OVERVIEWWHAT IS RHESUS FACTOR
The Rhesus factor is a blood group characteristic. This trait shows whether blood from two people is compatible when mixed, for example, blood from mother and child during birth. Most people possess the Rhesus factor (or antigen D) and are “RhD positive”. Every 6th to 7th person (14-17%) is “RhD negative”.
With non-invasive fetal Rhesus factor (RhD) determination, RhD negative pregnant women can have their blood tested to determine the RhD of their unborn child. Most people possess the RHD gene and are therefore RhD positive. About 17% of pregnant women are RhD negative.
IMPORTANCE OF THE TEST
There is a risk of RhD sensitization for a RhD negative woman pregnant with a RhD positive child. When this happens the pregnant RhD negative woman forms antibodies that can enter the bloodstream of the RhD positive child. This can have serious consequences for the fetus.
Fetal and maternal blood can mix during birth, for example, due to small injuries to the placenta or the umbilical cord. Mixing can also occur during pregnancy, either without any external influence or through interventions such as amniocentesis. Approximately 35-40% of RhD negative pregnant women do not need anti-D prophylaxis because the child is also RhD negative.
WHO SHOULD TAKE THE TEST
RhD negative pregnant women who
want to know whether they require
anti-D prophylaxis (with a RhD positive fetus)
RhD negative pregnant women
with evidence of anti-D antibodies
TEST PROCEDUREHOW DOES THE TEST WORK?
Fragments of fetal genetic material, which originate from the placenta, circulate in the maternal blood (cell-free DNA) during pregnancy. Using molecular biology methods (real-time PCR), these fragments are analyzed for the presence of the rhesus D gene. If this is detected, the unborn child is RhD positive. Only one blood sample from the pregnant woman is required for the test. This prenatal test has no discernible disadvantages for either the pregnant woman or the unborn child. The test has a high sensitivity (>99%) and specificity (>98%).
Fetal rhesus testing is recommended from the 19th week of pregnancy. The test can be performed from the 12th week of pregnancy at the earliest. However, a RhD negative result must be confirmed by a repeat test following the 17th week of pregnancy.
Please not that pregnant woman must give informed and written consent!
RESULT OF THE TEST
Regardless of the result of the fetal Rhesus factor determination test, umbilical cord blood is used immediately after birth to determine the Rhesus factor for every baby born to a RhD negative mother.
Positive result: The fetus is RhD positive and anti-D prophylaxis is recommended.
Negative result: Anti-D prophylaxis is not necessary if the fetus is RhD negative. (Note that a negative result before the 19th week of pregnancy should be considered preliminary).
*Our tests can be ordered through our local partners. Please choose one of the locations listed below
9-10ml EDTA tube labeled with surname, first name and date of birth
Specimen storage and transport: between +2°C and +25°C, max. 48 hours
5-8 working days after sample receipt
– Detection of cell-free fetal DNA from maternal plasma
– Analysis of the RHD sequences using real-time PCR with three target regions (exons) to detect as many variants of the RHD gene as possible
– Test (CE-IVD product) with high diagnostic sensitivity (>99%) and specificity (>98%)
– Only possible for single fetus pregnancy, not suitable for twin pregnancies
– Where the result is unclear (around 1%), anti-D prophylaxis is recommended
– Rare genetic rhesus variants can lead to false-positive results in 0.2-0.3% of cases
– False-negative results can be caused by a low level of fetal cell-free DNA, degradation of the cell-fee DNA or by hemolysis in the specimen tube