Doctors who earn money for cardiac stress testing are much more likely to prescribe the tests than those who don't, a new study has found. Researchers at Duke University studied data on 17,847 patients nationwide who had cardiac bypass surgery or coronary angioplasty, checking to see how often doctors prescribed nuclear stress tests and echocardiograms later than 90 days after discharge. Their results appeared Wednesday in The Journal of the American Medical Association. Among doctors who billed for administering and interpreting a stress test, 12.6% prescribed the test, compared with 5% of those who were not paid for testing. Results for echocardiograms were similar: 2.8% of patients were tested by doctors who billed for both test and interpretation, and 0.4% by those who were paid for neither.