C5b A Closer Look at Chlamydia Re-Infection Rates: Denominators Are Important

Wednesday, March 10, 2010: 10:45 AM
Cottonwood (M1) (Omni Hotel)
Elizabeth Torrone, MSPH, PhD1, Catherine Satterwhite, MSPH, MPH2, Thomas A. Peterman, MD, MSc1, Delia Scholes, PhD3, Onchee Yu, MS3 and Stuart Berman, MD, ScM2, 1Division of STD Prevention, Epidemiology and Surveillance Branch, Centers for Disease Control and Prevention, Atlanta, GA, 2Division of STD Prevention, Epidemiology and Surevillance Branch, Centers for Disease Control and Prevention, Atlanta, GA, 3Center for Health Studies, Group Health Research Institute, Seattle, WA

Background: Increases in chlamydia (CT) re-infection rates have been hypothesized as a reason for recent increases in reported CT infection rates. However, there are numerous methodological challenges that must be considered when analyzing and interpreting CT re-infection rates.

Objectives: To demonstrate how re-infection trends are influenced by choice of denominator.

Methods: Computerized data on women aged 15-44 enrolled in Group Health Cooperative, a Pacific Northwest health plan, were abstracted. CT re-infection was defined as a positive CT test ≥30 days after a previous CT infection. Crude annual CT re-infection rates, using person-years, were calculated with different denominators from the same population.

Results: From 1997-2007, CT infection rates approximately doubled. During this time, there were 676 CT re-infections during 816,543.8 person-years of follow-up. The number of re-infections per year increased four-fold from 20 to 89. Among all women in the plan, re-infection rates increased six-fold from 2.2 to 13.6 per 10,000 person-years. However, when the denominator was restricted to women previously infected, re-infection rates decreased 2-fold from 1,692.1 to 724.2 per 10,000 person-years.

Conclusions: Using all women in the plan as a denominator showed an increasing re-infection rate, but did not account for the changing size of the population at-risk for re-infection. Restricting the denominator to women with a prior infection is likely more appropriate; however, crude rates do not account for additional factors that could influence the number of re-infections diagnosed, including test and re-test coverage, who is getting retested (e.g. age, reason for re-testing), when they are getting retested, and changes in test technology. 

Implications for Programs, Policy, and/or Research: Trends in re-infection rates are influenced by denominator choice and are also likely to be confounded by the characteristics of the population re-tested, which is often unknown in sentinel surveillance databases. Further investigation is required to elucidate contributing variables.