Kleihauer-Betke test

Background

  • The Kleihauer-Betke (KB) test is the standard method for detecting and quantifying fetomaternal hemorrhage (FMH)[1]
  • First described in 1957 by Enno Kleihauer and Klaus Betke
  • Based on the principle that fetal hemoglobin (HbF) is resistant to acid elution, whereas adult hemoglobin (HbA) is not[1]
  • Mechanism: A maternal blood smear is exposed to an acid bath (citric acid–phosphate buffer), which removes HbA from adult red blood cells while HbF-containing fetal cells remain intact; subsequent staining causes fetal cells to appear dark/rose-pink while adult "ghost" cells appear pale[1]
  • Primary clinical purpose: Quantify the volume of FMH to guide dosing of Rho(D) immune globulin (RhoGAM) in Rh-negative mothers[2]
  • FMH occurs in up to 40% of trauma cases involving pregnant patients and can also occur during placental abruption, intrauterine fetal demise, and invasive obstetric procedures[1]
  • As little as 0.01–0.03 mL of fetal blood is sufficient to trigger maternal Rh isoimmunization[1]

Clinical Features

  • Fetomaternal hemorrhage is often clinically silent; the KB test is a laboratory screening/quantification tool rather than one prompted by specific symptoms
  • Clinical scenarios in which FMH should be considered:
    • Decreased fetal movement
    • Abnormal or nonreassuring fetal heart rate tracing (sinusoidal pattern is classic for fetal anemia)
    • fetal hydrops on ultrasound
    • Unexplained intrauterine fetal demise
    • Unexplained neonatal anemia at birth
    • Maternal trauma (blunt abdominal injury, motor vehicle collision)
    • Antepartum hemorrhage of uncertain etiology
  • Massive FMH (>30 mL of fetal blood) can cause:
    • Fetal tachycardia or bradycardia
    • Sinusoidal fetal heart rate pattern
    • Fetal anemia, hydrops, and death

Differential Diagnosis

Causes of a positive KB test / elevated fetal cells in maternal circulation:

  • True fetomaternal hemorrhage (trauma, abruption, procedures, spontaneous)
  • Hereditary persistence of fetal hemoglobin (HPFH) — false positive[3]
  • Sickle cell disease — false positive[1]
  • Thalassemia — false positive[1]
  • Maternal malignancy (rare cause of elevated HbF)

Conditions that may prompt KB testing:

Evaluation

Indications for KB Testing

  • Rh-negative mothers:
    • All Rh-negative mothers after a potentially sensitizing event to determine if additional RhoGAM doses (beyond the standard 300 mcg) are needed[2]
    • ACOG recommends KB testing in all Rh-negative pregnant trauma patients[1]
  • Additional indications (regardless of Rh status):[2]
    • Maternal trauma (the KB test predicts preterm labor risk after trauma more accurately than clinical assessment alone)[4]
    • Unexplained elevated maternal serum alpha-fetoprotein
    • Unexplained fetal distress or abnormal heart rate tracings
    • Intrauterine fetal death
    • Unexplained neonatal anemia
  • Not recommended for:

Test Procedure

  • Specimen: maternal peripheral venous blood (EDTA tube)
  • Timing: should be obtained as soon as possible after the suspected event; fetal cells are cleared from maternal circulation over hours to days
  • A blood smear is made, exposed to acid buffer, then stained
  • 2000 cells are counted; fetal cells (dark/pink) are distinguished from maternal ghost cells (pale)
  • Result is reported as a percentage of fetal cells

RhoGAM Dosing Calculation

  • One standard vial of RhoGAM = 300 mcg, which covers 30 mL of fetal whole blood (15 mL of fetal RBCs)[1]
  • Formulas:
    • Volume of fetal blood (mL) = % fetal cells × 50
    • Number of vials needed = volume of fetal blood ÷ 30
  • Rounding rule:[1]
    • If the decimal is < 0.5 → round down and add 1 vial
    • If the decimal is ≥ 0.5 → round up and add 1 vial
  • Example: KB test reports 1.5% fetal cells
    • Volume = 1.5% × 50 = 75 mL fetal blood
    • Vials = 75 ÷ 30 = 2.5 → round up to 3, add 1 = 4 vials

Screening Algorithm

  • Many institutions use a two-step approach[1]:
    • Step 1: Rosette test (qualitative screen) — if negative, standard 300 mcg RhoGAM dose is sufficient
    • Step 2: If rosette test is positive → KB test (quantitative) to determine additional RhoGAM dosing
  • In major trauma, many centers proceed directly to KB testing

