Editor's note: Dr. H. Gilbert Welch, M.P.H., is a professor of medicine at the Dartmouth Institute of Health Policy & Clinical Practice and the author of "Overdiagnosed: Making People Sick in the Pursuit of Health" (Beacon Press 2011).
(CNN) -- A simple blood test. It's able to detect minute quantities of cancer cells that might be circulating in your bloodstream.
It's reported to be able to detect a single cell. It's intended to allow cancer patients to start treatment much earlier.
It's supposed to save lives. It's a cancer breakthrough.
But it's not that simple. The test could just as easily start a cancer epidemic.
We've seen it before. Twenty years ago another simple blood test was introduced. Twenty years later over 1 million Americans had been treated for a cancer that was never going to bother them.
The test was the PSA. It is able to detect minute quantities of prostate specific antigen -- minute as in one-billionth of gram. Turned out a lot of men had "abnormal" PSAs. Many were found to have microscopic cancers, far more than would ever suffer from prostate cancer.
They were overdiagnosed.
Does it matter? Absolutely. Most were treated with either radical surgery or radiation. Roughly a third suffered side-effects of treatment -- generally related to bowel, bladder or sexual function. And a few have died from it.
Things are rarely simple in modern medicine. And while most think about complexities related to treatment, some of the most challenging relate to diagnosis.
The conventional wisdom is people either have a disease or they do not. But, in fact, there are a lot of people somewhere in between. As we doctors can see more on X-ray images -- like shadows as small as the head of a pin -- and can measure things that were previously unmeasureable -- like a single cancer cell in the blood -- we diagnose more of these in-between people as being sick.
You might think we doctors would want to find all the in-between people with any evidence of cancer. That's probably because you think of cancer as a disease that invariably leads to death if not treated. That's what we were taught in medical school.
But our understanding of cancer is changing. As doctors increasingly try to find cancer early, before it causes symptoms, it has become clear the word "cancer" encompasses a broad array of cellular abnormalities: Some grow extremely rapidly, others do so more slowly, others stop growing completely and some even regress.
It also has become clear that an increasing number of our early cancer diagnoses -- particularly those involving the breast, prostate, thyroid, kidney and skin -- are made in people who are not destined to ever have symptoms or die from their cancer.
They are overdiagnosed.
The problem goes well beyond cancer. The conventional wisdom is that the more diagnoses doctors make, the more healthy we will all be. Not surprisingly, Americans get more diagnoses today than we did in the past. In fact, we are in the midst of an epidemic of diagnosis.
Most assume there are no downsides to looking for things to be wrong. But the truth is that early diagnosis is a double-edged sword. While it has the potential to help some, it always has a hidden side-effect: overdiagnosis, the detection of abnormalities that are not destined to ever bother people in their lifetime.
Becoming a patient unnecessarily has real human costs. There's the anxiety of being told you are somehow not healthy. There's the problem that getting a diagnosis may affect your ability to get health insurance. There are the headaches of renewing prescriptions, scheduling appointments and keeping them. Finally, there are the physical harms of treatments that cannot help (because there is nothing to fix): drug side-effects, surgical complications and even death. Not to mention it can bankrupt you.
Americans don't need more diagnoses, they need the right diagnoses.
I don't know whether this test will help some patients. It might, but it will take years to figure that out.
But I do know that the test will lead more people to be told they have cancer (or that their cancer has returned). That will lead more people to receive cancer treatment. Because these new patients are bound to be less severely ill (if they are ill at all), they will appear to do better. Many will assume that their doing better is because of the new test and early treatment. So the test will be performed more often. And a lot of money will be made along the way.
Ironically, what this test might actually teach us is that it's not that unusual for healthy people to have an occasional cancer cell in their blood.
The opinions expressed in this commentary are solely those of H. Gilbert Welch.