Clinical information
Diagnostic Utility:
Determination of fetal RHD genotype from cell-free DNA in the plasma of pregnant women allows for non-invasive determination of the Rh(D) Group of the fetus, always in the context of pregnant women with RhD negative phenotype.
- In RhD negative pregnant women with anti-D antibodies, this determination allows to identify truly at-risk pregnancies, where the fetus is confirmed as RhD positive, avoiding invasive procedures that could increase maternal sensitization. If, on the contrary, the fetus is D negative, the risk of haemolytic disease is ruled out and therefore monitoring is limited to that of a normal pregnancy.
- In the case of unsensitized RhD negative pregnant women, the main advantage of applying this technique is the ability to restrict the administration of anti-D immunoglobulin only to those pregnant women carrying an RhD positive fetus, avoiding unnecessary exposure in those women who do not require it.
Method:
Prior to determining the fetal RHD genotype in maternal plasma, DNA extraction must be performed from a plasma aliquot separated under suitable conditions to minimize the risk of contamination.
The determination is carried out using real-time PCR technique, using specific TaqMan primers and probes for exons 5 and 10 of the RHD gene. The determination also includes amplification of the SRY gene as a specific marker of the Y chromosome.
Reference Values
Possible results are: Fetus RHD Negative / Fetus RHD Positive.
In case the result is inconclusive, it is reported as such and a new sample from the pregnant woman is requested to repeat the determination.
Diagnostic Algorithm:
Not applicable
Turnaround Time:
10 days.
Specimen information
Sample: Total Blood (sample obtained from week 12 of gestation)
Tube: EDTA K3 10 ml tube
Minimum volume: 8 ml
Precautions:
- Sample extracted exclusively for this determination
- Untouched vacutainer tube
Stability:
- At room temperature: 5 days
- In refrigeration: 7 days
Transport instructions: room temperature
Reasons for rejection:
- Highly hemolyzed sample
- Sample with insufficient volume
- Manipulated sample
- RhD positive pregnant sample
Administrative information
BST Code: 4950
Test Description: Fetal Rh(D) genotype in maternal plasma
Synonyms: FETAL GENOTYPE RhD - BLOOD DNA
Section: Immunohematology
BST Rate: Check the updated rates here.
Profiles:
Test 4950 is not included in any profile.
References
- Lo YMD, Hjelm NM, Fidler C, Sargent IL, Murphy MF, Chamberlain PF, et al. Prenatal diagnosis of fetal RHD status by molecular analysis of maternal plasma. N Engl J Med 1998;339:1734-8
- Finning KM, Martin PG, Soothill PW, Avent ND. Prediction of fetal D status from maternal plasma: introduction of a new noninvasive fetal RHD genotyping service.Transfusion 42;1079-1085