.

Limitations and important caveats

The test has known variability: the staining result depends on technique and the reading microscopist, and the 1.22 correction factor is an average. Women with raised fetal haemoglobin (HbF) for non-pregnancy reasons — such as sickle-cell disease or haemolytic anaemia — may give falsely elevated readings, because adult cells with high HbF resist elution just as fetal cells do. This estimator reproduces the standard BCSH-style calculation for education and cross-checking; actual anti-D dosing must follow the laboratory’s quantified result and local transfusion guidance.