What is a KB (Kolmer-Bishop) test?

The Kleihauer-Betke (KB) Test

The Kleihauer-Betke (KB) test is a laboratory assay that detects and quantifies fetal red blood cells in maternal circulation, primarily used to assess fetal-maternal hemorrhage in obstetric care and to guide Rh immune prophylaxis dosing.

Purpose and Clinical Applications

The KB test serves several important clinical functions:

• Primary use: Detecting and quantifying fetal-maternal hemorrhage, particularly in:

  • Rh-negative mothers with Rh-positive infants to determine appropriate RhIG dosing
  • Cases of maternal trauma
  • Unexplained elevated maternal alpha-fetoprotein levels
  • Fetal distress with abnormal heart tracings
  • Intrauterine fetal death
  • Unexplained neonatal anemia 1

• Predictive value: A positive KB test following maternal trauma strongly predicts risk of preterm labor, with research showing it to be the single most significant risk factor associated with preterm labor (p<0.001) 2

Test Methodology

The KB test works based on the principle that fetal hemoglobin (HbF) is resistant to acid elution while adult hemoglobin is not:

1. A maternal blood smear is prepared and fixed
2. The slide is exposed to an acid buffer solution
3. Adult hemoglobin (HbA) is eluted, appearing as "ghost" cells
4. Fetal hemoglobin (HbF) resists acid elution and retains staining
5. Technologists count a minimum of 2,000 red blood cells to determine the percentage of fetal cells 3

Interpretation and Limitations

• Quantification: Results are reported as percentage of fetal cells or volume of fetal blood in maternal circulation
• Limitations:
  • False positives can occur due to maternal hemoglobin F 4
  • Poor sensitivity (only 4.4%) for detecting placental abruption 5
  • Manual counting is time-consuming and subject to human error 3

Clinical Recommendations

1. Perform KB testing in:

   • All Rh-negative mothers of Rh-positive infants following a positive screening test
   • Cases of maternal trauma (regardless of Rh status)
   • Unexplained elevated maternal alpha-fetoprotein
   • Fetal distress with abnormal heart tracings
   • Intrauterine fetal death
   • Unexplained neonatal anemia 1

2. Do not use KB testing for:

   • Suspected placental abruption (poor sensitivity of only 4.4%) 5, 1

3. Post-trauma monitoring:

   • With a negative KB test: Limited electronic fetal monitoring may be sufficient
   • With a positive KB test: Extended monitoring is mandatory due to significant risk of preterm labor 2

Technological Advances

Automated systems for KB testing have been developed that can:

• Count over 60,000 cells (vs. ~2,000 manually)
• Complete analysis in 5 minutes (vs. ~15 minutes manually)
• Provide improved accuracy through spatial-color pixel classification and supervised learning algorithms 3

These advances may improve the reliability and efficiency of KB testing in clinical practice, potentially reducing human error and increasing throughput by approximately 90 times compared to manual counting.