The number needed to screen (NNS) is the number of patients that need to undergo screening to prevent one death by the disease being screened for1,2. The number is calculated by taking the reciprocal of the absolute risk reduction as follows:
calculate the risk (probability) of death from the disease in question in the unscreened group
calculate the risk (probability) of death from the disease in question in the screened group
subtract the risk of death in the screened group from the risk in the unscreened group to get the absolute risk reduction
take the reciprocal of the absolute risk reduction
Example
Results from the National Lung Screening Trial (NLST) indicate an NNS for low-dose CT for lung cancer screening of 320. That means that 320 subjects need to be screened to save one person from dying of lung cancer3. The number was calculated using data from subjects who were screened at least once. There were 425 lung cancer deaths out of 26,232 subjects in the group screened with chest X-ray (considered the unscreened group) and 346 lung cancer deaths out of 26,455 subjects in the low-dose CT group. NNS calculations for the NLST were:
risk of death from lung cancer in the chest X-ray group is 425/26,232=0.01620
risk of death from lung cancer in the low-dose CT group is 346/26,455=0.01308
absolute risk reduction is 0.01620-0.01308=0.00312
reciprocal of the absolute risk reduction is the NNS: 1/0.00312=320.5
Limitations and assumptions
The calculation assumes that there are no harms due to screening. In addition, the effects of competing causes of death are ignored.
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