CT angiography: A scambuster update
Published Dec 4, 2008
Not long ago, I recommended you avoid CT angiography as a screening test for heart disease unless you were absolutely convinced that it was necessary and not just being ordered either because it’s a new, extremely cool, technology, or because your doctor has an equity position in the machine (meaning he or she or the practice profits every time the test is run because they own or lease the equipment). I noted that as a screening test for people without symptoms of heart disease, CT angiography has yet to be proven effective. Moreover, even though the test is quite short, there is significant radiation exposure from the X-rays used to obtain the high-tech images (orders of magnitude greater than that used in a simple chest X-ray).But let’s say that you actually have symptoms of coronary artery disease. Is CT angiography an appropriate diagnostic test for you? Since it’s faster and less invasive, can it be done instead of a conventional angiogram? Not according to a new study, just published in the November 27, 2008 issue of the New England Journal of Medicine, which has found that CT angiography is less accurate than conventional angiography. The study’s authors concluded that because of this lower accuracy, “…CT angiography cannot replace conventional coronary angiographyat present.”
The study was done in 291 people with symptoms of coronary artery disease who would have been recommended to undergo conventional angiography, which is an X-ray examination using a catheter inserted into an artery in the groin and passed upwards to the heart. Once at the heart, a dye is injected into each coronary artery and the blood flow is examined using X-ray technology. In the study, however, each patient had CT angiography first, which was then followed by conventional angiography. The results of the CT scan were then compared to those of the definitive conventional exam. While the CT angiogram did pretty well, it fell short of conventional angiography. The CT test showed a 91 percent positive predictive value and an 83 percent negative predictive value. This means that if the test showed an obstruction then nine times out of 10 there really would be an obstruction present and if the test showed no obstructions then eight times out of 10 there really would be no obstruction. Yes, that’s a bit confusing, but suffice it to say that the closer to 100 percent both numbers are the better and that the values obtained (91 and 83) are just not high enough to routinely recommend CT angiography instead of conventional angiography.
Compounding the lower accuracy of CT angiography is the much higher level of radiation exposure required by this test compared to the conventional angiogram. If CT angiography were to have been found equal in accuracy to conventional angiography, you would still have to weigh the risks of the extra radiation received. Radiation exposure increases one’s risk of cancer and one study in the New England Journal of Medicine estimated that 1.5 – 2 percent of all cancers in the U.S. are caused by CT scans (that includes all CT scans not just CT angiography). CT scans should only be used when they are clearly indicated to be the best diagnostic test available.
It is likely that in the future, CT angiography will be more accurate than it is today because of improvements in technology. It is also likely that the amount of radiation exposure will decrease. The companies that make these units have a huge incentive to make them better and safer. But until that time, CT angiography cannot be recommended either for routine screening of people who have no symptoms of coronary artery disease or for diagnostic testing of those that do.