As doctors try to stem abuse of prescription drugs, they are relying more on sophisticated urine-screening tests to learn which drugs patients are taking and which ones they're not. The New York Times says the result has been a boom in profits for diagnostic testing laboratories. This year, sales are expected to reach $2 billion, up from $800 million in 1990, says the Frost & Sullivan consulting firm. The tests have prompted vexing questions about what doctors should do with information they obtain, about the accuracy of urine screens, and about whether some companies and doctors are financially exploiting the testing boom. The tests show that many pain patients do not take prescribed drugs or are taking substances not given them by a doctor. A new study of 800 pain patients at a Veterans Affairs unit in North Carolina found that one-quarter tested negatively in a urine-screen test for a drug they had been prescribed, while 20 percent tested positively for an illicit drug or a narcotic painkiller that was not prescribed.