BMJ 1998;317:307-312 ( 1 August )

Papers

Number needed to screen: development of a statistic for disease screening

Christopher M Rembold, associate professor. 

Cardiovascular Division, Department of Internal Medicine, University of Virginia Health Sciences Center, Charlottesville, Virginia 22908, USA

Correspondence to: Professor Rembold crembold{at}virginia.edu

Objectives: To develop the number needed to screen, a new statistic to overcome inappropriate national strategies for disease screening. Number needed to screen is defined as the number of people that need to be screened for a given duration to prevent one death or adverse event.
Design: Number needed to screen was calculated from clinical trials that directly measured the effect of a screening strategy. From clinical trials that measured treatment benefit, the number needed to screen was estimated as the number needed to treat from the trial divided by the prevalence of heretofore unrecognised or untreated disease. Directly calculated values were then compared with estimate number needed to screen values.
Subjects: Standard literature review.
Results: For prevention of total mortality the most effective screening test was a lipid profile. The estimated number needed to screen for dyslipidaemia (low density lipoprotein cholesterol concentration >4.14 mmol/1) was 418 if detection was followed by pravastatin treatment for 5 years. This indicates that one death in 5 years could be prevented by screening 418 people. The estimated number needed to screen for hypertension was between 274 and 1307 for 5 years (for 10 mm Hg and 6 mm Hg diastolic blood pressure reduction respectively) if detection was followed by treatment based on a diuretic. Screening with haemoccult testing and mammography significantly decreased cancer specific, but not total, mortality. The number needed to screen for haemoccult screening to prevent a death from colon cancer was 1374 for 5 years, and the number needed to screen for mammography to prevent a death from breast cancer was 2451 for 5 years for women aged 50-59.
Conclusion: These data allow the clinician to prioritise screening strategies. Of the screening strategies evaluated, screening for, and treatment of, dyslipidaemia and hypertension seem to produce the largest clinical benefit.

Key messages

  • Number needed to screen is a new statistic defined as the number of people that need to be screened for a given duration to prevent one death or one adverse event. It can be directly calculated from clinical trials of disease screening, and can also be estimated from clinical trials of treatment and the prevalence of so far unrecognised or untreated disease

  • For prevention of all cause death, 418 people need to be screened with a lipid profile if detection of dyslipidaemia was followed by pravastatin treatment for 5 years

  • The estimated number needed to screen for hypertension to prevent all cause death was 274 to 1307 for 5 years if detection was followed by treatment with thiazide diuretic

  • Screening with haemoccult testing or mammography did not significantly prevent all cause death. Haemoccult screening significantly decreased deaths from colon cancer with a number needed to screen of 1274 for 5 years. Mammography significantly reduced deaths from breast cancer with a number needed to screen of 2451 for 5 years of women aged 50-59 



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Rapid Responses:

Read all Rapid Responses

Cost information vital for deciding population screening priorities
Christopher Payne
bmj.com, 18 Aug 1998 [Full text]
Conclusions are unjustified
Nereo Segnan
bmj.com, 19 Aug 1998 [Full text]
Number needed to screen not enough
L S Lewis
bmj.com, 24 Aug 1998 [Full text]
Number needed to screen: a long overdue concept
Tim Reynolds, et al.
bmj.com, 2 Sep 1998 [Full text]
The Number Needed to Screen depends upon what you are screening.
Gay J Canaris
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