NUMBER NEEDED TO SCREEN - DEVELOPMENT OF A STATISTIC FOR DISEASE SCREENING

Objectives: To develop the number needed to screen, a new statistic to overcome inappropriate national strategies for disease screening. Num ber needed to screen is defined as the number of people that need to b e screened for a given duration to prevent one death or adverse event. Design: Number needed to screen was calculated from clinical trials t hat directly measured the effect of a screening strategy. From clinica l trials that measured treatment benefit, the number needed to screen nas estimated as the number needed to treat from the trial divided by the prevalence of heretofore unrecognised or untreated disease. Direct ly calculated values were then compared with estimate number needed to screen values. Subjects: Standard literature review. Results: For pre vention of total mortality the most effective screening test was a lip id profile. The estimated number needed to screen for dyslipidaemia (l ow density lipoprotein cholesterol concentration > 4.14 mmol/1) was 41 8 if detection was followed by pravastatin treatment for 5 years. This indicates that one death in 5 years could be prevented by screening 4 18 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 b lood pressure reduction respectively) if detection was followed by tre atment based on a diuretic. Screening with haemoccult testing and mamm ography significantly decreased cancer specific, but not total, mortal ity. The number needed to screen for haemoccult screening to prevent a death from colon cancer was 1374 for 5 years, and the number needed t o screen for mammography to prevent a death from breast cancer was 245 1 for 5 years for women aged 50-59. Conclusion: These data allow the c linician to prioritise screening strategies. Of the screening strategi es evaluation, screening for, and treatment of, dyslipidaemia and hype rtension seem to produce the largest clinical benefit.