Ordinary patients don’t understand about pre-test probability and positive and negative predictive values. Indeed, all physicians were taught to consider Bayesian theory when ordering diagnostic tests. This is very tough concept for patients to grasp. A critical principle of proper diagnostic testing can be summarized in a single sentence.
If an individual is unlikely to have the medical condition under consideration, then a diagnostic test that yields a positive result is likely to be a false reading.
Here is an illustration demonstrating why patients need to understand this issue. While the forthcoming example is hypothetical, I guarantee that every physician has seen very similar patients in their practices. While the patient presented here has a presumed cardiology issue, every medical specialist and primary care physician can land in the same trap. When this occurs, patients suffer.
A 30-year-old non-smoker sees me in the office with chest pain that is readily relieved with antacids. It is very unlikely to be angina, and probably represents simple heartburn. If I arrange for this person to undergo a cardiac stress test, and the result is positive, then it is much more likely that the test result is wrong than that the individual has true heart disease. This is not simply my opinion, but a conclusion based upon mathematical and statistical principles. However, try explaining this to a patient with a false positive stress test result. Despite the physician’s reassurance that the test result is erroneous, the patient will likely become anxious and remain unconvinced. Such a patient can easily slide, or be pushed, down a medical cascade that may include cardiac catheterization, or even stenting of a coronary artery that was not responsible for the patient’s symptoms, and should have been left alone.
The key is that diagnostic tests need to be ordered when the patient has a reasonable chance of having the condition under consideration. (If the physician is nearly certain of the disease, then the test may not be needed.) This determination is made on the basis of a careful history and physical examination. When stress tests and various scans are ordered casually by physicians, or requested by patients, then this opens a pathway into a medical labyrinth with no easy way out. Would you prefer to agonize over a false positive test result that pushes you toward medical quicksand, or avoid an unnecessary test in the first place?
Of course, there are rare individuals who have benefited from a scan that was ordered for the wrong reasons. These folks understandably are convinced that the scan saved their lives. These anecdotes, however, which make for potent testimonials, should not change established medical diagnostic principles. Every day, folks become millionaires after purchasing winning lottery tickets. Since nearly 100% of lottery tickets become bookmarks or end up in landfills, we know that this is a poor strategy to accumulate wealth. Should every person undergo a CAT scan of the head every year because it is theoretically possible that a few might benefit by accident? Dumb luck should not be our diagnostic touchstone.
Total body scans, and all of their cousins, are examples of medicine at its worst. It is a commercial enterprise that bypasses sound medical principles and judgment. These entrepreneurs proffer a promise that they know they cannot fulfill. It’s a scam clad in a white coat. For the majority of their unsuspecting customers, a positive result will be wrong and a negative result will guarantee nothing. I realize that an ordinary patient will celebrate when his total body scan is negative, but this is not how medicine works. You can have a normal EKG performed weekly, but this will not prevent a heart attack or exclude significant coronary artery disease.
Still thinking about that cardiac scan being advertised in the newspaper or on television? Do yourself a favor. Buy some snake oil instead. The result will be the same, but you won’t waste nearly as much money and you won’t end up with a stent.
Many patients who have endured a ride on the medical cascade may feel that they were rescued from certain disaster. I'd rather rescue folks from the cascade.