Kleihauer-Betke test

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Kleihauer-Betke test


K-B test

A test to identify fetal red blood cells in the maternal circulation, an abnormal finding identified when fetomaternal hemorrhage has occurred.

Patient care

The test is used in several settings: 1. In Rh– mothers to determine if they have been exposed to Rh+ fetal blood. If they have, it is used to determine the dose of Rh immune globulin needed by the exposed mother to prevent future episodes of hydrops fetalis; 2. in pregnant women who have experienced traumatic injury, it is used to identify the presence of fetomaternal hemorrhage; 3. in amniocentesis, to determine whether any blood found to be present is maternal or fetal in origin. A sample of maternal blood is obtained from a vein and treated with acid. Maternal blood cells develop a pale, ghostly appearance when exposed to acid; fetal cells do not. The number of fetal cells identified microscopically per a specified number of low-power fields is used to estimate the mL of fetal bleeding into the maternal circulation.

Medical Dictionary, © 2009 Farlex and Partners

Kleihauer-Betke Test

Synonym/acronym: Fetal hemoglobin, hemoglobin F, acid elution slide test.

Common use

To assist in assessing occurrence and extent of fetal maternal hemorrhage and calculate the amount of Rh immune globulin to be administered.


Whole blood (1 mL) collected in a lavender-top (EDTA) tube. Freshly prepared blood smears are also acceptable. Cord blood may be requested for use as a positive control.

Normal findings

(Method: Microscopic examination of treated and stained peripheral blood smear) Less than 1% fetal cells present.


The Kleihauer-Betke test is used to determine the degree of fetal-maternal hemorrhage (FMH) and to help calculate the dosage of Rh immune globulin (RhIG)—Rho(D) RhoGAM IM or Rhophylac IM or IV—to be given in some cases of Rh-negative mothers. Administration of RhIG inhibits formation of Rh antibodies in the mother to prevent Rh disease in future pregnancies with Rh-positive children. The test is also used to resolve the question of whether FMH was the cause of fetal death in the case of stillbirth. A sample of maternal blood should be collected within 1 hr of delivery. A blood film of maternal red blood cells (RBCs) is prepared, treated with an acid buffer, and stained. The acid solution causes hemoglobin to be leached from the maternal cells, giving them a ghostlike appearance. Fetal cells containing hemoglobin F retain their hemoglobin and are stained bright red. Approximately 2,000 cells are examined microscopically and counted. A percentage of fetal cells is reported. Enumeration of a total of 2,000 cells is important to achieve the accuracy and precision to detect an FMH of 15 mL of fetal red blood cells or 30 mL of fetal whole blood, which is the amount of FMH that corresponds to a 300-mcg dose of RhIG. Recommendations for initial RhIG doses range from 100 to 300 mcg to cover 10 to 30 mL of fetal blood volumes. Many manufacturers recommend additional 50 mcg doses for each 2.5 mL of fetal blood. Calculation of RhIG dosage is based on the calculated size of FMH and should only be done after reviewing the information in the manufacturer’s package insert. The formula to calculate quantity of fetal bleed in milliliters of fetal blood is to multiply the percentage of fetal cells in maternal circulation by 50, based on the assumption that maternal blood volume is 5 liters. For example, if the percentage of fetal cells counted is 0.6%, then FMH = 0.6 x 50 = 30 mL. The formula to calculate FMH in relation to fetal blood cell volume is to multiply the percentage of fetal cells in maternal circulation by 2, based on the assumption that the hematocrit of fetal whole blood is 50%. For example, if the quantity of fetal bleed is 30 mL of fetal whole blood, then FMH = (30/2) = 15 mL fetal red blood cells. Postpartum RhIG should be given within 72 hours of delivery. The test can also be used to distinguish some forms of thalassemia from the hereditary persistence of fetal hemoglobin, but hemoglobin electrophoresis and flow cytometry methods are more commonly used for this purpose.

This procedure is contraindicated for



  • Assist in the diagnosis of certain types of anemia
  • Calculating dosage of RhoGAM
  • Determine whether FMH was a potential cause of death in stillborn delivery
  • Screening postpartum maternal blood for the presence of FMH

Potential diagnosis

Positive findings in:

  • Fetal-maternal hemorrhage (related to leakage of fetal RBCs into maternal circulation)
  • Hereditary persistence of fetal hemoglobin (the test does not differentiate fetal hemoglobin from neonate and adult)

Critical findings


Interfering factors

  • Specimens must be obtained before transfusion.

Nursing Implications and Procedure


  • Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
  • Patient Teaching:   Inform the patient this test can assist in identifying how much medication should be given after delivery.
  • Obtain a history of the patient’s complaints, including a list of known allergens, especially allergies or sensitivities to latex.
  • Obtain a history of the patient’s hematopoietic and reproductive systems, symptoms, and results of previously performed laboratory tests and diagnostic and surgical procedures.
  • Note any recent procedures that can interfere with test results.
  • Obtain a list of the patient’s current medications, including herbs, nutritional supplements, and nutraceuticals (see Effects of Natural Products on Laboratory Values).
  • Review the procedure with the patient. Inform the patient that specimen collection takes approximately 5 to 10 min. Address concerns about pain and explain that there may be some discomfort during the venipuncture.
  • Sensitivity to social and cultural issues,  as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
  • Note that there are no food, fluid, or medication restrictions unless by medical direction.


  • Potential complications: N/A
  • Avoid the use of equipment containing latex if the patient has a history of allergic reaction to latex.
  • Instruct the patient to cooperate fully and to follow directions. Direct the patient to breathe normally and to avoid unnecessary movement.
  • Observe standard precautions, and follow the general guidelines in Patient Preparation and Specimen Collection. Positively identify the patient, and label the appropriate specimen container with the corresponding patient demographics, initials of the person collecting the specimen, date, and time of collection. Perform a venipuncture.
  • Remove the needle and apply direct pressure with dry gauze to stop bleeding. Observe/assess venipuncture site for bleeding or hematoma formation and secure gauze with adhesive bandage.
  • Promptly transport the specimen to the laboratory for processing and analysis. Sample must be less than 6 hr old.


  • Inform the patient that a report of the results will be made available to the requesting health-care provider (HCP), who will discuss the results with the patient.
  • Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP. Answer any questions or address any concerns voiced by the patient or family.
  • Depending on the results of this procedure, additional testing may be performed to evaluate or monitor progression of the disease process and determine the need for a change in therapy. Evaluate test results in relation to the patient’s symptoms and other tests performed.

Related Monographs

  • Related tests include amniotic fluid analysis, blood group and type, hemoglobin electrophoresis, and US biophysical profile obstetric.
  • Refer to the Hematopoietic and Reproductive systems tables at the end of the book for related tests by body system.
Handbook of Laboratory and Diagnostic Tests, © 2013 Farlex and Partners