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Sex Transm Dis. 1998 May;25(5):251-3.

Chlamydia positivity versus prevalence. What's the difference?

Author information

1
Division of Sexually Transmitted Diseases Prevention, Centers for Disease Control and Prevention, Atlanta, Georgia 30333, USA.

Abstract

BACKGROUND:

Data on chlamydia screening collected as part of Regional Infertility Prevention Projects often do not include personal identifiers, therefore repeat tests for patients during a year cannot be identified. Consequently, positivity is calculated and used to monitor chlamydia prevalence.

GOALS:

To assess how well positivity can estimate prevalence in family planning and sexually transmitted disease (STD) clinic settings.

STUDY DESIGN:

Analyzed data from chlamydia screening programs in three geographic areas of the United States that used unique patient identifiers.

RESULTS:

The relationship between positivity and prevalence is related to both the percentage of tests that are repeat tests and the percentage of repeat tests that are positive. On average, the percentage of positive repeat tests was the same as or higher than prevalence in family planning clinics; thus, positivity was the same as or higher than prevalence. In STD clinics, the percentage of positive repeat tests was consistently lower than prevalence; thus, positivity underestimated prevalence. However, the absolute difference between positivity and prevalence was less than 0.5% in family planning and STD clinics.

CONCLUSIONS:

Positivity can be used to monitor chlamydia prevalence in women screened in family planning and STD clinic settings.

PIP:

Data collected from US family planning (FP) and sexually transmitted disease (STD) programs that offer screening for chlamydia are used to monitor trends in chlamydia prevalence and identify high-risk groups. Because personal identifiers are often not collected and repeat tests for patients during the year cannot be identified, the proportion of total tests that are positive is used to estimate prevalence. To determine how well positivity estimates prevalence, data that used personal identifiers was analyzed from 16 states that are part of US Regional Infertility Prevention Projects in 3 geographic areas. In 1988-96, a total of 880,069 chlamydia tests were performed in FP clinics in the 3 regions; the percentage of women having a repeat test in a given year ranged from 2.7% to 11.9%. On average, the percentage of positive repeat tests was the same as or higher than the chlamydia prevalence in FP clinics. Over 26,000 tests for chlamydia infection were performed in STD clinics in 1 of these regions (VIII) in 1994-96; about 11% of women were tested more than once. In STD clinics, the percentage of positive repeat tests was much lower than chlamydia prevalence. Overall, however, the absolute difference between positivity and prevalence was less than 0.5% in both settings, confirming that positivity can be used to monitor chlamydia prevalence. As the positivity of initial and repeat tests diverges and the percentage of repeat tests increases, the difference between positivity and prevalence will widen.

[Indexed for MEDLINE]

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