Rh factor testing, also known as Rhesus factor testing, is the procedure of determining the Rhesus D status of an individual (see Rh blood group system).
Background Rhesus factor testing utilizes genotyping to detect the presence of the RhD gene. By checking the existence of the RhD gene in the individual's genome, the presence of Rhesus D (RhD) antigens can be inferred. Individuals with a positive RhD status have RhD antigens expressed on the cell membrane of their red blood cells, whereas Rhesus D antigens are absent for individuals with a negative RhD status. Rhesus factor testing is usually performed on pregnant women to determine the RhD blood group of the mother and the fetus. By confirming the RhD status of both mother and fetus, precautions can be made, if necessary, to prevent any medical complications caused by Rhesus incompatibility.
Rhesus factor The entire Rh blood group system involves multiple antigens and genes. For Rh factor testing, however, only the Rhesus factor correlated to the RhD antigen is assayed. The RhD gene that codes for the RhD antigen is located on chromosome 1. This chromosome contains gene instructions for making proteins in the body. RhD is a dominant gene, meaning that as long as at least one RhD gene is inherited from a single parent, the RhD antigen is expressed. Vice versa, if no RhD gene is inherited from either parent, no RhD antigen is produced.
Extraction of test samples Non-invasive extraction Blood plasma is commonly used as test samples for verifying the maternal RhD status. Blood plasma can also be used for determining the fetal RhD status if the mother is RhD- as maternal blood plasma contains maternal DNA and trace amounts of fetal DNA. In early pregnancy, around 3% of the mother's free-cell DNA is from the fetus, and raises to 6-7% by late pregnancy. Blood samples can be obtained through venipuncture of the mother. Since plasma and other components of blood have different densities, centrifugation of blood samples with added anticoagulant (such as EDTA) can segregate blood contents into multiple layers. Blood plasma can then be isolated from the other components. It can be genotyped using real time PCR to determine the RhD status of the fetus. The method of extracting fetal DNA from maternal blood plasma is considered to be a type of non-invasive prenatal testing.
Invasive extraction Non-invasive prenatal testing can be used if the mother is RhD-. However, in the case of maternal RhD status being positive, invasive prenatal testing may be used to determine the fetal RhD status instead. The two most common invasive methods of extracting fetal DNA are chorionic villus sampling (CVS) and amniocentesis (AMC). These invasive procedures can be conducted on both RhD+ and RhD- mothers. After the invasive procedure, medications that prevent the Rh immunization are usually prescribed to RhD- mothers. This is done to avoid the production of maternal anti-D antibodies which may attack the fetal blood cells should the fetus be Rh incompatible with the mother.
Chorionic villus sampling Chorionic villus sampling is usually performed between the 10th and 13th week of pregnancy. It samples chorionic villi, which are tiny projections of placental tissue. The placental tissues are derived from embryonic cells, hence, they contain fetal genetic information that can be used to determine the child's RhD status. There are two types of chorionic villus sampling. Trans-cervical sampling involves inserting a catheter through the cervix into the placenta to obtain villi; an ultrasound is used to guide the catheter to the site of sampling. Trans-abdominal sampling requires the insertion of a needle through the abdomen and uterus to obtain placental tissue. Local anesthesia can be applied to reduce the pain from invasive procedures.
Amniocentesis Amniocentesis is another invasive procedure which can be used to collect fetal DNA samples.[medical citation needed] This procedure is usually done between the 15th and 20th week of pregnancy. The purpose of AMC is to extract a small amount of amniotic fluid as fetal cells may be shed from the fetus and are suspended in the amniotic fluid. Since the fetal genome can be found in these cells, extracting amniotic fluid provides the required fetal genetic material for the genotyping of the RhD gene. Before amniocentesis commences, the doctor will inject local anesthetics to the mother's abdomen. The doctor will then use an ultrasound to locate the fetus in the uterus. Under the guidance of the ultrasound imaging, a long, thin, hollow needle will be inserted through the skin of the abdomen to the uterus of the mother. The needle is used to withdraw a trace amount of amniotic fluid. It is then removed from the maternal body and the extracted amniotic fluid is sent to the laboratory for further testing.
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