An Investigation Into Swab and Urine Samples for the Detection of Chlamydia Trachomatis Genital Infections

Abstract

Chlamydia trachomatis was first isolated in tissue culture in 1965 and urogenital swabbing followed by culture has remained the standard by which other, more recent, non-cultural methods are judged. Viable C.trachomatis organisms do not survive well in urine. However, with the advent of non-cultural methods, where the presence of viable material is not a requirement, the potential of urine for the detection of chlamydial antigen is under scrutiny. In order to remain competitive, manufacturers of chlamydia immunoassays and more recently DNA amplification tests, have sought validation of existing and new diagnostic methods, for use with urine samples, by hospital based clinical trials. The aims of this study were to investigate the inhibitory effect that some urogenital samples have on non-cultural test outcome and to undertake a series of clinical trials. In addition a simplified culture method was evaluated. In two investigations prior to the clinical trials, the little described “hook effect”, observed in some enzyme immunoassays, was researched. It was found that an optical density parabola could be induced in all of the tests under scrutiny depending on the number of leucocytes present and the volume of urine centrifuged. Methods were developed to exclude the possibility of excess antigen causing a prozone effect. In the other investigation, undertaken at Unipath Limited Research and Development department, sample interference in the Clearview™ Chlamydia test, possibly caused by cations within the specimen and anionic groups or hydophobic components in Chlamydia lipopolysaccharide, were explored. Cationic interference was reduced by the addition of heparin to the test extraction reagent. The eleven clinical trials resulted in successful validation of the methods for diagnostic use. The problems with discrepant results and sampling error were resolved by repeat testing or further analysis by alternative methods.