Explanation

Irregular red cell antibodies (indirect Coombs): what is measured?

Red blood cells carry many features on their surface: the blood groups. If someone else's blood enters your bloodstream, for example through a blood transfusion or during pregnancy or childbirth, your immune system can make antibodies against blood groups you do not have yourself. These are called irregular red cell antibodies; the natural antibodies of the ABO blood group are not among them. The test (indirect Coombs test or antibody screen) brings your blood into contact with test red cells of known blood groups and checks whether antibodies bind to them.

What can it detect?

Antibodies against red blood cells after a previous blood transfusion, pregnancy, miscarriage or childbirth
Antibodies that can cause a reaction at a future blood transfusion or affect the baby in a pregnancy

Result

What does the result mean?

Positive result

A positive result means antibodies against red blood cells were found in your blood. You do not notice this yourself. The screen does not yet say which blood group the antibody is directed against; that requires further testing (antibody identification). Not every antibody matters: some cause no problems, others do. For a blood transfusion, blood without that blood group then has to be chosen, and in a pregnancy the antibody can reach the baby through the umbilical cord and cause anemia. A positive result can also be caused by an anti-D injection during pregnancy or after childbirth, or by antibodies against your own red blood cells.

Negative result

A negative result means no irregular antibodies were found at this time. Over the years, antibodies can fall so low that they are no longer measurable, and rise quickly again after a new transfusion or pregnancy. A new antibody can also form after the blood draw. That is why the hospital always tests for antibodies again before a transfusion, and why this is checked during pregnancy by the midwife or gynecologist.

After an abnormal result

With a positive result, antibody identification follows to determine which blood group the antibody is directed against and whether it matters. If you are pregnant, the midwife or gynecologist discusses the next steps; sometimes a blood test of the biological father is also needed. For a later transfusion or pregnancy, it is important that healthcare providers know about the antibody.

Reliability

How reliable is the test?

The antibody screen is a common and reliable laboratory test. Hospitals use it before a blood transfusion, and in the Netherlands every pregnant woman has it around week 12 through the midwife or gynecologist, as part of the RIVM (Dutch National Institute for Public Health and the Environment) population screening PSIE. The result only applies to the moment of the blood draw. This test does not replace the blood test during pregnancy, nor the test the hospital does itself before a transfusion.

What can affect the result?

The result is a snapshot: new antibodies can form after a transfusion, pregnancy or childbirth, and old antibodies can fall below the detection limit.
An anti-D injection, for example around 30 weeks of pregnancy or after childbirth, gives a positive result for weeks to months without you having made antibodies yourself.
Antibodies against your own red blood cells and some medicines (certain antibody treatments) can interfere with the test.
For this test, the laboratory needs a separate tube labeled with your name and date of birth; without unambiguous labeling the test cannot be done.

Who it is for

Who is the test useful for?

Useful for

In the Netherlands, every pregnant woman already has this test free of charge through the midwife or gynecologist, around week 12 and for some again around week 27; this test does not replace that. Useful for anyone who wants to know after a previous blood transfusion, pregnancy, miscarriage or childbirth whether antibodies are present, for example with a future pregnancy in mind, and before a planned procedure if the treating doctor asks for it. The hospital always does its own recent test before a transfusion. In people who have never had someone else's blood in their bloodstream, the chance of a positive result is small.

How common is it?

In the national blood test for pregnant women (PSIE) in 2024, antibodies that may matter for the baby were found at the first blood test in about 3 in 1,000 pregnant women; in about 2 in 1,000 they turned out to really matter. In about half, the antibody was already known from a previous pregnancy or other testing. In people who have never had a transfusion or pregnancy, irregular antibodies are rare.

RIVM, PSIE process monitor 2024

Drawbacks and risks

The result can wrongly suggest that the blood test during pregnancy or the hospital test before a transfusion is unnecessary.
A positive result due to an antibody without significance, or due to an anti-D injection, can cause unnecessary worry.

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