How accurate are stress tests? How many of them come out with false positives? What can cause a false positive if the heart is fine?

Fair. False positives are possible fairly frequently (maybe up to about 25-30% of the time, but this number can vary based on how likely a person is to have the disease before the test. So a positive stress test as a screen for a patient with no symptoms is more likely a false (+) than one done in someone with pain. Caused of fp are prior ekg changes, certain medications and other issues.