Pregnancy · Risk
Kleihauer-Betke / FMH Calculator
Calculate fetal-maternal haemorrhage (FMH) volume from the Kleihauer-Betke % fetal cell count, and the required RhIg vials per ACOG PB 181 dosing (with +1 vial safety margin).
Last reviewed 25 May 2026
Kleihauer-Betke / FMH calculator
Fetal-maternal haemorrhage volume + RhIg dose
%
mL
Enter the % fetal cells from the Kleihauer-Betke stain to calculate.
Educational tool only — not medical advice. Used in Rh-negative mothers after sensitising events (delivery, trauma, antepartum bleeding, ECV, amniocentesis, abdominal trauma, intrauterine procedure). RhIg dosing decisions are made by blood-bank and obstetric teams. Standard antepartum prophylaxis (28 weeks) for Rh-negative mothers does not require KB testing.
What does this mean?
The Kleihauer-Betke test detects fetal red blood cells in maternal circulation and quantifies a fetomaternal haemorrhage (FMH). It matters for two reasons: (1) RhIg dosing in an Rh-negative mother after a sensitising event — one standard 300 mcg dose covers up to 30 mL of fetal whole blood; large FMH needs additional vials (each extra 30 mL FMH = 1 extra vial). Under-dosing risks alloimmunisation that can cause severe haemolytic disease of the newborn in this or future pregnancies (ACOG PB 181, RCOG GTG 65). (2) Recognising occult FMH as a cause of unexplained fetal demise, anaemia, or decreased fetal movement after abdominal trauma. Alternative quantification methods (flow cytometry) are more precise and increasingly replace KB in big labs. Routine antepartum anti-D at 28 wk and post-delivery prophylaxis do not need a KB — KB is for sensitising events with potentially large FMH.
Introduction
The Kleihauer-Betke (KB) test quantifies fetal red blood cells in maternal circulation. Developed in 1957, it remains the most widely-used test (along with flow cytometry) for determining fetal-maternal haemorrhage (FMH) volume after sensitising events in Rh-negative pregnant women. The FMH volume drives anti-D RhIg (Rh immune globulin) dosing — additional protection against Rh sensitisation when haemorrhage exceeds the standard 30 mL coverage of a single 300 mcg vial.
The formula
FMH volume (mL fetal whole blood) = (% fetal cells / 100) × maternal blood volume Standard maternal blood volume ≈ 5,000 mL Number of 300 mcg RhIg vials = ceiling(FMH / 30) + 1 (safety margin)
When to test
Rh-negative pregnant women after any sensitising event:
- Delivery (especially caesarean, manual placental removal, retained placenta).
- Antepartum bleeding (placental abruption, placenta praevia).
- Abdominal trauma (motor vehicle collision, fall, blunt assault).
- External cephalic version (ECV).
- Amniocentesis, chorionic villus sampling, intrauterine procedure.
- Intrauterine fetal demise.
- Unexplained fetal anaemia / hydrops on ultrasound.
How RhIg dosing works
- Standard 300 mcg RhIg vial covers 30 mL of fetal whole blood (15 mL fetal red cells).
- FMH less than 30 mL → single standard vial sufficient.
- FMH greater than 30 mL → divide by 30, round up to next whole vial, then add 1 extra vial for safety margin (per ACOG PB 181).
- Example: FMH = 45 mL → 45/30 = 1.5 → round up to 2 → +1 safety = 3 vials = 900 mcg.
Routine vs sensitising-event dosing
Routine prophylaxis
- 28 weeks: standard 300 mcg RhIg vial (covers spontaneous antepartum FMH up to delivery).
- Within 72 hours of delivery: standard 300 mcg vial (covers labor / delivery FMH).
- No KB test routinely required.
Sensitising-event dosing
- KB or flow cytometry FMH test ordered.
- RhIg given within 72 hours.
- Dose adjusted based on FMH volume.
- If event > 72 hours ago, RhIg still given — it’s less effective but better than none.
KB vs flow cytometry
Flow cytometry with anti-D or anti-HbF antibodies is increasingly preferred — it’s more accurate at low FMH percentages (where KB has 10-20 % CV). ACOG PB 181 endorses either method. Larger centres often have both available; smaller centres rely on KB. The “+1 vial safety margin” in this calculator compensates for laboratory variability.
