Work out number screened to detect one case instantly with clear inputs, formula shown and shareable results.
Combines prevalence with test sensitivity to give the number of people who must be screened to detect one case, and the cases the test will miss.
Number needed to screen
NNS = 1 / (prevalence x sensitivity); cases missed = people screened x prevalence x (1 - sensitivity)
Halving prevalence doubles the number needed to screen, whereas realistic sensitivity gains change it only modestly.
No. Benefit also needs the proportion of detected cases in whom early treatment changes the outcome.