A pregnant 38 year old mother of two presents at the office concerned about her pregnancy. She is Rh-negative and her husband is Rh-positive. Both of her children are also Rh-negative. She has had two spontaneous aboions and carried a third pregnancy to term, but the child died at bih, diagnosed with erythroblastosis fetalis. The test of choice to determine the presence of circulating anti-Rh antibody in the mother is a(n)
The indirect Coombs test would be the test of choice to detect the presence of IgG (warm agglutinin) anti-Rh antibody circulating in the mother's blood. Serum is collected from the mother and mixed with Rh-positive red blood cells, followed by incubation with complement. If the mother had circulating IgG anti-Rh, it would bind to the Rh antigen on the Rh-positive red blood cells, fixing the complement and lysing the target cells. A titer of greater than or equal to 1:8 is considered diagnostic. The direct Coombs test would be used to determine if anti-IgG or anti-C3 antisera can agglutinate the baby's RBCs. This would not be useful for determining whether circulating IgG antibody to the Rh factor is present in the serum from the mother. An indirect Coombs test to measure IgM anti-Rh antibody would not be informative, since IgM anti-Rh antibody will not cross the placenta. Ref: Brooks G.F., Carroll K.C., Butel J.S., Morse S.A., Mietzner T.A. (2013). Chapter 8. Immunology. In G.F. Brooks, K.C. Carroll, J.S. Butel, S.A. Morse, T.A. Mietzner (Eds), Jawetz, Melnick, & Adelberg's Medical Microbiology, 26e.
Direct Coombs test to measure IgG anti-Rh antibody
Direct Coombs test to measure IgM anti-Rh antibody
Indirect Coombs test to measure IgG anti-Rh antibody
Indirect Coombs test to measure IgM anti-Rh antibody