Number Screened To Detect One Case Calculator

Work out number screened to detect one case instantly with clear inputs, formula shown and shareable results.

About Number Screened To Detect One Case Calculator

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.

Formula

Number needed to screen

NNS = 1 / (prevalence x sensitivity); cases missed = people screened x prevalence x (1 - sensitivity)

Worked example

With the default values below, Number Screened To Detect One Case Calculator returns Number screened per case detected of 294.1. Change any input above and every figure here is recalculated in your browser.

Inputs used

Prevalence in the screened groupaccepts 0.001–50: 0.4
Test sensitivityaccepts 10–100: 85
People screenedaccepts 1–10,000,000: 10,000

Results

Number screened per case detected: 294.1
Cases detected in this cohort: 34.0
Cases missed by the test: 6.0
Cases actually present: 40.0

Step by step

1Values used
2Number needed to screen
3Number screened per case detected= 294.1
4Cases detected in this cohort= 34.0
5Cases missed by the test= 6.0
6Cases actually present= 40.0

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why does prevalence dominate?

Halving prevalence doubles the number needed to screen, whereas realistic sensitivity gains change it only modestly.

Does NNS measure benefit?

No. Benefit also needs the proportion of detected cases in whom early treatment changes the outcome.