An at-home test using a device called a Rigiscan is done to check for the occurrence of erections during sleep, which typically occur four or five times a night in men with a physically normal erection response.

The unfortunate truth is that the F.D.A. has no effectiveness standards for professionally performed tests, let alone at-home tests.

A good way to pin down the reason is to take a simple at-home test that involves the use of four to six postage stamps.

From Lake Consumer Products of Vernon Hills, Ill., this at-home test sells for $39.99, lower than the cost of tests in hospitals and doctors' offices.

An easy at-home test?

Dr. Spielberg said that she surveyed 240 people infected with HIV and found that more than half said that they would have preferred to have found out about their infection with a rapid at-home test.

At-Home Genetic Tests: A Healthy Dose of Skepticism May Be the Best Prescription [PDF] Cautions consumers about the marketing, sale, and use of at-home genetic tests.

The in-office tests detected blood only 4.9 percent of the time in patients later found to have cancer or precancerous growths, compared with 23.9 percent for the at-home test.

In an at-home test, all three kits worked admirably - if stickily.