The evaluation of a positive direct antiglobulin test (autocontrol) in pretransfusion testing revisited

Abstract

Direct antiglobulin tests (DATs) using anti-IgG were performed on 65,049 blood samples from prospective transfusion recipients; 3570 tests (5.49%) were positive. Using criteria published previously (primarily excluding patients not transfused within the preceding 14 days), 778 samples from other than neonatal patients were selected for further evaluation. Eluates that did not react were obtained on 518 (66.6%) of these samples. Warm-reactive autoantibodies were apparent in 192 eluates, while 16 contained drug-related antibodies, anti-A or anti-B from prior transfusion with ABO mismatched blood components, or anti-D passively acquired from immune serum globulin. Fifty-two eluates contained alloantibodies; however, in only six of these cases did the corresponding serum lack unexpected alloantibodies, as determined by routine pretransfusion studies. Three additional weakly reactive clinically significant alloantibodies were detected solely through additional serum tests performed on DAT-positive samples. On the basis of these findings, the DAT had a low predictive value when used to detect the early manifestations of an immune response to recently transfused red cells. Elimination of the autocontrol from routine pretransfusion testing, therefore, carries minimal risk to patients yet will undoubtedly contribute to the containment of health care costs. Moreover, the risk is lower than that associated with the elimination of the antiglobulin crossmatch.

Revisiting the issue: can the reading for serologic reactivity following 37°C incubation be omitted?

“… † Data derived from Judd et al 6 In 29 months, seven potentially significant alloantibodies (3 anti‐K1, 3 anti‐Jk a , and 1 anti‐E) were detected solely through evaluation of a positive DAT. …”

“… ¶ The percentage of incompatible units will vary with antibody specificity; the actual antigen frequencies used in the calculations were those corresponding to the antibodies missed following test elimination, as shown in our earlier studies 1,6,7 …”

“…Retrospective review of our experience with the pretransfusion DAT led us to eliminate this test in June 1984 6 . At the same time, there was a national interest in eliminating the IAT‐XM for all patients except those known to have unexpected antibodies 11 .…”

“…For both equations, n equals wanted antibodies missed, and 5 represents the average number at our institution of units given per patient transfused. The percentage of antigen‐positive units will vary with antibody specificity; the actual antigen frequencies used in the calculations were those corresponding to the antibodies missed following test elimination, as shown in our earlier studies 1,6,7 . Cases involving anti‐D were excluded, because the patients would normally receive Rh‐negative blood.…”

Blood Grouping Techniques

“…Such a test very seldom detects AIHA (Kaplan and Garratty 1985) and very seldom detects an alloantibody bound to transfused red cells: in a series of almost 800 patients with a positive DAT following transfusion within the preceding 14 days, only six were due to alloantibodies bound to transfused red cells and not detectable in the patient's serum (Judd et al 1986). When, as at present, crossmatching is carried out only at 37°C, an IAT carried out as an autocontrol on the patient's own red cells is only dubiously worthwhile.…”