Tests to diagnose a condition rather than monitor it may be worthwhile when they are simple and accurate, don't require a lab for further analysis, and follow-up treatment based on the result is clear. For example, breath tests for blood alcohol levels are clear-cut: If the test is positive, someone else should drive.

Home-pregnancy tests also tend to be accurate. "In fact, it's the same test we use in the office," Curtis says. As long as the woman waits for a week after a missed period to do the test, the results are reliable. Of course, those who test positive should go to a health-care professional for the appropriate care.

The benefits of other home diagnostic tests are less certain. For example, women who suspect a vaginal yeast infection can safely test and treat themselves with over-the-counter drugs as long as they see a doctor if symptoms don't improve or recur. But if you're sure of the symptoms you don't need the test at all and can go straight to the treatment. And while home tests for urinary-tract infections may be accurate, the results are helpful only if your provider is willing to phone in a prescription for antibiotics without seeing you—something many doctors may be reluctant to do unless you have a history of recurrent infections.

Other tests can be hard to interpret, our medical consultants say. For example, it can be difficult for an untrained eye to discern the subtle color changes that indicate the possible presence of blood in the stool in some home colon-cancer tests. And in any case, that test isn't nearly as reliable at detecting malignancies as a colonoscopy. So a negative result on a home fecal-occult blood test, or, for that matter, in the doctor's office, can be falsely reassuring.