• No FAQs found
  • No FAQs found
  • No FAQs found
  • No FAQs found
  • No FAQs found
  • No FAQs found
  • No FAQs found
  • No FAQs found

ABO Group and Rh Type

Test code(s) 7788

Question 1. Why is Rh type tested during pregnancy?

Rh type is needed during pregnancy to assess the risk of hemolytic disease of the newborn (HDN). In this condition, fetal red blood cells are destroyed by antibodies that are formed in the pregnant woman and that cross the placenta and into the fetal blood.

The most common severe HDN is due to Rh group incompatibility in which the pregnant woman lacks the D antigen of the Rhesus system (Rh negative or D-), while her fetus inherits the D antigen from the father and carries it on the red blood cells (Rh positive or D+). Other components of the Rhesus blood group system are C and E antigens. Each has a big letter and a small letter allele (C,c and E,e). Incompatibility in these alleles between the pregnant woman and fetus can also cause HDN in rare instances. However, the pregnant woman’s cells are not routinely tested for presence of these rhesus antigens. Rarely, if a maternal antibody screen reveals the presence of an antibody that may be directed to C, c, E or e antigen, then documenting the absence of such antigen from the maternal cells would aid in the characterization of this antibody.

Question 2. Why is ABO type tested during pregnancy?

ABO incompatibility between the pregnant woman and fetus can also cause hemolytic disease of the newborn (HDN). In this condition, the pregnant woman’s blood type is group O. Persons with O blood group have naturally occurring antibodies to A and B blood group antigens. However, these are usually of the IgM antibody class and are too large to cross the placental barrier into the fetal blood. Rarely, smaller IgG class antibodies to A or B antigens can form in the pregnant woman and can cross the placental barrier, causing HDN with a variable degree of severity when the fetus has inherited either A or B blood group antigens from the father.

Question 3. How are antibodies formed in the pregnant woman to red blood cells in the fetus?

There are 3 possible mechanisms by which the pregnant woman can develop antibodies to fetal red blood cells: 1) Fetal-maternal hemorrhage, during which fetal red cells penetrate the maternal blood. This can occur due to abortion, childbirth, placental rupture, accidents, or medical procedures carried out during pregnancy. 2) The pregnant woman had a blood transfusion with blood that has the same blood group as the fetus but not the pregnant woman. This may happen with blood groups that are not commonly tested for in routine transfusion work-up. 3) Naturally occurring antibodies as in ABO incompatibility (see Question 2).

Question 4. Why is Rhogam®given to Rh negative pregnant women and how does it affect lab results?

Rhogam®[Rho(D) immune globulin] contains antibodies to the D antigen in Rh positive cells. It is given by intra-muscular injection to Rh negative pregnant women during the 28th week of pregnancy, following childbirth, and following any of the events described in Question 3 as a cause fetal-maternal hemorrhage. This immunoglobulin attaches to fetal cells in maternal blood and causes their destruction. Rhogam®administration inhibits the pregnant woman’s immune system from producing antibodies to the D antigen and thus protects against HDN. Since Rhogam®contains anti-D antibodies, it interferes with an RBC antibody screen (test code 795) and causes a low-titer positive result if the test is performed shortly after injection.

Question 5. My patient has a weak D Rh positive blood group. What does this mean and should she be given Rhogam®?

Some individuals have RHD gene alleles that cause diminished expression (rather than full or no expression) of the D antigen on their red blood cells. Testing of these individuals may variably result in a D+ or D- report, depending upon the sensitivity of the testing method and the strength of the reagents. Generally individuals who are “weak D+” do not produce antibodies to the D antigen and do not need Rhogam®treatment. There are rare exceptions in cases of partial D expression. However, there is no harm in giving Rhogam to a “weak D+” patient if she is found to be D- by a less sensitive testing platform.

Question 6. Are there other reasons for ABO-Rh blood group testing?

Testing for blood groups is essential in preparation for blood transfusion and organ transplantation to determine compatibility between the recipient and the donor. Blood groups are also useful in paternity testing and forensic investigations. Epidemiological association between certain blood groups and several diseases, eg, peptic ulcer and gastric carcinoma, has been described. However, this test is not used in the diagnosis of these disorders.


  1. Roback JD, Grossman BJ, Harris T, and Hillyer CD, eds. Technical Manual. 17th ed. Bethesda, MD: American Association of Blood Banks; 2011. 
This FAQ is provided for informational purposes only and is not intended as medical advice. A clinician’s test selection and interpretation, diagnosis, and patient management decisions should be based on his/her education, clinical expertise, and assessment of the patient.
Document FAQS.111 Revision: 0