Rh factor testing: Difference between revisions

Revision as of 20:06, 9 March 2025

Latest revision as of 21:53, 15 June 2025

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== Background ==

Rhesus factor testing utilizes [[genotyping]] to detect the presence of the [[RHD (gene)|RhD gene]]. By checking the existence of the RhD gene in the individual's [[genome]], the presence of Rhesus D (RhD) [[Antigen|antigensantigen]]s can be inferred. Individuals with a positive RhD status have RhD antigens expressed on the [[cell membrane]] of their [[Redred blood cell|red blood cells]]s, 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.

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==== 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 [[Placenta|placental tissue]]. The placental tissues are derived from [[Embryonicembryonic cell|embryonic cells]]s, 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 [[Minimally invasive procedure|invasive procedures]].

==== Amniocentesis ====

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== Genotyping of RhD gene ==

The presence of the RhD gene in an individual's genome is determined by [[genotyping]]. Firstly, the body fluid containing an individual's DNA will be extracted. DNA will then be isolated from unwanted impurities. The isolated DNA will then be mixed with various reagents to prepare the [[Polymerasepolymerase chain reaction|polymerase chain reactions]]s (PCR) mixture. The PCR mixture usually contains [[Taq polymerase|Taq DNA polymerase]], [[DNA primer|DNA primers]]s, [[Deoxyribonucleotide|deoxyribonucleotidesdeoxyribonucleotide]]s (dNTP) and [[buffer solution]]. The DNA primers are specific for [[exon]] 7 and exon 10. Under different circumstances, primers for other regions of the RhD gene, such as [[intron]] 4 and exon 5, may also be used. The mixture will be subjected to a series of PCR which is performed by a [[thermal cycler]]. By the end of the PCR process, the amount of RhD gene will be amplified if it is present. The product of the PCR will be analyzed by [[gel electrophoresis]]. Before gel electrophoresis, [[Molecular-weight size marker|DNA reference ladder]], a [[Positive control group|positive control]] containing DNA with RhD gene, and the PCR product will be loaded onto the wells of the gel. An [[electric current]] will be applied and the DNA fragments will migrate to the positive terminal as they are negative in charge. Since DNA fragments have different molecular sizes, the larger they are, the slower they migrate. Utilizing this property, DNA fragments with different molecular masses can be segregated. With the help of gel staining and visualizing devices such as [[Transillumination|UV trans-illuminators]], RhD gene DNA fragments, if present, will be visible as a band with its corresponding molecular mass. Further DNA sequencing can be conducted to confirm that the sequence of product DNA fragments matches that of the RhD gene sequence.

== Clinical Applications ==

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=== Prevention of Rh group incompatibility in blood transfusion ===

When RhD antigens on red blood cells are exposed to an individual with RhD- status, high-frequency of [[Immunoglobulin G|IgG]] [[Rho(D) immune globulin|anti-RhD]] [[Antibody|antibodies]] will be developed in the RhD- individual's body. The antibodies then attack red blood cells with attached RhD [[Antigen|antigensantigen]]s and lead to the destruction of these cells. This condition is known as a [[Hemolytic reaction|haemolytic reaction]]. The destruction of red blood cells releases [[hemoglobin]] to the bloodstream. Hemoglobin may be excreted through [[urine]], causing [[Hemoglobinuria|haemoglobinuria]]. The sudden release of hemoglobin will also pass through the liver and be metabolized into [[bilirubin]], which in high concentrations, accumulates under the skin to cause [[jaundice]]. Liberation of blood cell debris into the circulation will also cause [[disseminated intravascular coagulation]].

==== Symptoms of Rh group incompatibility in blood donation ====

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=== Medical Intervention ===

Normally, no extra medical intervention is required when maternal Rh status is RhD+, nor RhD- mothers going through first pregnancy. However, in the case of a sensitized RhD- mother (previously conceived an RhD+ child) and the fetus being Rh+, medication such as an [[Rho(D) immune globulin|anti-D immunoglobulin]], called RhoGAM, will be given to the RhD- mother. Injecting RhD- mother with RhoGAM has been proven effective in avoiding the sensitisation of RhD+ antigen, even though the mechanism of how this medication works remains obscure.

This injection is given to the RhD- mother during the second trimester when there is incompatibility between her and the father. Another injection is given a couple days after delivery if the baby is found to be RhD+.  These injections may also be given to RhD- mothers after a miscarriage/abortion, after injury to the abdomen, or after the prenatal tests mentioned before of amniocentesis and chorionic villus sampling (cite1).   Anti-D immunoglobulin injection is also offered to RhD- individuals who have been mistakenly transfused with RhD+ blood.

== References ==