(FORTUNE Magazine) – Once or twice a week I get a call from a patient who's just heard about a new early-warning test for cancer, heart disease, or some other serious condition and wants to know why my office hasn't called to schedule an appointment. After all, the more tests, the greater the chance of catching something before it becomes a problem, right?

Not necessarily. We're talking about screening--testing people who have no symptoms. For screening to really work, we need an appropriate disease, an effective treatment, and a good test. An appropriate disease would be one that's present a long time before symptoms develop. Example: colon cancer. Pancreatic cancer, in contrast, is rare and often too far advanced to treat effectively after detection.

That said, all sorts of new tests bear watching, including genetic and DNA varieties. The jury is still out for most, but here are a few that might help certain individuals.

COMPUTED TOMOGRAPHY (CT) SCAN OF THE HEART In general, more calcium means more plaque, which can lead to a heart attack; this test (above right) looks for calcification in the three arteries of the heart. It has become quite popular, and some centers offer it to all comers. Scientific data don't support routine screening, however, and factors other than calcium in the coronary arteries also play a role in heart disease. I use this test, but mainly in cases where it will affect treatment, such as deciding whether to start cholesterol- lowering medication.

A variant of the test, used to look for lung cancer, is the subject of ongoing studies. Yet another variant, a "lip to hip" CT scan that examines most of the body, might turn out to be useful--but it might also discover things we're better off not knowing, information that leads to anxiety and follow-up tests without necessarily improving health.

BONE DENSITY A few years ago it didn't make sense to order a bone-density test because hormone replacement, exercise, and calcium supplements were the only treatment options. Now, however, new drugs have been developed that can increase bone strength. (Fosamax, Actonel, and Evista are three, and more are on the way.) The National Osteoporosis Foundation recommends bone-density testing for all postmenopausal women over 65 and those under 65 who have risk factors for osteoporosis (family history, smoking, low body weight, bone fracture with minor trauma). I order it at times for women who are not taking hormone replacement.

NEW RISK FACTORS FOR HEART DISEASE Homocysteine, lipoprotein (a), and C-reactive protein are fancy names for blood substances, but the bottom line is that they're similar to cholesterol: High levels increase the risk of heart disease. Treatment with appropriate drugs or vitamins is promising, but there's no direct proof that it results in reduced heart disease. I usually order the tests for patients with a family history of heart disease or patients who have heart disease but few of the traditional risk factors (high cholesterol, smoking, hypertension, and diabetes mellitus).

HEMOCHROMATOSIS TESTING Hemochromatosis, which can lead to diabetes and heart and liver disease, is a condition in which the body absorbs and stores too much iron. Although it's the most common abnormal genetic condition among Caucasians, few people have heard of it. A blood iron test is an easy, but not accurate, screen. If it turns out to be abnormal, a blood ferritin test will indicate how much iron is stored in the body. If both tests are abnormal, I may consider genetic testing or a liver biopsy. As with the other tests I've mentioned, there's no proof that mass screening is beneficial. On the other hand, if hemochromatosis is discovered early, the treatment is easy--just donate blood often enough to prevent iron buildup.

HAIR ANALYSIS FOR NUTRITIONAL DEFICIENCIES Forget about it. Shampoo, dirt, and other extraneous factors can affect the results. You're better off consulting a Ouija board.

FOR MORE INFORMATION ON THIS TOPIC, go to mayoclinic.com. Mayo Clinic offers Executive Health Programs at Mayo Clinics in Jacksonville, Fla.; Rochester, Minn.; and Scottsdale, Ariz.