We intuitively believe that medical diagnostic tests are more likely to banish uncertainty than they really are.
Here’s an example. Many executives have an “executive physical” that includes an exercise stress test to be sure they don’t have coronary artery disease, and many patients pay to have a CT scan to assess coronary artery calcification.
Take a 45 year old executive with a normal blood pressure and a normal cholesterol – his “pretest” probability of having a heart attack or cardiac death in the next 10 years is about 1%. Here is a link that lets you put in age, gender, blood pressure and cholesterol and calculates risk based on data from the Framingham
Imagine 1000 such executives, each with a 1% chance of coronary disease. That means that ten of them will have a heart attack or cardiac death in the next ten years. If they all had exercise stress tests, with a 70% sensitivity (chance a person with disease will have a positive) and a 90% specificity (chance a positive is a “true” positive” we would have 107 positives – but only 7 of them would be correct. The overwhelming majority of positive tests would be false positives. There would be almost 900 negatives, and of these only 3 would be “false” negatives.
This calculation of “posterior probability” is called Bayes Theorem
So – even with a positive test a patient has a low chance of serious cardiac event. A negative test is much more reliable. However, the executive had a 1% chance of heart disease before the test, while after the test the probability remains about 1/3 as high.
So – we have more data – but not much more information. Here is a graphic way to look at this:
Worst of all, there is not evidence that people with no symptoms benefit from invasive therapy to correct cardiac disease. Even the “lucky” executive whose hidden cardiac disease is discovered through this testing might not be so lucky either!
We desire more information – and insist on more diagnostic tests in a vain effort to banish uncertainty. Alas, tests done under the wrong circumstances don’t do much to diminish uncertainty. We want exercise stress tests, mammograms and prostate specific antigen tests, even when our risk of disease is low. The “cascade” of follow-up tests costs substantial sums (the point of this blog). This cascade also causes discomfort and anxiety –and sometimes real damage to patients.
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