More and more americans are playing doctor in the privacy of their own bathrooms, using a few drops of blood or a urine sample to test for cholesterol, blood glucose, or evidence of colon or rectal cancer. In fact, a snippet of a child's hair now can confirm the use of illicit drugs.

Often seen as a less expensive and a more convenient alternative to a trip to the doctor's office, self-testing diagnostic and monitoring devices are booming in sales. Devices such as blood-glucose tests and blood-pressure kits make it easier for people to self-monitor conditions such as diabetes and hypertension. However, this technology-driven trend is not without limits and could result in serious problems for those who rely on the tests instead of on the expertise of their health-care provider. A recent shift in the home diagnostics market--from monitoring chronic illnesses to diagnosing serious or potentially fatal diseases--is raising red flags among health professionals.For years, pregnancy tests and ovulation predictors dominated the home test kit market. While these devices still generate large numbers of self-care sales, other tools of the medical trade are fast becoming available outside the doctor's office--no prescription needed. Spiraling health-care costs, increased interest in preventive health care, and a desire for privacy are paving the way for products that now include screening for the virus that causes AIDS and for drugs of abuse.

Screening tests often are used at home to check for symptoms of a disease when they may not be readily apparent. For example, people can measure their cholesterol and triglyceride levels--two types of fats in the blood--to help minimize the risk of cardiovascular disease.  Home Diagnostic