The detection of Rhesus D (RhD) is a blood test finding related to a specific protein on the surface of red blood cells. The Rh factor is the second most important blood typing system after the ABO system. When a test result indicates “RHD is detected,” it confirms the presence of this protein, meaning the individual is Rh-positive. This finding is especially significant in the context of pregnancy and blood transfusions, as it determines potential compatibility issues. The presence or absence of the RhD factor defines a person’s Rh status.
Understanding the Rhesus D Factor
The Rhesus D factor is a protein known as an antigen that resides on the membrane of red blood cells. If a person carries this RhD antigen, they are classified as Rh-positive, which includes approximately 85% of the population. Conversely, if the RhD antigen is absent, the individual is Rh-negative. This status is inherited genetically, with the Rh-positive trait being dominant.
A person’s Rh status generally does not affect their health unless their blood comes into contact with Rh-incompatible blood, such as during a blood transfusion or pregnancy. For an Rh-negative person, exposure to Rh-positive blood triggers an immune response. The body recognizes the RhD protein as foreign and begins to produce specialized antibodies against it.
This immune response is called sensitization, and it is the central issue in Rh incompatibility. The RhD antigen is highly immunogenic, meaning it is very likely to provoke an immune reaction in someone who lacks it. Once a person is sensitized, the antibodies they produce are permanent and can pose a risk in future exposures.
Interpreting “RHD Detected” in Testing
The phrase “RHD detected” can appear in several clinical contexts. In basic blood typing, “RHD detected” simply means the person is Rh-positive. This is the most common result and indicates they do not face the risk of Rh sensitization.
For an Rh-negative mother, the detection of anti-D antibodies in her blood, often through an antibody screen, means sensitization has already occurred. This indicates the mother’s immune system has previously been exposed to Rh-positive red blood cells and developed antibodies. This crucial finding requires close monitoring throughout the pregnancy, as the antibodies could affect a future Rh-positive fetus.
A third context involves Non-Invasive Prenatal Testing (NIPT), which analyzes cell-free fetal DNA found in the mother’s blood. If the fetal RHD gene is detected in the maternal sample, it predicts the fetus is Rh-positive. This accurate test helps determine if the mother is carrying an Rh-positive baby, guiding the need for preventive treatment.
Risks Associated with Rh Incompatibility
Rh incompatibility is a medical concern only when an Rh-negative mother is carrying an Rh-positive fetus. Sensitization occurs when fetal Rh-positive red blood cells enter the mother’s circulation, most likely during childbirth, miscarriage, or abdominal trauma. The mother’s immune system then produces anti-D antibodies against the fetal cells.
These antibodies are typically of the IgG type, meaning they are small enough to pass through the placenta into the fetal bloodstream. Once in the fetus, these maternal anti-D antibodies attach to the RhD antigens on the fetal red blood cells, marking them for destruction. This immune attack results in the breakdown of fetal red blood cells, a process called hemolysis.
The resulting condition is known as Hemolytic Disease of the Fetus and Newborn (HDFN). Hemolysis leads to fetal anemia, causing the fetus’s heart and other organs to work harder to compensate. In severe cases, the fetus can develop hydrops fetalis, characterized by widespread tissue swelling and organ failure, often leading to death in utero or shortly after birth.
After birth, the rapid breakdown of red blood cells produces high levels of bilirubin, a yellow pigment. The newborn’s immature liver may not be able to process this excess bilirubin quickly enough, leading to severe jaundice. Uncontrolled high bilirubin levels can cross the blood-brain barrier, causing permanent brain damage called kernicterus, which can result in long-term neurological dysfunction.
Medical Management and Preventive Treatment
Modern medical practice focuses on preventing Rh sensitization in Rh-negative mothers. The cornerstone of this prevention strategy is the administration of Rh immunoglobulin (RhoGAM or Anti-D Globulin). This injection contains purified anti-D antibodies that act as a temporary shield.
When given to an Rh-negative mother, the injected anti-D antibodies circulate in her bloodstream and destroy any Rh-positive fetal red blood cells that may have entered her system. This prevents the mother’s immune system from becoming sensitized and producing its own permanent antibodies.
Rh immunoglobulin is administered prophylactically around the 28th week of pregnancy for all Rh-negative mothers carrying a potentially Rh-positive baby. A second dose is given after delivery if the newborn is confirmed Rh-positive. The injection is also necessary following any event that risks maternal-fetal blood mixing, such as miscarriage, amniocentesis, or abdominal trauma.
For mothers who are already sensitized, Rh immunoglobulin is no longer effective. Instead, the pregnancy is managed through regular monitoring, including checking maternal antibody titer levels. Specialized Doppler ultrasound scans assess blood flow in the fetal brain, which indirectly detects the severity of fetal anemia, guiding decisions for potential in-utero transfusions.