The Coombs test (indirect antiglobulin test) is the standard test for the diagnosis of anti-Rh antibodies in women sensitized to Rh positive blood.
Coombs test: what it is and what it is used for
As part of pregnancy tests, the Coombs test, also known as an antiglobulin test, is used to detect antibodies in the woman’s blood that – with the passage of blood through the placenta – can attack the red blood cells of the future unborn child, developing a hemolytic disease of the fetus and newborn (MEFN).
This screening is primarily aimed at looking for antibodies directed against the D antigen of the Rh factor.
The Coombs test can be of two types: direct or indirect.
When it is performed in pregnancy
The indirect Coombs test should be performed on all pregnant women by the 16th week of pregnancy, especially when the mother’s blood type is Rh negative and that of the father is Rh positive: in this case it is likely that the fetus has the D antigen and that, consequently, the mother develops anti-Rh antibodies.
Based on this logic, later in pregnancy the test is repeated every month in Rh negative women, and during the third trimester in Rh positive women.
If antibodies are detected, tests will be repeated throughout pregnancy to monitor the titer of the antibodies: an increase in antibody titer could indicate that hemolytic anemia of the fetus and newborn is developing.
Hemolytic disease of the fetus and newborn
It is a blood disorder in the fetus or newborn that in some cases can be fatal. The one caused by anti-RhD antibodies, the most frequent in terms of incidence and severity.
Normally red blood cells last about 120 days. In the presence of this disorder they are destroyed quickly and therefore do not last long.
Hemolytic disease in the fetus occurs when the mother’s immune system sees the fetus’ red blood cells as foreign and develops antibodies that attack them and cause them to break down too soon.
Pregnancies at risk
MEFN rarely occurs during the first pregnancy, unless the mother
has been previously sensitized by transfusions.
Usually, during the first pregnancy, primary maternal immunization takes place, characterized by the production of a small amount of IgM antibodies, immunoglobulins that do not cross the placenta.
In subsequent pregnancies, and after further exposure to the antigen, as a result of secondary immunization, IgG antibodies are produced, which can cross the placenta and cause hemolysis (the process of destruction of red blood cells).
Statistics
Without any type of intervention the fetus:
- in 50% of cases, they have only mild signs of illness and will recover without any treatment;
- in 25% of cases it can present hemolysis and kernicterus (pathological neonatal jaundice with deposit of free bilirubin in the brain tissue) if not adequately treated at the time of birth;
- in the remaining 20-25% of cases, anti-D MEFN can present in its most severe form (hydrops fetalis and death) before the 34th week of gestational age.
However, currently, with the improvement of maternal fetal surveillance and
the possibility of treatment in utero, severe cases (hydrops and death) are reduced
to about 10%
Katherine Johnson, M.D., is a board-certified obstetrician-gynecologist with clinical expertise in general obstetrics and gynecology, family planning, women’s health, and gynecology.
She is affiliated with the Obstetrics and Gynecology division at an undisclosed healthcare institution and the online platform, Maternicity.com.