Impact of switching laboratory tests on reported trends in Chlamydia trachomatis infections.

Improvements in the sensitivity and specificity of laboratory testing methods for Chlamydia trachomatis infections in recent years have created potential problems with interpreting data on chlamydia prevalence trends. A switch to a more sensitive test can result in an increase in chlamydia positivity even with no increase in the true disease prevalence. To examine the impact of switching laboratory testing methods on chlamydia positivity trends among women, the authors analyzed data from chlamydia screening programs in family planning clinics in two geographic areas of the United States. Data from 7,287 tests performed in Philadelphia, Pennsylvania, indicated a 46% increase in positivity (from 4.1% to 6.0%) when the clinics switched from a nucleic acid probe assay to a ligase chain reaction test. Data from 35,306 tests performed in Oregon and Washington State laboratories showed a 21% increase in positivity (from 3.3% to 4.0%) when clinics switched from a direct immunofluorescent antibody testing procedure to an enzyme immunoassay with negative gray zone confirmation. These increases were within ranges consistent with the variability of the testing methods and occurred primarily in asymptomatic women and in women over age 20 years. Any switch in laboratory testing methods must be considered when interpreting data on chlamydial infection trends.