Problems in determining immune status in borderline specimens in an enzyme immunoassay for rubella immunoglobulin G antibody.

A total of 374 sera, found by enzyme-linked immunosorbent assay (Rubazyme; Abbott Laboratories, North Chicago, Ill.) to have borderline rubella antibody levels, were tested by hemagglutination inhibition. All sera had Rubazyme indexes in the range of 0.500 to 1.499. Rubazyme sensitivity was 59.0%, and specificity was 80.8%. The predictive value of Rubazyme-positive result was 91.4%, and that for a negative result was 36.4%. Immune and nonimmune results were significantly different between the two methods (P less than 0.001). The same sera were retested with the Rubazyme test, with an inter-run agreement of 75.3%. A significant difference in Rubazyme indexes between runs (P less than 0.001) was observed. An alternative method of testing specimens in the range close to the Rubazyme index cutoff value of 1.000 may be indicated.