Use of blocking reagent to confirm enzyme immunoassay results in chlamydial conjunctivitis.

A blocking reagent (Abbott) was used to test 153 conjunctival and nasopharyngeal samples from children with signs of early trachoma and from neonates and adults with conjunctivitis caused by Chlamydia trachomatis in whom positive results had been obtained on at least one occasion in an enzyme immunoassay (EIA) (Abbott). EIA and cycloheximide-treated McCoy cell cultures of the eye sample were equally often positive in the non-trachoma cases (70 versus 71), the EIA results being confirmed by the blocking reagent test in these cases. The corresponding figures for the nasopharyngeal samples were 31 versus 25. There was full agreement between the results of the EIA and blocking reagent tests, except in two eye samples and one nasopharyngeal sample where the blocking reagent test was negative. Three EIA results were within the equivocal zone around the cutoff point, two of which were positive and one negative in the blocking test.