Limitations

  • Sensitivity: Poor sensitivity for small hemorrhages; interobserver variability is significant[5]
  • False positives: Maternal conditions with elevated HbF (HPFH, sickle cell disease, thalassemias) cause false elevations — one study found 32% of maternal samples had high HbF-containing cells, and 69% of those yielded clinically significant false positives[1]
  • Poor prognostic value: Larger retrospective studies have demonstrated limited correlation between KB test positivity and adverse pregnancy outcomes; its primary value is in quantifying FMH for Rh immune prophylaxis rather than predicting outcomes[6][7]
  • Not useful for diagnosing abruption: A retrospective cohort study found no positive KB tests among placentas later confirmed to have abruption on pathologic review[2]
  • Operator-dependent: Manual counting introduces subjectivity; results can vary between technicians
  • Flow cytometry is a more precise alternative that uses anti-HbF antibodies to quantify fetal cells, with improved sensitivity and reproducibility, but is limited by higher cost and availability[8][5]

Management

  • Rh-negative mothers:
    • Administer standard 300 mcg RhoGAM for all potentially sensitizing events
    • If KB test indicates FMH > 30 mL fetal whole blood, administer additional vials per calculation above
    • RhoGAM should ideally be given within 72 hours of the sensitizing event
    • Before 12 weeks GA: mini-dose (150 mcg) is appropriate[1]
  • Trauma:
    • All pregnant trauma patients > 20 weeks GA should have continuous fetal monitoring (minimum 4–6 hours; extended to 24 hours if KB-positive or other concerning features)[4]
    • A positive KB test after trauma predicts increased risk of preterm labor and warrants extended monitoring[4]
    • Rh-negative trauma patients should receive RhoGAM regardless of KB result; KB test guides additional dosing
  • Massive FMH (fetal compromise):
    • Immediate OB consultation
    • Continuous fetal monitoring
    • Consider emergent delivery if fetal heart rate tracing is nonreassuring
    • Intrauterine transfusion may be considered in select preterm cases in consultation with maternal-fetal medicine

Disposition

  • All Rh-negative patients with a potentially sensitizing event should receive RhoGAM prior to discharge from the ED
  • Patients with a positive KB test should be admitted for continuous fetal monitoring and serial labs
  • Patients with a negative KB test after minor trauma and reassuring fetal heart rate monitoring for ≥ 4–6 hours may be considered for discharge with close OB follow-up[4]
  • OB consultation should be obtained for all cases of suspected significant FMH, abnormal fetal monitoring, or intrauterine fetal demise
  • Transfer to a facility with obstetric and neonatal intensive care capabilities if not available on site

See Also

External Links

References

  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 Krywko DM, Yarrarapu SNS. Kleihauer Betke Test. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2025. PMID 28613626.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 Emery CL, Morway LF, Chung-Park M, et al. The Kleihauer-Betke test: clinical utility, indication, and correlation in patients with placental abruption and cocaine use. Arch Pathol Lab Med. 1995;119(11):1032-1037. PMID 7487403.
  3. ↑ Kush ML, Muench MV, Harman CR, Baschat AA. Persistent fetal hemoglobin in maternal circulation complicating the diagnosis of fetomaternal hemorrhage. Obstet Gynecol. 2005;105(4):872-874. PMID 15802420.
  4. ↑ 4.0 4.1 4.2 4.3 Muench MV, Baschat AA, Reddy UM, et al. Kleihauer-Betke testing is important in all cases of maternal trauma. J Trauma. 2004;57(5):1094-1098. PMID 15580038.
  5. ↑ 5.0 5.1 Pelikan DM, Mesker WE, Scherjon SA, Kanhai HH, Tanke HJ. Improvement of the Kleihauer-Betke test by automated detection of fetal erythrocytes in maternal blood. Cytometry B Clin Cytom. 2003;54(1):1-9. PMID 12827463.
  6. ↑ Dhanraj D, Lambers D. The incidences of positive Kleihauer-Betke test in low-risk pregnancies and maternal trauma patients. Am J Obstet Gynecol. 2004;190(5):1461-1463.
  7. ↑ Cahill AG, Bastek JA, Engel SM, et al. Minor trauma in pregnancy — is the evaluation warranted? Am J Obstet Gynecol. 2008;198(2):208.e1-5.
  8. ↑ Chen JC, Davis BH. Detection of fetal red cells in fetomaternal hemorrhage using a fetal hemoglobin monoclonal antibody by flow cytometry. Transfusion. 1998;38(8):749-756.
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