# Number needed to screen (and number needed to diagnose)

Source: https://onco.cc/terms/number-needed-to-screen/  
OnCo record `number-needed-to-screen` (Term). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

How many men have to be offered the test for one man to be saved from dying of prostate cancer, and how many extra cancers have to be found along the way. In the big European trial the answer at sixteen years was 570 men invited and 18 extra cancers diagnosed per death prevented, and the figures get better the longer the trial runs.

## Summary

The number needed to screen is the reciprocal of the absolute risk difference in disease-specific mortality between an invited group and a control group: how many people must be invited to, or must undergo, screening for one death from that disease to be prevented over a stated follow-up. Its companion, the number needed to diagnose, is how many extra cancers must be found to prevent that one death, and is the arithmetic of overdiagnosis. Neither is a property of the test. Both depend on the follow-up length, on the disease risk of the population, on the screening interval and threshold, and on whether the denominator counts men invited or men actually screened.

The European Randomized study of Screening for Prostate Cancer is the reference. At a median 9 years of follow-up in the predefined core age group of 162,243 men aged 55 to 69, the rate ratio for prostate cancer death was 0.80 (95 percent confidence interval 0.65 to 0.98), the absolute risk difference 0.71 deaths per 1,000 men, and 1,410 men had to be screened and 48 additional cases treated to prevent one prostate cancer death. At 16 years, in 162,389 men of the same core group, the rate ratio was 0.80 (0.72 to 0.89), the absolute mortality difference had grown from 0.14 percent at 13 years to 0.18 percent, the number needed to be invited had fallen to 570 from 742 at 13 years, and the number needed to diagnose had fallen to 18 from 26. The direction of travel is the point: the benefit accrues with time while the overdiagnosed cancers are all counted up front, so a number needed to screen quoted without its follow-up is close to meaningless.

Three cautions. First, ERSPC's 1,410 counts men screened and its 570 counts men invited, which are different denominators, and the second is the one that matches how a programme is offered. Second, PLCO reported no significant mortality difference in 76,693 United States men, but screening in its control group rose from 40 percent in the first year to 52 percent in the sixth, so its comparison is organised against opportunistic screening and no number needed to screen can be computed from it. Third, none of these figures came from a modern pathway: the trials randomised a blood test, not magnetic resonance imaging triage followed by targeted biopsy and active surveillance, and both the numerator and the denominator move when the pathway changes. The 2018 United States task force expresses the same quantity the other way up, as about 1.3 prostate cancer deaths and about 3 metastatic cases prevented per 1,000 men screened over about 13 years.

## Fields

- Kind: Term
- Last checked: 2026-09-25
- Also known as: NNS; number needed to invite; number needed to diagnose; NND; number needed to treat to prevent one prostate cancer death
- Tags: gu; prostate-glossary

## Notes

- Invited, screened or attended: three denominators, three numbers. ERSPC's 16-year report gives 570 as the number of men needed to be invited; its 2009 report gives 1,410 as the number needed to be screened. Reports adjusted for non-participation give a third, smaller figure, because they estimate the effect in men who actually attended rather than in everyone offered. A programme is offered to a population, so the invited denominator is the one that describes what a health service would buy.
- The number needed to diagnose is the overdiagnosis cost in the same units as the benefit. ERSPC's fall from 48 extra cases treated at 9 years to 18 extra diagnoses at 16 years is not a change in the disease; it is the benefit catching up with a harm that was banked at the start.

## Sources

- Schroder et al., New England Journal of Medicine 2009 (ERSPC): screening and prostate cancer mortality in a randomised European study: https://doi.org/10.1056/nejmoa0810084
- Hugosson et al., European Urology 2019: a 16-year follow-up of the European Randomized study of Screening for Prostate Cancer: https://doi.org/10.1016/j.eururo.2019.02.009
- Andriole et al., New England Journal of Medicine 2009 (PLCO): mortality results from a randomised prostate cancer screening trial: https://doi.org/10.1056/nejmoa0810696
- US Preventive Services Task Force, JAMA 2018: screening for prostate cancer, recommendation statement: https://doi.org/10.1001/jama.2018.3710

## Connected records

- terms: [Hazard ratio (HR)](https://onco.cc/terms/hazard-ratio/), [Lead time, and lead-time bias](https://onco.cc/terms/lead-time-bias/), [Overdiagnosis](https://onco.cc/terms/overdiagnosis/), [Overtreatment](https://onco.cc/terms/overtreatment/), [Polygenic risk score (PRS)](https://onco.cc/terms/polygenic-risk-score/), [PSA (prostate-specific antigen)](https://onco.cc/terms/psa/), [Screening](https://onco.cc/terms/screening/)
- key papers: [A 16-yr Follow-up of the European Randomized study of Screening for Prostate Cancer](https://onco.cc/key-papers/paper-hugosson-eur-urol/), [ERSPC: screening and prostate cancer mortality in a randomised European study](https://onco.cc/key-papers/paper-schroder-erspc-screening-mortality-nejm-2009/), [Lead time and overdiagnosis in prostate-specific antigen screening: importance of methods and context](https://onco.cc/key-papers/paper-draisma-lead-time-overdiagnosis-psa-jnci-2009/), [Measurement of prostate-specific antigen in serum as a screening test for prostate cancer](https://onco.cc/key-papers/paper-catalona-psa-screening-test-nejm-1991/), [PLCO: mortality results from a randomised prostate cancer screening trial](https://onco.cc/key-papers/paper-andriole-plco-prostate-screening-nejm-2009/), [Prostate Cancer Screening with PSA and MRI Followed by Targeted Biopsy Only](https://onco.cc/key-papers/paper-goteborg-2-n-engl-j-med-2022/), [USPSTF 2018: screening for prostate cancer, recommendation statement (grade C at 55 to 69, grade D at 70 and over)](https://onco.cc/key-papers/paper-uspstf-prostate-screening-jama-2018/)
- technologies: [PSA and MRI-first prostate cancer screening](https://onco.cc/technologies/prostate-screening-psa-mri/)
- cancers: [Localised prostate cancer, high and very high risk](https://onco.cc/cancers/prostate-high-risk/), [Localised prostate cancer, intermediate risk](https://onco.cc/cancers/prostate-intermediate-risk/), [Localised prostate cancer, very low and low risk](https://onco.cc/cancers/prostate-low-risk/), [Prostate cancer](https://onco.cc/cancers/prostate/)
- fronts: [Early Detection & Screening](https://onco.cc/fronts/early-detection/), [Prevention & Risk](https://onco.cc/fronts/prevention/)
- bottlenecks: [Overdiagnosis and false alarms](https://onco.cc/bottlenecks/b-overdiagnosis/), [The hardest cancers are found late](https://onco.cc/bottlenecks/b-early-detection/), [Trial design, endpoints and cost](https://onco.cc/bottlenecks/b-trial-design/)
- ideas: [Judge a prostate screening programme on metastatic presentation, not on incidence or mortality](https://onco.cc/ideas/idea-prostate-metastatic-presentation-as-the-screening-endpoint/)

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