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Indirect Coombs test in pregnancy: what does this test detect?

All pregnant women, at the first prenatal visit, undergo a blood test to determine if they are Rh positive or Rh negative and are screened for antibodies using the indirect Coombs test .

It is one of the basic tests that is carried out in prenatal check-ups, since it is of great importance for the monitoring of pregnancy and the treatment to be followed will depend on its result.

It is a preventive test performed on all pregnant women who are Rh negative, since if the mother is Rh negative and the baby is Rh positive inherited from the father, maternal-fetal Rh incompatibility occurs.

What is the Coombs Test?

The indirect Coombs test or indirect Coombs test (also known as the indirect antiglobulin test) is a screen that looks for antibodies that can bind to red blood cells and cause their premature destruction .

The test detects antibodies that are present in the mother’s bloodstream and could attach to the baby’s red blood cells, leading to problems if mixing of blood between mother and baby occurs during pregnancy or delivery. Birth. If this happens, it would cause hemolytic disease of the newborn or erythroblastosis fetalis.

When it’s made?

It is a non-invasive test that is performed through a blood sample from the mother in the first trimester, at the 8th or 9th week of pregnancy , and is repeated in the second trimester, at the 28th week of pregnancy .

Maternal-fetal Rh incompatibility

During pregnancy, the mother and the baby can have different blood groups and this is not a problem, since the blood of both does not actually mix, since the exchange of nutrients and waste products in the placenta is carried out through a membrane.

But at the time of delivery, the placenta ruptures, and the blood of the baby and the mother mix. It doesn’t matter at that point, but if the mother is Rh negative and the baby is Rh positive, the mother will begin to make defensive antibodies against the baby’s Rh positive blood.

The risk will be for the next pregnancy , since they can cross the placenta in a future pregnancy and harm the fetus if it is Rh positive.

What happens if the indirect Coombs test is positive?

When the mother is Rh negative, she can develop these antibodies if she is exposed to the red blood cells of an Rh positive baby. As prevention, all Rh-negative mothers are screened for anti-erythrocyte antibodies during pregnancy (at 28 weeks) and again at delivery.

If Rh antibodies are not detected at 28 weeks, the pregnant woman is given an injection of Rh-type immunoglobulin (Rh-Ig) in order to eliminate any trace of Rh-positive fetal red blood cells from her circulation, thus preventing the development of Rh-type antibodies from the mother.

At birth, the newborn’s Rh system is determined. If the baby is Rh negative, the mother will not require any further Rh-Ig injections; If the baby is Rh positive and the mother does not have antibodies to the D antigen, Rh-Ig will be given within 72 hours after delivery to prevent the mother from making antibodies.

If the woman has already developed the antibodies, the pregnancy should be followed closely to check that the Rh levels are not too high. Only in exceptional cases, if the incompatibility is very serious and can put the baby at risk, are blood transfusions called exchange transfusions performed during pregnancy and after delivery.

Exchange transfusions replace the baby’s blood with Rh-negative red blood cells to minimize the damage that can be caused by the circulation of Rh antibodies already present in your bloodstream.

In short, the indirect Coombs test allows you to determine if the mother has produced antibodies against red blood cells that have the Rh-positive factor.

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