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)