False Positive — what’s not there can kill you.

An x-ray, biopsy or other medical test can have a number of possible results.  The uninformed patient or the unwary doctor can be fooled by some of the possibilities.  How could a little test hurt you — it’s just a harmless x-ray or small biopsy or just a few drops of blood?  There is some underlying truth about a condition we want to know.  Do we have a problem or not, yes or no, cancer or no cancer, pregnant or not pregnant, AIDS or no AIDS and many other questions.

Most people understand if the weatherman predicts rain tomorrow, but no rain comes, we laugh because we know prediction of weather is not always accurate.  Yet we don’t apply the same common sense to medical tests.  If a test predicts a person has cancer we believe it — the sad fact is medical tests are not always accurate.  A test can predict a condition when none is present which is called a false positive.  A test can predict a condition is absent when it really is present which is called a false negative.  A test can predict a condition which turns out to be correct which is a true positive.

So how can a little test hurt?  If the test is a false positive which leads to surgery which has a complication then a person could die.  This is not just theoretical, it happens.  The problem is made worse by a tendency of health care professionals to over-diagnose disease.  For example, if a pathologist is not certain if a biopsy shows cancer the safe thing to say is “possibly cancer” rather than “I don’t know”.  The surgeon says if it is “possibly cancer” the best thing is to “take it out”.  Later, when the patient is missing some body part and no cancer is found the surgeon says “you were lucky”.  Really?  Perhaps the patient should not have had the test in the first place.

In certain circumstances the chance of getting a false positive is higher than the chance of a true positive.  A good example is finding a small spot on a chest x-ray.  Most spots on chest x-rays are not cancers.  Because of the unreliable nature of the chest  x-ray for cancer screening, routine chest x-rays are not advised.

People who study the statistics of medical testing can figure out which tests have the best chances of true results.  This is important information for patients and doctors.  Recently, the PSA test has come under criticism.  It seems the test has a high rate of false positive results and to make it worse, positive results may lead to unnecessary surgery.  In view of the statistics, experts now do not recommend PSA tests for routine screening for prostate cancer.  Hopefully, this will lead to better tests while the old PSA test heads to the history books.

Another important question is whether the result of a test will change treatment.  If not, then don’t do the test.  For example, surgery in the very elderly is dangerous.  So if no surgery would be recommended for a 100 year old person then don’t do a CT scan of the brain.  Is that mean and uncaring?  No, it is taking care not to do tests that lead to harmful procedures.

So, more information is not always better.  It is smart to avoid tests with a high false positive rate.  National guidelines do exist for many tests so search for them on the Internet and as they say “ask your doctor”.

, , , , , ,