Comparison of four serological methods for detecting antibodies to Chlamydia trachomatis

The ability of four serological methods (enzyme-linked immunosorbent assay, single radial haemolysis, micro-immunofluorescence and complement fixation) to detect chlamydial antibodies was compared. Antigen was prepared from Chlamydia trachomatis serotype E. Sixty-two sera known to be positive and 12 negative reference sera were screened. The four tests yielded 71.6% (53/74) concordant results (Chi-square = 14.57, p less than 0.01). Samples giving discordant results had only low levels of antibodies. Analysis of results showed a strong correlation between enzyme-linked immunosorbent assay and immunofluorescence (r = 0.98), and complement fixation and single radial haemolysis (r = 0.75). Antibodies were detected in 97% of the 62 positive sera by immunofluorescence and in 91% by enzyme-linked immunosorbent assay, whereas the results for complement fixation and single radial haemolysis were only 69.3% and 70.9% respectively. Extrapolation of the regression curve to the abscissae indicated a marginal superiority of immunofluorescence over enzyme-linked immunosorbent assay, although the reading of the results of the former test was more cumbersome. Good linearity was obtained between haemolytic zone diameter and reciprocal serum dilution in single radial haemolysis. Immunofluorescence was found to be the most sensitive and complement fixation the least sensitive of the four tests.