SPADA

Pre-event/Post-event

•

Test > 90% sensitivity

•

Pre-event prevalence of smallpox is zero:

–

If test has 95% specificity, 10 tests done per month,

in 6 to 8 sites, every month there will be 3 or 4 false

positives

•

Post-event prevalence of smallpox is finite:

–

If test has 95% specificity, 1000 tests done per week,

every week 50 results will be false positives

–

If test has 99% sensitivity, 1000 tests done per week,

10 results will be false negatives

Test parameters

•

Sensitivity and specificity are independent of

the prevalence of disease

•

Predictive value positive and negative vary

with disease prevalence (Bayes’Theorem)

•

Implications for smallpox testing “pre-event”

and“post-event”: use of tests for decision

making