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Table 2. Balance Sheet: Screening for Chlamydia in 10,000 Women Aged 18-34 Years

  Low risk
(questionnaire)a
Low risk
(no questionnaire)b
High riskb
Base case assumptions
Prevalence of chlamydia in population 0.03 0.03 0.094
Compliance with chlamydial testing 0.64 0.64 0.64
Sensitivity of test 100% 100% 100%
Results
Mailed questionnaire 10,000    
Responded to questionnaire 5,701    
Identified as high-risk 713    
Prevalence in risk group 0.07 0.03 0.094
Tested for chlamydia 457 6400 6400
Cases of chlamydia diagnosed and treated 32 192 602
Cases of pelvic inflammatory disease prevented 9 53 167
Number needed to screen (using questionnaire) to prevent one case of pelvic inflammatory disease 1,130 not applicable not applicable
Number needed to invite for screening (using chlamydial test) to prevent one case of pelvic inflammatory disease 81 (57)c 188 (120) 60 (39)
[a] Assumptions based on the results of a randomized controlled trial conducted at Group Health of Puget Sound46 described in the text.
[b] Assumptions based on theoretical populations. The proportion of all patients who meet the criteria for "high risk" varies with practice setting, patient population, and the criteria used to define high risk.
[c] Numbers in parentheses indicate number needed to screen based on actual number of women tested for chlamydia.

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