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Kleihauer-Betke test

What Is It, Interpretation, and More

Author: Anna Hernández, MD

Editors: Alyssa Haag,Ian Mannarino, MD, MBA

Illustrator: Jillian Dunbar

Copyeditor: Joy Mapes

Modified: 24 Sep 2021

What is the Kleihauer-Betke test?

The Kleihauer-Betke (KB) test is a blood test used during pregnancy to screen maternal blood for the presence of fetal red blood cells. It is mainly used to assess the severity of a fetomaternal hemorrhage (FMH), a condition in which a disruption in the placental barrier allows fetal blood to enter the maternal circulation. Fetal blood may enter the pregnant individual’s blood circulation naturally throughout pregnancy and childbirth, when there are traumatic or obstetric complications to pregnancy, or due to invasive prenatal procedures (e.g., amniocentesis).

Pregnant individuals are considered “Rh negative” when they do not have the Rhesus (Rh) factor on the surface of their red blood cells. For these individuals, the introduction of Rh-positive blood into their blood circulation may induce the formation of anti-Rh antibodies. This process can occur when fetal blood enters the maternal circulation, thereby inducing the formation of maternal anti-Rh antibodies against fetal red blood cells. These antibodies are not able to cross the placenta during the first pregnancy, so they cannot harm the first baby. However, in future pregnancies, pre-formed anti-Rh antibodies may cross the placenta and destroy the infant’s Rh-positive red blood cells, resulting in fetal anemia (i.e., low levels of red blood cells). 

If an Rh-negative individual is pregnant with an Rh-positive fetus, anti-Rh immune globulin (e.g., RhoGAM) is given at 28 weeks gestation and again after childbirth to prevent the pregnant individual from forming anti-Rh antibodies. Individuals may be given an additional dose of anti-RH immune globulin in situations where a fetomaternal hemorrhage may have occurred. The anti-Rh immune globulin binds to any Rh-positive red blood cells from the fetus that sneak into the pregnant individual’s circulation, preventing the formation of maternal antibodies. A standard dose of anti-Rh immune globulin is enough to prevent the formation of anti-Rh antibodies in response to 15 milliliters (about 0.3 ounces) or less of Rh-positive blood. The results of the KB test can help quantify the size of the fetomaternal hemorrhage, thereby assisting in calculating the needed dose of anti-Rh immune globulin.

How do you interpret a Kleihauer-Betke test?

The Kleihauer-Betke test distinguishes between fetal and maternal blood cells based on the type of hemoglobin that is present. Red blood cells containing fetal hemoglobin are more resistant to acid than those containing adult hemoglobin. When a sample of maternal blood is exposed to an acid, the maternal hemoglobin washes away, while the fetal hemoglobin remains intact. After this acid elution, the slide is washed, stained, and examined under the microscope. The fetal cells will be red and easy to see, while the maternal cells will look pale because they have lost most of their hemoglobin. The number of fetal cells is then counted to estimate the total volume of fetal blood.

What is a positive Kleihauer-Betke test?

A positive KB test means there is fetal blood in the maternal circulation. The sensitivity of the KB test is 5 mL, meaning a fetomaternal hemorrhage of less than 5 mL between the fetus and the pregnant individual’s circulation may not be detected with this method. Because the KB test is a quantitative test, results are often reported as the total amount of fetal blood in the maternal circulation, rather than as positive or negative.

What are the most important facts to know about the Kleihauer-Betke test?

The Kleihauer-Betke test (KB) is a blood test used during pregnancy to quantify the amount of fetal blood found in the maternal circulation. Most commonly, the KB test is used to assess the severity of fetomaternal hemorrhage (FMH) so pregnant Rh-negative individuals may receive an appropriate dose of prophylactic anti-Rh immune globulin. The KB test is based on the principle that fetal hemoglobin is more resistant to acid elution than adult hemoglobin.

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