False Positivity and False Negativity in Health Screening

Recent blogs have indicated that the PSA test should be individualized according to doctor and patient preference, in part because of the false positivity rate of 40 percent. This means that forty percent of patients who have a positive PSA test (an elevated PSA count) will go through potentially dangerous, uncomfortable or expensive secondary testing that can be invasive. Such patients will have later found that they do not in fact have prostate cancer but may simply have an enlarged prostate or nothing at all.

Potentially all tests have a false positive rate that can be as low as 1 percent or less or much higher. It occurs because other conditions can contribute to the finding of a positive test or because the cutoff rate for a positive test is set too low. In this matter, doctors have a dilemma to face. To have a high false positive rate means inherently that you have a low false negative rate.

Another term you want to pay attention to is the sensitivity of a test. It is a measure of the ability of the test to pick up a finding consistent with the disease at a certain level. Mathematically, the sensitivity equals the number of true positives divided by the summation of the true positives and the false negatives. A sensitive test won’t miss the disease but the chances of a false positive tend to be higher in such cases.

The other important thing to pay attention to is the specificity of a test. A highly specific test means that a positive test means you have disease being screened for and not some other disease. Mathematically, specificity equals the number of true negatives divided by the summation of true negatives and false positives. Specificity of a test can be as high as 100 percent and won’t define another disease accidentally as being the disease being tested for.

If a test affects five percent of the population and the test screens 90 percent of the population, that means that, out of a thousand people, 950 will be completely cleared and fifty will need a more detailed evaluation. The higher the false positive rate, the more patients will have to go through additional testing. Ideally, you want that number to be low enough to inconvenience as few people as possible, but high enough so that you don’t have too many false negatives. False negatives are a problem as well because you don’t want to miss too many people who have the disease. It’s all a fine balance that doctors and those that make health policy must consider.