Limitations
- Standard maternal blood volume of 5,000 mL is an estimate; actual volume varies with maternal height, weight, and pregnancy progression (volume expands ~40 % through pregnancy).
- KB requires manual cell counting — operator-dependent variability.
- Maternal HbF elevations (e.g. hereditary persistence of fetal haemoglobin, sickle-cell trait) can cause false-positive KB results.
- This calculator does not replace blood-bank and obstetric clinical input. RhIg dosing decisions are protocol-driven and team-coordinated.
Sources
- Kleihauer E, Braun H, Betke K. Demonstration of fetal hemoglobin in erythrocytes of a blood smear. Klin Wochenschr 1957;35:637-8.
- ACOG. Practice Bulletin 181: Prevention of Rh D Alloimmunization. 2017 (reaffirmed 2022).
- RCOG. Green-top Guideline 65: The Management of Women with Red Cell Antibodies during Pregnancy.
- AABB. Technical Manual. Current edition.
- Bowman JM. The prevention of Rh immunization. Transfus Med Rev 1988.
Frequently asked questions
What is the Kleihauer-Betke test?
A haematology test that identifies and quantifies fetal red blood cells in maternal circulation. Developed by Kleihauer, Braun and Betke in 1957. The principle: fetal haemoglobin (HbF) resists acid elution while adult HbA does not — so after acid treatment a peripheral smear shows fetal RBCs as 'ghost' cells. Count of fetal cells / 1,000 maternal cells gives the % fetal cells, which is used to calculate the volume of fetal blood that has crossed into maternal circulation (fetal-maternal haemorrhage, FMH).
When is the KB test ordered?
In Rh-negative pregnant women after a sensitising event that may have caused FMH: delivery (esp. caesarean, manual placental removal), antepartum bleeding (placental abruption, placenta praevia), abdominal trauma, external cephalic version (ECV), amniocentesis or other intrauterine procedure, intrauterine fetal demise. Also when unexplained fetal anaemia or hydrops is found. The test guides additional RhIg dosing beyond the standard 300 mcg vial.
How is RhIg dosing calculated?
Standard 300 mcg vial covers 30 mL of fetal WHOLE BLOOD (15 mL fetal RBCs). For larger FMH: divide the calculated FMH volume by 30 and round UP to next whole number. ACOG PB 181 recommends ADDING ONE EXTRA VIAL to the calculated dose to account for laboratory variability. Example: FMH = 45 mL → 45/30 = 1.5 → round up to 2 → add 1 safety = 3 vials (900 mcg total).
Why does Rh sensitisation matter so much?
Before RhIg prophylaxis was developed in the 1960s, Rh haemolytic disease of the newborn affected ~1 % of pregnancies and killed ~10,000 babies per year in the US alone. RhIg works by passively neutralising fetal Rh-positive cells before the maternal immune system mounts an antibody response. Universal antepartum prophylaxis (300 mcg at 28 weeks) plus postpartum prophylaxis within 72 hours has reduced Rh sensitisation to ~0.1 % of pregnancies. KB-directed dosing for sensitising events extends this protection.
What's the difference between routine prophylaxis and KB-directed dosing?
Routine: 300 mcg RhIg at 28 weeks (covers spontaneous antepartum FMH up to delivery) and 300 mcg within 72 hours of delivery (covers labor / delivery FMH). NO KB test needed for routine timing. KB-directed: ordered after sensitising events to determine if MORE than the standard 300 mcg is needed (FMH > 30 mL). About 1 % of deliveries have FMH > 30 mL requiring additional dosing.
Is the KB test accurate?
Operator-dependent and has 10-20 % CV at low percentages. Modern alternatives — flow cytometry with anti-D or anti-HbF antibodies — are more accurate especially at low FMH percentages. ACOG PB 181 endorses either method; flow cytometry is increasingly preferred in larger centres. The 'add 1 vial' safety margin compensates for KB variability.
What if I'm Rh-positive — do I need this test?
Usually no. Rh-positive women don't develop anti-D antibodies because their own RBCs carry the D antigen. However, KB testing is occasionally used in Rh-positive women to investigate large FMH suspected from fetal hydrops, anaemia, or after major trauma — to quantify the extent. The RhIg prophylaxis pathway specifically does not apply.