The gel test: investigation into the aetiology and the significance of a positive auto-control.

Background: Shortly after the gel test was introduced into routine immunohaematology, an increased percentage of patients were reported to show a positive auto-control in the indirect antiglobulin test (IAT) of uncertain significance as the direct antiglobulin test (DAT) in the tube technique was negative.

Materials and methods: In our study 13,280 randomized patient blood samples were screened and additional investigations carried out including an analysis of patient histories in the 97 blood samples that were auto-control positive in the gel test.

Results: In 87.4%, a re-testing with polyspecific antiglobulin serum (83.2% with anti-IgG) showed positive results in contrast to only 52.9% re-tested by the tube test. Neither nonspecificity nor cold agglutinins were significant. None of the patients examined showed any signs of haemolysis except for one with pernicious anaemia. We concluded that the increased number of positive auto-controls and DATs is due to the greater sensitivity of the gel test and thus the detection of minute quantities of specific cell-bound IgG molecules, i.e. warm auto-antibodies or drug-induced antibodies.

Conclusion: Prior to transfusion, a positive result should be confirmed by a tube DAT. If this test is negative and there is no history of a previous transfusion or of haemolysis, the transfusion should not be delayed by carrying out further time-consuming investigations.