False positive tests for anti-hepatitis C antibodies and the problem of notifying blood donors.

Background: All donated blood in Israel is tested for anti-hepatitis C virus (HCV) antibodies by enzyme immunoassay (EIA) and donors are notified of the result. There is evidence that at low antibody titres, the percentage of false positives may be high, with consequent labelling of healthy people as being infected with HCV.

Aim: In this study we examined the correlation between anti-HCV antibody titres determined by a second generation EIA test with supplemental EIA tests and evidence of abnormal liver function.

Methods: Blood samples of 201 Israeli civilians who donated blood during 1992 and were repeat reactive for anti-HCV antibody based on second generation EIA, were tested by a supplemental test. Follow-up data were obtained from the donors and their family physicians.

Results: Results of anti-HCV EIA tests on two separate occasions of blood donation were highly correlated with each other (r = 0.86). Positive supplemental tests and abnormal liver function tests were found only in those subjects with high antibody titres. Furthermore low antibody titres were more prevalent during the winter months, suggesting that seasonal intercurrent infections may increase the percentage of false positives.

Conclusions: A high proportion of blood donors labelled as anti-HCV antibody positive based on low antibody titres, may not be at increased risk of carrying HCV. Since labelling would result in creating unnecessary anxiety among blood donors, it is important to confirm such results with test such as radioimmunoblot assay (RIBA).