(Money Magazine) -- A few decades ago, playing doctor was a game. Now it's a $3 billion industry, as the growing variety of home medical tests on the market enable millions of consumers to take their health into their own hands.

Fans of these direct-to-consumer diagnostic kits contend that they cost less, are more convenient and offer more privacy than testing by a doctor.

With no appointment or prescription required, you can diagnose allergies, hepatitis or menopause; monitor cholesterol or blood pressure; and even screen for some types of cancer. All from the comfort of home.

But before you skip your annual physical in favor of a trip to the pharmacy, you need to ask: Do the tests really work? Are they worth your money?

The home-diagnostics market has come a long way since DIY pregnancy kits made their debut 30 years ago. Today you can get a wide range of tests from storefront and online drugstores.

Some give immediate results; others require you to take a sample of blood, urine or cheek cells, then mail it to a lab for analysis. Many are approved for safety and effectiveness by the FDA (but before you buy, check at the Web site). All aim to do one of the following:

Diagnose Generally $10 to $90, tests in this category detect illness or a medical condition, such as HIV or pregnancy.

Monitor Typically $20 to $150 and often covered by insurance, these are designed to track diagnosed conditions. Example: glucometers for diabetics.

Screen These aim to determine how susceptible you are to a condition. Among them: genetic tests, which sell for up to $1,200 and claim links between DNA and disease.

Those for ovarian and breast cancer genes have the support of some medical experts. Many other gene tests, however, are said to lack scientific value.

As long as the test has the FDA's okay and you follow the directions, the results should be reliable. But there are other risks:

Lack of complete analysis Consider these examples: Home prostate cancer tests detect a specific antigen, only one potential marker of the disease. A cholesterol test that sells for $12 reveals "total cholesterol level" but not the equally important LDL (bad cholesterol) and HDL (good). It's like getting half a weather report - you learn the temperature but nothing about the chance of rain.

For the price of a co-pay, you'd get the full workup from your doc, plus his advice.

Chance of a misread Tests are only as good as their interpreter. "If used without consulting a physician, the chance of them being misunderstood goes up," says University of Maryland public policy professor Robert Sprinkle, M.D., Ph.D.

Lack of a follow-up Even if a doctor delivers your result (as with many HIV kits), he can't prescribe treatment if you're not his patient.

So then what? Treat yourself? Or worse, delay needed care?

So when should you consider using home medical tests? They make sense if...

You're looking for an early read Home tests can give you quick, convenient answers about specific medical conditions from infections to infertility as soon as the first symptoms appear or even before. That way you can get early treatment if needed. Remember, though: The tests are just a preliminary check. See your doctor so she can make the diagnosis and come up with a medical course of action.

You need to monitor an existing condition Glucose kits, blood-pressure monitors and similar devices can serve as a valuable bridge between doctor visits for people with chronic conditions like diabetes and heart disease.

For someone who's just started to take medication (a statin for high cholesterol, say), interim home testing can help evaluate how well the drug is working. Your doctor will likely welcome the additional readings. Just make sure you let her write the prescriptions.

Curtis Pesmen is the author of The Colon Cancer Survivors' Guide.