Pregnancy · Rh Disease
Kleihauer-Betke Test — FMH Quantification
Blood test that quantifies fetal red cells in maternal circulation after sensitising events ≥20 wk in Rh-neg women. Determines if extra anti-D needed beyond standard 1500 IU dose. RCOG Green-top 22.
Last reviewed 2 June 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.
What is Kleihauer-Betke?
Blood test detecting + quantifying fetal red cells in maternal circulation. Determines extra anti-D dose needed beyond standard 1500 IU after sensitising events ≥20 weeks in Rh-negative women.
When done
- After sensitising events ≥20 weeks (bleeding, trauma, ECV, etc.).
- Routinely after delivery in Rh-neg mothers.
- Suspected FMH (reduced movements, fetal anaemia, hydrops).
- Unexplained IUFD.
How result determines dose
- Standard 1500 IU UK covers ~6 mL fetal blood.
- Extra: 125 IU per mL beyond 6 mL.
- Very large FMH: specialist haematology input; IM/IV combinations.
Calculation: % fetal cells × 50 (maternal volume) = mL fetal blood.
Massive FMH (>20-30 mL)
Rare but serious. Causes:
- Severe abruption.
- Vasa praevia.
- After ECV.
- Chronic feto-maternal bleeding.
- Unexplained.
Presentation: reduced movements, fetal anaemia on MCA-PSV, hydrops, unexplained stillbirth. Management: gestation-dependent; IUT or delivery.
Flow cytometry alternative
More sensitive; more automated. Equivalent clinical decisions. Same thresholds for additional anti-D.
Sensitising event protocol (Rh-neg)
- Call maternity unit immediately.
- Blood sample for Kleihauer + group/screen.
- Anti-D within 72 hours.
- Fetal assessment.
- Record event + dose.
After birth in Rh-neg mum
- Baby’s blood group + DCT (direct Coombs).
- If baby Rh-pos: maternal anti-D within 72h.
- Kleihauer determines if standard dose sufficient.
Different scenarios
Scenario 1: Rh-neg, abdominal trauma 28 wk, Kleihauer 4 mL
Standard 1500 IU anti-D sufficient. CTG + ultrasound check baby.
Scenario 2: After ECV, Kleihauer 15 mL
9 mL extra fetal blood; 1125 IU additional anti-D = total ~2625 IU.
Scenario 3: Postpartum, baby Rh-pos, routine Kleihauer 2 mL
Standard 1500 IU anti-D within 72h adequate.
Scenario 4: Stillbirth + Kleihauer 40 mL
Massive FMH likely cause. Haematology input. Large anti-D dose. Investigate cause.
Scenario 5: Bleeding at 18 wk in Rh-neg
<20 wk — Kleihauer not usually needed; standard anti-D dose appropriate.
Care guidance — Kleihauer-Betke
- Rh-neg pregnancy: tell midwife of any bleeding, trauma, procedure.
- Anti-D within 72h essential.
- Kleihauer for events ≥20 wk.
- Standard 1500 IU covers ~6 mL.
- Larger FMH: extra anti-D + haematology consult.
- cffDNA can determine baby’s Rh upfront — avoids unnecessary anti-D.
Sources
- RCOG Green-top Guideline 22. The use of anti-D immunoglobulin for rhesus D prophylaxis.
- BCSH. Guidelines for the use of prophylactic anti-D immunoglobulin.
- NICE NG201. Antenatal care.
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Frequently asked questions
What is the Kleihauer-Betke test?
BLOOD TEST that detects + QUANTIFIES FETAL RED CELLS in maternal circulation. USED to determine how much BABY'S BLOOD has entered MUM'S BLOODSTREAM during a sensitising event. INFORMS ANTI-D DOSING in Rh-negative mothers — bigger bleeds need bigger anti-D doses. PROCEDURE: maternal blood sample; lab acid-elution treatment; fetal cells resist acid (different haemoglobin) → counted. RESULTS in mL of fetal blood; informs anti-D dose calculation.
When is it done?
(1) AFTER SENSITISING EVENTS ≥20 weeks in Rh-negative women: bleeding, abdominal trauma, abruption, ECV, antepartum haemorrhage, procedures; (2) ROUTINELY after delivery in Rh-negative mothers (decides need for extra anti-D beyond standard postpartum dose); (3) SUSPECTED feto-maternal haemorrhage in any pregnancy: reduced movements, fetal anaemia signs, hydrops, unexplained intrauterine death; (4) RHESUS DISEASE management — though now MCA-PSV more commonly used. NOT NEEDED for <20 weeks events typically (standard anti-D dose sufficient).
How is the result used?
(1) STANDARD ANTI-D DOSE 1500 IU (UK) covers ~6 mL fetal blood; (2) IF KLEIHAUER shows >6 mL: extra anti-D given (~125 IU per additional mL fetal blood); (3) LARGE FETO-MATERNAL HAEMORRHAGE (>10-20 mL): may need very large anti-D doses; consider IM/IV combinations; specialist haematology input. CALCULATION: % fetal cells × 50 (maternal blood volume estimate) = mL fetal blood. EXAMPLE: 0.5% = 25 mL fetal blood → significantly more anti-D needed.
