Effects of the Transition to Chlamydia Nucleic Acid Amplification Testing in a Public Health Screening Program

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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

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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 years end

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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

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Methods

• Jan. - Jun., 2000 Syva Microtrak EIA with
  negative gray zone by Abbott LCR (n13636)
• Jan. - Jun., 2001 Probe Tec ET SDA (NAAT),
  Becton Dickinson (n13720)
• 2001 rates adjusted for volume

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Chlamydia Positivity
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Discussion

• Overall positivity increase from 6.3 to 7.2 is
  consistent with previous data on EIA and NAAT
  performance in this population1
• Low-prevalence and low-risk groups showed largest
  gains in positivity
• Trend did not hold true for age
• Many possible contributing factors

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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

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Positivity Changes, Low Prevalence by Age
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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

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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

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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

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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

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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