“Doctor, I really want to stay healthy and I just got a big promotion/had a baby/had a grandchild, so I really don’t want to end up with some horrible illness. Please test me for everything.”

Primary care doctors hear requests like this all the time. It’s an impossible request to fulfill because it assumes two premises that are usually false. It assumes that we have a test for all illnesses, and that being diagnosed early with a dreaded illness makes a difference.

Monday’s NY Times published a terrific op-ed about the myth of early diagnosis. I highly recommend it. It’s brilliant and short, and the rest of my post will make a lot more sense if you read the op-ed first. Go ahead. I’ll wait.


I hope you found that illuminating, and I assume you also found it counterintuitive. That’s because for over a generation we have seen doctors on TV dramas shake their heads in sorrow and say “If only we had caught it earlier”. We have also been urged to get tested for the very few diseases in which early diagnosis makes a difference. For example high cholesterol and high blood pressure cause no symptoms, but detecting and treating them prevent strokes and heart attacks. So we assume that most other diseases work the same way – catch them early, before they cause symptoms, and you’ll have a better outcome.

But it just isn’t so. We’ve proven that screening for breast cancer and colon cancer saves lives, but for the vast majority of diseases, early diagnosis makes absolutely no difference in outcomes. So if I’m going to get lymphoma or lupus or pernicious anemia or myriad other illnesses, there’s absolutely no reason for me to do a thing about it until I feel sick. Even writing this feels sacrilegious because we are constantly inundated with messages that being proactive is praiseworthy. But in terms of health, being proactive means exercising, getting enough sleep, maintaining a normal weight, and abstaining from unhealthy habits like drinking too much or smoking. Add to that a handful of tests for the diseases in which testing helps, and you just can’t get more proactive.

It doesn’t make sense, does it?

There are actually two reasons that screening for many diseases doesn’t help. (Remember, screening means testing for an illness in someone with no symptoms or signs of the illness.)

The first reason is just that the best treatments we have for many illnesses work the same whether the illness is diagnosed before or after it starts causing symptoms. Why test everyone for a disease that only a few people have if those few people would do as well if they just waited until they got sick? If you’re going to get leukemia, catching it early won’t help. Some leukemias are cured, and some aren’t, but it doesn’t much matter when the diagnosis is made. So it makes sense to diagnose leukemia after it makes people sick.

The second reason has to do with the harms done by testing errors.

To explain this, indulge me in a little thought experiment. Let’s pretend there’s a disease called RBD (Rare Bad Disease) that is curable if caught before symptoms start, but is rapidly fatal otherwise. But it’s rare; only one in 10,000 people has it. That sounds like a perfect opportunity for screening, right? If we just test everybody then we can cure the ones with RBD. Now the treatment must be either expensive or dangerous, because otherwise it would be simpler to just treat everyone. (That’s why we just add folic acid to flour rather than test everyone for folic acid deficiency. It’s easier and safer to treat everyone in that case.) So let’s assume that the treatment of RBD if given to a person without RBD has a one percent fatal complication rate. And let’s also imagine that we have a test for RBD that is 99% accurate.

So in a city of a million people, one hundred of them have RBD and 999,900 don’t. If we test everyone in the city, because the test is inaccurate 1% of the time, one person with RBD will falsely test negative, but almost 10,000 healthy people will test positive. If we give everyone who tests positive the treatment for RBD, we’ll be treating a hundred times more healthy people than people with RBD and we’ll be killing as many people from the treatment as we’re saving. Better to forget the screening.

Are people in real life actually harmed by screening tests? Absolutely. Primary care doctors have all seen many patients go through unnecessary angiograms because of falsely-positive screening stress tests, unnecessary biopsies because their whole-body CT scan found some benign lumps, unnecessary sleepless nights because unproven blood tests suggested cancer that wasn’t there. The number of patients actually helped from these tests is much smaller, and the peace of mind that patients have when such tests are normal is entirely illusory. They could still develop leukemia or be hit by a truck the next day.

So keep yourself healthy. And whatever you do, don’t get tested for everything.

Learn more:

If You Feel O.K., Maybe You Are O.K. (NY Times op-ed by Dr. H. Gilbert Welch)

For a wonderful review of randomness and probability which has no math, and has a section explaining the dangers of false positives even with very accurate tests, I highly recommend The Drunkard’s Walk – How Randomness Rules Our Lives by Leonard Mlodinow.

In the RBD example, above, the probability that I have RBD if I test positive is 1%, but the probability of the test being positive if I have the disease is 99%. The fact that these two numbers are not the same is very counterintuitive. We owe our understanding of these related probabilities to Thomas Bayes, an eighteenth century English mathematician and minister. Bayes’ theorem and Bayesian statistics has transformed our understanding of risk in general and medical testing in particular.