Something else, it's true that the Thallium test might be more accurate, however for most screening purposes the regular threadmill test might be sufficient. Besides, the doctor does not make his diagnosis based only in the stress test alone, he uses other tests, the history, the symptoms and the physical examination. If it's a case in which the doctor strongly suspects something wrong with the coronary arteries, he will insists in doing the Thallium test, if on the contrary, the doctor only needs a screening test for a case that doesn't look like a coronary obstruction, he will order the regular threadmill. The insurance companies have made extensive and meticulous studies to determine that the extra cost of the Thallium test does not increase sustantially the number of correct diagnoses, therefore they consider it "not cost effective". Many times, insurance companies know more about the effectiviness of tests, medicines and treatments than the medical profession, because they collect information by the millions.