Why does fetal blood get into mum's circulation?
PLACENTA usually separates the two circulations. SOMETIMES BARRIER BREACHED: (1) NORMAL pregnancy + delivery: small leaks (<0.1 mL most), occasionally larger; (2) ABRUPTION; (3) VAGINAL BLEEDING; (4) ABDOMINAL TRAUMA (fall, car accident); (5) ECV (external cephalic version turning breech baby); (6) AMNIOCENTESIS / CVS / cordocentesis; (7) IUFD (intrauterine fetal death) — release of fetal cells; (8) MOLAR pregnancy. LARGE BLEEDS rare but possible.
What's massive feto-maternal haemorrhage (FMH)?
RARE but serious. >20-30 mL fetal blood into maternal circulation (out of ~150-300 mL fetal total volume). CAUSES: (1) SEVERE abruption; (2) UNEXPLAINED (sometimes no clear cause); (3) AFTER ECV; (4) CHRONIC fetal-to-maternal bleeding (vasa praevia, placental tumour). PRESENTATION: REDUCED FETAL MOVEMENTS; FETAL ANAEMIA on MCA-PSV; HYDROPS; UNEXPLAINED stillbirth. INVESTIGATION when above features present. MANAGEMENT: depends on gestation; intrauterine transfusion vs delivery + neonatal transfusion.
What's the alternative to Kleihauer?
FLOW CYTOMETRY for fetal cell quantification — more accurate; increasingly available in UK NHS. ADVANTAGES: more sensitive at low FMH levels; automated; less operator-dependent. KLEIHAUER traditional + still widely used (especially smaller labs); generally adequate. RESULTS comparable for clinical purposes (anti-D dosing).
How long does the result take?
USUALLY 4-24 HOURS for urgent samples; routine 24-48 hours. URGENT (suspected significant FMH, large abruption): processed quickly to inform anti-D dosing within 72-hour window. NHS labs may have specific reflex protocols. RESULT shared with clinical team + recorded in maternity notes.
What if the result is normal / negative?
STANDARD ANTI-D DOSE (1500 IU UK) sufficient. NO FURTHER anti-D needed for that event. NORMAL test reassuring for fetal-maternal interface. CONTINUE pregnancy monitoring as planned. IF SYMPTOMS persist (continued bleeding, reduced movements), repeat assessment + Kleihauer if event recurs.
Does my baby need testing too?
AFTER BIRTH (in Rh-neg mothers): baby's blood group + DCT (direct Coombs test, also called direct antiglobulin test) — checks for antibodies on baby's red cells. RESULT informs: (1) NEED for anti-D dose to mother (only if baby Rh-positive); (2) BABY'S risk of newborn anaemia / jaundice from any maternal antibodies. STANDARD postnatal procedure.
What happens during a 'sensitising event' if I'm Rh-negative?
(1) CALL maternity unit IMMEDIATELY; (2) BLOOD SAMPLE for Kleihauer + group/screen; (3) ANTI-D within 72 HOURS (sooner ideal — first 72 hours window of effectiveness); (4) FETAL ASSESSMENT (CTG, ultrasound if late gestation); (5) RECORD event + dose given for future care. DON'T MISS the 72-hour window. SAFE pregnancy continuation usually. SUBSEQUENT pregnancies + events follow same protocol.
Can Kleihauer be done at home?
NO — laboratory test. REQUIRES specialised acid-elution staining + microscopic counting (or flow cytometry equivalent). LAB-BASED. RESULTS interpreted by haematologists / lab staff. CLINICALLY shared with obstetric team to inform anti-D dose.
What about flow cytometry alternative?
FLOW CYTOMETRY for fetal cell quantification using FETAL HAEMOGLOBIN antibody. MORE SENSITIVE than Kleihauer (detects very small amounts); MORE AUTOMATED; better quality control. INCREASINGLY available NHS hospitals. EQUIVALENT clinical decisions. SAME thresholds for anti-D additional dosing.
Is the test painful?
NO — just standard blood draw from your arm. SAME procedure as any maternal blood test. SLIGHT bruise possible at venipuncture site. NO RECOVERY time. CAN BE done at maternity unit, GP, phlebotomy department. SOMETIMES added to other routine bloods (avoids extra needle stick).
What's my anti-D dose if Kleihauer is high?
STANDARD 1500 IU UK = covers up to 6 mL fetal blood. EXTRA DOSE: 125 IU per mL beyond 6 mL fetal blood detected. EXAMPLE: Kleihauer shows 15 mL fetal blood = needs additional 9 mL × 125 = 1125 IU + standard 1500 = 2625 IU total. LARGER FMH: may need multiple sites (IM) or IV administration; haematology consult. UK NHS dosing tables guide.
How does this relate to other calculators on BumpBites?
Companion: /calculators/anti-d-dosing for full Rh management; /calculators/blood-type; /calculators/mca-psv (anaemia detection); /calculators/methotrexate-ectopic; /calculators/newborn-bilirubin; /calculators/recurrent-miscarriage.