Most women have only very small amounts of fetal blood in their circulations following pregnancy and delivery: the volume is less than 0.5 mL of whole blood in 93 percent of women, less than 1 mL in 96 percent, and less than 2 mL in 98 percent. FMH of 30 mL or more occurs in just 3 of 1000 women. When the FMH was 150 mL or more, 15 of 41 infants did not survive Rh-negative women with FMH of more than 30 mL of Rh-positive whole blood are at increased risk of Rh immunization, and thus the outcome of their future pregnancies also may be affected. ABO-compatible fetal red cells that have entered the maternal circulation have a life span similar to that of adult cells. ABO-incompatible fetal red cells may be cleared rapidly, but in some cases they circulate for weeks. Most FMHs of 30 mL or more occur before labor, delivery, or cesarean section. The majority occur with minimal clinical signs and symptoms in apparently normal pregnancies. The identification of postpartum Rh-negative women who have 30 mL or more of Rh-positive fetal blood in their circulation is important so that sufficient RhIG for immune suppression can be administered. It appears that more than one-half of women with FMH of 30 mL or more would not be identified if protocols were adopted to test only women in pregnancies considered to be at high risk.

Current serologic tests occasionally fail to identify women needing more than one vial of Rh immune globulin. We compared the indirect antiglobulin test after incubation with anti-D and a rosetting technique using enzyme treated Rh2Rh2 erythrocytes as methods for identifying significant fetal maternal hemorrhage (FMH). Artificial mixtures containing 0.05 to 1.2 percent Rh1rh (CcDe) fetal red blood cells mixed with rh (ce) adult red blood cells were tested. The indirect antiglobulin test of the 0.6 percent mixture (approximately 30 ml FMH) was reported to be microscopically positive by 17/20 technologists; whereas, 20/20 found the rosetting test to be strongly positive. The volume of FMH in 118 postpartum Rh immune globulin candidates was quantified using Kleihauer's test and formula. The results of the rosetting and Kleihauer tests of blood specimens from these patients were negative 1.4 ml for two, and strongly positive rosetting test and FMH of 6.5 ml for one. The rosetting test utilizes routine blood banking skills and requires 5 minutes more "hands on" time than an indirect antiglobulin test. Confirmation and quantification of positive results by an acid-elution test is necessary.

Anti-D causing mild hemolytic anemia was found in the serum and on the red cells of a D-positive patient who had received a kidney transplant from his D-negative mother. Anti-D had been detected in the donor's serum before transplantation. Gm allotyping of the patient's serum, donor's serum, donor's anti-D, and the unexpected anti-D in the posttransplantation serum showed the antibody to be of maternal origin. The patient was Gm(fb), the donor Gm(agfb), the maternal anti-D Gm (afb), and the anti-D in the posttransplantation serum was Gm(a).

Alloantibodies of A and B specificity were detected in the serum and on the red cells of 4 non-0 recipients of group 0 solid organ transplants. The antibodies were assumed to be produced by ‘passenger’ lymphocytes introduced at the time of transplant. Gm allotyping of donor serums, recipient serums and the unexpected antibodies was done. Gm typing was informative in 1 case, when the anti-A(1) found in the serum of a liver transplant recipient was shown to be of phenotype Gm(fb) and the Gm phenotype of the recipient to be Gm(axg). Gm allotyping was not informative in the other 3 cases.

Microscopic examination of 219 indirect anti‐human globulin tests using a 1: 1,000 dilution of RhoGAM®* and 259 indirect antihuman globulin tests using appropriate slide test anti‐Rho(D) sera on specimens simulating fetal maternal hemorrhages (FMHs) of from 15 to 200 ml showed that agglutination was more frequently observed using the slide test anti‐Rho(D) sera. Tests done on coded specimens sent to several hospital laboratories indicated the need for technologist education in the interpretation of results of these tests on postpartum women. Since the amount of Rh immune globulin needed to treat women with FMHs depends on the volume of fetal bleed, suspected FMHs should be confirmed and quantitated using Kleihauer‐Betke smears.