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Roberta A. McDonald and John Pfister Wisconsin State Laboratory of Hygiene

Effects of the Transition to Chlamydia Nucleic Acid Amplification Testing in a Public Health Screening Program. Roberta A. McDonald and John Pfister Wisconsin State Laboratory of Hygiene National STD Prevention Conference San Diego, CA, March 4-7, 2002. Chlamydia Screening in Wisconsin.

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Roberta A. McDonald and John Pfister Wisconsin State Laboratory of Hygiene

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  1. Effects of the Transition to Chlamydia Nucleic Acid Amplification Testing in a Public Health Screening Program Roberta A. McDonald and John Pfister Wisconsin State Laboratory of Hygiene National STD Prevention Conference San Diego, CA, March 4-7, 2002

  2. Chlamydia Screening in Wisconsin • Well-established Chlamydia Control Program • Culture (~1978), DFA (‘84), EIA (‘85; 1° in ‘91) • Family planning, STD clinics, other settings • NAAT incorporated for male urine and gray zone analysis in 1997 • EIA positivity in females consistent over time • Transition from EIA to NAAT began in September of 2000, completed by year’s end

  3. Objective and Study Design • Retrospective analysis of routine testing data from 64 family planning clinics • Throughout state, urban and rural • Majority are long-term screening program participants • Selected six month time periods before and after transition to NAAT • Female urine and cervical swabs included

  4. Methods • Jan. - Jun., 2000: Syva Microtrak EIA with negative gray zone by Abbott LCR (n=13636) • Jan. - Jun., 2001: Probe Tec ET SDA (NAAT), Becton Dickinson (n=13720) • 2001 rates adjusted for volume

  5. Chlamydia Positivity

  6. Specificity and Positive Predictive Value (PPV) • Changes in specificity can have large impact on PPV in low prevalence (low risk) settings • No indicators of reduced specificity are evident • Lack of comparative data to suggest that SDA has lower specificity than EIA • Experienced lab, no increase in disputed results • Largest gains in low-prevalence/risk groups were in younger women; lowest prevalence/risk women over 24 showed little or no increase in positivity

  7. Positivity Changes, Low Prevalence by Age

  8. Other Possible Contributing Factors • Lower EIA sensitivity in low-prevalence setting: • Consensus that NAAT sensitivity is superior to EIA sensitivity • Reported EIA sensitivities vary widely (42% in Van Dyck et al1; 80% in WI method comparison2) • Changes in population / screening practices at participating clinics: • Some clinics showed greater variation in positivity or numbers tested • Unable to trace specific protocol changes to determine full impact on positivity

  9. Other Effects of the Transition • Gonorrhea test requests increased by 16.3% (from 37% to 43% of CT requests) • Previously required a second swab • GC criteria expanded to include chlamydia-positive individuals lacking other criteria • Urine submissions from females increased from zero to 2.6% of specimens in FP clinics • Female urine less sensitive than swabs • Urine testing (male and female) may increase screening in high and low risk settings

  10. Limitations • Impossible to determine actual sensitivity or specificity of either assay • Population consistent over time, but cannot be considered identical between the time periods • Prevalence estimates for some low-volume clinics had wide confidence intervals • Procedural changes in individual clinics could not be thoroughly evaluated • NAAT new in this setting; reassess after longer period in use

  11. Implications and Conclusions • Simple percent increases in overall positivity may not tell whole story • Many positives missed by EIA are in lower risk settings • Specificity monitors remain important to assure acceptable PPV • Convenience-based increases in GC testing may adversely affect PPV • Availability of urine tests can expand screening

  12. References 1. Detection of Chlamydia trachomatis and Neisseria gonorrhea by Enzyme Immunoassay, Culture, and Three Nucleic Acid Amplification Tests. E. VAN DYCK, M. IEVEN, S. PATTYN, L. VAN DAMME, and M. LAGA.JCM.39.5.1751-1756,2001 2. Comparison of the Performance of PCR, LCR, EIA and EIA with Supplemental Testing for Detection of Chlamydia trachomatis Infections in Women Attending Family Planning Clinics in Wisconsin. R. A. MCDONALD, J. R. PFISTER and L. BOETTCHER-TROXELL. Poster presented at ASM 99th General Meeting, Chicago, IL, 1999 3. Comparison of the Performance of Two Nucleic Acid Amplification (NAA) Assays for the Detection of Chlamydia trachomatis and Neisseria gonorrhea Infections. R. A. MCDONALD and J. R. PFISTER. Poster presented at 2000 National STD Prevention Conference, Milwaukee, WI

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