Fetal-maternal erythrocyte distribution

How the test is performed:

Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill with blood). A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the tourniquet is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

The sample is taken to the laboratory where the hemoglobin is removed and both fetal and maternal red blood cells are counted. Based on that count a ratio of fetal to maternal blood cells is determined.

How to prepare for the test:

No special preparation is necessary for this test.

How the test will feel:

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed:

It is not uncommon for some fetal red blood cells to be transferred to the mother's blood stream during a normal delivery, or abortion. However, if a large number of fetal red blood cells are transferred to an Rh-negative mother, then Rh incompatibility may occur in future pregnancies.

All Rh-negative mothers should be screened during their first prenatal visit, and subsequent visits as indicated by the health care provider.

RhoGAM is a drug that is given after birth where there has been maternal and fetal blood exchange. This test determines the quantity of RhoGAM that should be administered. The RhoGAM will attack the fetal cells in the maternal blood stream and prevent maternal formation of anti-Rh antibodies that could endanger future pregnancies.