Kleihauer-Betke-Test

Abstract

Fetomaternal hemorrhage (FMH) occurs when there is a break in the placental barrier, allowing blood from the fetal circulation to enter the maternal circulation.  This disruption in the placental barrier may occur for many reasons, including intra-uterine fetal demise and trauma.  Trauma is the number one cause of pregnancy-associated maternal deaths in the United States.[1]. It occurs in as many as 40% of traumas, increasing in frequency and amount with high-force trauma, blunt force trauma, abdominal trauma, and with anterior placental placement in the uterus. When FMH occurs, fetal hemoglobin (HgF) is mixed with maternal blood.  In response to this exposure, the maternal immune system is activated, and isoimmunization (formation of Anti-RhD antibodies) may occur if the mother is Rhesus-D protein (RhD) negative and the blood type of the fetus is RhD positive. It takes only 0.01 ml to 0.03 ml of FMH to isoimmunize the mother.   Future pregnancies may be at risk for RhD disease if the fetus is RhD positive. The maternal antibodies bind to fetal RhD +ve erythrocytes, leading to hemolysis, anemia, hydrops fetalis and possible fetal death.   To prevent the formation of Anti-RhD antibodies, RhoGAM (IgG Anti-D (Anti-Rh)) is indicated.  Before 12-weeks gestational age, in the setting of a RhD negative mother and FMH, a mini-dose of 150 mcg (MICRhoGAM) is given.  This dose will suppress the immune response to 2.5 mL of Rh-positive red blood cells.  After 12-weeks gestational age, a dose of 300 mcg is recommended. This standard dose of RhoGAM (300 mcg) covers FMH up to 15 mL of fetal red cells (30 mL of whole fetal blood).  However, there are times when the additional dose is necessary due to massive red blood cell FMH and subsequent maternal immune response.  This is when the Kleihauer-Betke (KB) test is essential. (See Potential Diagnosis section for preliminary Rosette testing).