Table 2. Outcomes of Screening 10 000 Asymptomatic Women for Chlamydial Infectiona

Outcome Risk for Chlamydia trachomatis Infection
Low Moderate Moderate to High High
Epidemiologic
Prevalence, % 0.1 1 5 10
New cases, n 10 100 500 1000
Expected PID in untreated C. trachomatis rateb 0.3 0.3 0.3 0.3
Expected cases of PID in untreated women without screening, n 3 30 150 300
Screening
Urine nucleic acid amplification test
Sensitivity 0.9 0.9 0.9 0.9
Specificity 0.99 0.99 0.99 0.99
Screening results, %
True positive 9 90 450 900
False negative 1 10 50 100
False positive 100 99 95 90
Total positive 109 189 545 990
Positive predictive value, % 8.25 47.6 82.3 90.9
Effect of therapy
Adherence to azithromycin therapy 0.8 0.8 0.8 0.8
C. trachomatis infection treatedc 7.2 72 360 720
C. trachomatis infection cured with treatment 0.96 0.96 0.96 0.96
Cases of C. trachomatis infection cured 6.9 69 345.6 691.2
Cases of C. trachomatis infection not cured 0.3 3 14.4 28.8
Total cases of C. trachomatis infection after screening and treatmentd 1.3 13 64.4 128.8
PID
Expected cases with screening, n 0.39 4 19 39
Cases avoided by screening, n 2.6 26 131 261
Number needed to screen to avoid 1 case of PID 3846 384.6 76.3 38.3
Infertility
Expected cases resulting from C. trachomatis infection-related PID, n 0.08 0.8 3.8 7.8
Cases avoided by screening, n 0.52 5.2 26.2 52.2
Number needed to screen to avoid 1 case of infertility 19 231 1923 382 192

Notes:
a. Assumptions made in this table, which are derived from a review of the literature by Hu and colleagues24 include that 80% of women with a positive result on chlamydia testing will be contacted and will receive and take azithromycin; 96% of women treated with azithromycin will be cured of infection; and 20% of women who experience PID will become infertile. PID = pelvic inflammatory disease.
b. Back from reference 1.
c. True-positive rate multiplied by 0.8.
d. Persons with a false-negative rate plus persons in whom infection is not cured.

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