Quantitative Chlamydia trachomatis cultures: Correlation of chlamydial inclusion-forming units with serovar, age, sex, and race

The number of inclusion-forming units (IFUs) observed in quantitative chlam ydial cultures may be a surrogate for infectivity or transmissibility. Ther efore, we conducted a cross-sectional study of 11,034 patients with Chlamyd ia trachomatis infection who presented to the Seattle-King County public he alth department clinics between 1988 and 1996, to determine relationships b etween the number of IFUs observed in culture and sex, age, race, and serov ar class. Of the 11,034 cases of infection we studied, 6801 (62%) were cerv ical infections in women, and 4233 (38%) were urethral infections in men. T he median count was 450 IFU for women and 72 IFU for men (P < .001), Overal l, both men and women infected with B-class serovars had significantly high er IFU counts than did those infected with C-class serovars (P < .001). The median IFU count fell consistently with increasing age for both women (625 IFU for those <16 years old to 185 IFU for those >30 years old; P < .001) and men (210 IFU for those <16 years old to 40.5 IFU for those >30 years ol d; P < .001), We found, by use of multiple regression analysis, that sex, a ge, race, and serovar class remained independently related to IFU count, wi th counts being highest among young white women infected with B-class serov ars.