What is the Rh test?

Rh Test: Purpose, Process, and Clinical Significance

The Rh test is a blood test that determines whether a person's red blood cells have the Rh(D) antigen, categorizing them as either Rh-positive or Rh-negative, which is crucial for preventing potentially life-threatening complications in pregnancy and transfusion medicine. 1

What is the Rh Factor?

• The Rh (Rhesus) system is one of the most important and complex blood group systems, second only to ABO in clinical significance 2
• The Rh factor refers specifically to the D antigen, which is a protein found on the surface of red blood cells 2
• People who have the D antigen are classified as Rh-positive (approximately 85% of the population), while those without it are Rh-negative (approximately 15%) 1
• The Rh proteins form a core complex that is critical to the structure of the erythrocyte membrane and may play a physiological role in ammonia transport 2, 3

The Rh Test Process

• The Rh test is typically performed as part of routine blood typing 4
• The test involves mixing a blood sample with anti-D antibodies and observing for agglutination (clumping) 4
• If agglutination occurs, the blood is Rh-positive; if no agglutination occurs, the blood is Rh-negative 4
• Modern testing methods include column agglutination technology used in immunohematology reference laboratories and rapid ABO/RhD tests with high accuracy (99.5% concordance with reference methods) 4

Clinical Significance in Pregnancy

• The primary clinical importance of Rh testing is in pregnancy, where Rh incompatibility between an Rh-negative mother and Rh-positive fetus can lead to maternal alloimmunization 1
• Fetal red blood cells display Rh antigens from as early as 6 weeks of gestation, making maternal sensitization possible even in early pregnancy 1
• If an Rh-negative mother is exposed to Rh-positive fetal blood cells (through delivery, miscarriage, abortion, or trauma), her immune system may produce anti-D antibodies 5
• These antibodies can cross the placenta in subsequent pregnancies and attack the red blood cells of an Rh-positive fetus, causing hemolytic disease of the fetus and newborn (HDFN) 1, 5

Prevention of Rh Sensitization

• RhD immune globulin (RhIg) is administered to prevent sensitization in Rh-negative women 5
• Standard prophylaxis protocol includes:
  • Administration at 28 weeks gestation 1
  • Administration within 72 hours after delivery of an Rh-positive infant 1, 5
  • Administration after potential sensitizing events (miscarriage, abortion, amniocentesis, abdominal trauma) 1, 5
• This two-dose protocol reduces the rate of RhD alloimmunization from approximately 1.8% to between 0.1% and 0.2% 1, 5

RhIg Dosing Guidelines

• For events before 12 weeks gestation, a 50 μg dose of RhIg within 72 hours is adequate 1
• If the lower dose is unavailable, the standard 300 μg dose should be used 1
• For significant fetomaternal hemorrhage, quantitative testing may be needed to determine if additional doses are required 1

Monitoring Rh Sensitization

• The Rh antibody titre test measures the concentration of anti-Rh antibodies in maternal serum to monitor alloimmunization during pregnancy 6
• Serial measurements track antibody levels over time, with increasing titres suggesting higher risk of fetal anemia 6
• The Direct Coombs test (direct antiglobulin test) can identify antibody-coated red blood cells, suggesting immune-mediated hemolysis 6

Common Pitfalls to Avoid

• Not recognizing that fetal RBCs display red cell antigens from as early as 6 weeks gestation, making maternal sensitization possible in early pregnancy 1
• Failing to administer RhIg within the critical 72-hour window after potential sensitizing events 5
• Overlooking the need for RhIg in cases of threatened abortion with heavy bleeding, abdominal pain, or minor trauma during pregnancy 1
• Not considering RhIg administration for spontaneous or induced abortion at less than 12 weeks gestation 7, 1

Special Considerations

• If RhIg supply is limited, postpartum patients and antenatal patients at later gestational ages should be prioritized 1
• The Society for Maternal-Fetal Medicine recommends offering RhD testing and RhIg administration for spontaneous and induced abortion at <12 weeks gestation in unsensitized Rh negative individuals 7, 1
• Guidelines for RhD testing and RhIg administration must balance the prevention of alloimmunization with access to abortion care 7