The diagnostic efficacy of an enzyme-linked immunosorbent assay (ELISA) using gonococcal pili 6650 as antigen was studied in asymptomatic women attending an STD clinic. Of the 182 women studied, gonorrhoea was diagnosed in 29%. The value of the ELISA was calculated on the basis of four arbitrary cut-off points in the test. The best predictive values for a positive result (PV(+)) were obtained at an absorbance value of A greater than or equal to 1.15 and A greater than or equal to 1.30 and that for a negative result at A greater than or equal to 0.85. When patients with a history of gonorrhoea were excluded, the PV(+) rose only at A greater than or equal to 1.15 (not at A greater than or equal to 1.30) and the PV(-) rose slightly. To be of use in the diagnosis of gonorrhoea in asymptomatic patients the ELISA should be used as follows: the result is positive at A greater than or equal to 1.15 and negative at A less than 0.85, the PV(+) then being 0.52 and the PV(-) 0.85. Whenever sera give a result between A = 0.85 and A = 1.15, the test should be repeated.
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