Published 15 July 2009, doi:10.1136/bmj.b2828
Cite this as: BMJ 2009;339:b2828

Letters

TYM and Alzheimer’s disease

Too many false positives

The first 150 words of the full text of this article appear below.

Brown and colleagues report 93% sensitivity and 86% specificity for their self administered screening test for Alzheimer’s disease.¹ They conclude that "it is a powerful and valid screening test for the detection of Alzheimer’s disease."

However, doctors and lay people are often confused by such conditional probabilities,^{2 3} and the value of a screening test is best appreciated when statistics are expressed in natural frequencies. For infrequent disorders, even high values of sensitivity and specificity can lead to an unacceptable number of false positives.

The accompanying editorial gives an expected prevalence of 13 cases of Alzheimer’s disease in 1000 people aged 65-69.⁴ Using this as the base rate together with the figures for sensitivity and specificity, I calculated test results in natural frequencies. For every individual correctly declared to have Alzheimer’s disease, about 11, on average, will be falsely declared to have this dread disease.

This test is likely to become . . . [Full text of this article]

¹ Bishop’s University, Sherbrooke, Quebec, Canada J1M 1Z

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