Screening for Disease, Part 2
Talk with your doctor about being appropriately screened based on your own family history and risk profile, and based on the best available evidence.
By Bob Bernstein, MD
In the last issue, we began to look at screening for disease. This article explores more about screening.
There are four kind of preventive medicine.
Primary prevention—the prevention of illness before it occurs.
An example is immunization. Get a flu shot every year, get a pneumonia shot if you have a chronic illness or you are over 65. Immunize your children, use seatbelts always, and use helmets for biking. This is true prevention; you are well, and you stay well.
Halfway between primary and secondary prevention is the PAP test. Cervical cancer was common when I was a medical student, but I have not seen a case of full-blown invasive cervical cancer in the last 25 years in my practice. An abnormal PAP actually indicates the presence of the virus that causes cervical cancer. It catches changes before they become cancer. The progression from early precancerous changes to invasive cancer is slow, and so it is possible to screen every 2-3 years and still catch all the potentially risky lesions.
Secondary prevention- identifying a disease early while it is curable.
Mammography is a good example. The intent is to find disease before it is producing symptoms (in effect, when the patient is unaware of it) and early enough to make a difference to the outcome.
Tertiary prevention—the prevention of complications and disability from established illness.
This is the work doctors do to manage acute and chronic illness—really, most of what doctors do to prolong life and improve its quality.
Quaternary prevention—the prevention of unnecessary and even harmful effects of overdiagnosis and over-treatment.
This is a relatively new type of prevention, as we have only become aware of the harmful effects of screening fairly recently. The point here is that screening is not a benign activity. There are consequences.
Murray McLauchlan, a Canadian folk singer, told his story at a concert I attended. He flies small planes, and every year, Transport Canada requires him to undergo a treadmill exercise stress test. One year, this screening test was positive. To follow up, he had coronary angiography, a procedure where a thin flexible plastic tube is threaded through his arteries until it comes to the heart’s blood supply. The good news was that his arteries were normal. The bad news was that during the procedure, the plastic tube accidentally pierced one of the arteries necessitating emergency heart surgery and a prolonged recovery. Public safety dictates that pilots have to take this kind of risk for the greater good, but one could argue that not everyone should submit to such risks. This is why we do not do angiograms as a screening test. This is why, when I have a worried patient who wants a specific test, I only do it if the conditions are right and the benefit outweighs the risk.
So what about testing for cancer? It seems that early detection ought to be a good thing, but this idea does not always hold up under the scrutiny of science. Sometimes we detect cancers early so it looks as if people live longer, but what we have done actually is make them aware earlier without influencing the actual age at which they die. Sometimes we detect cancers that will never kill you in the rest of your natural lifespan. Screening for prostate cancer in an 89 year old man is silly. By the time the cancer could kill him, he would have long since died of other natural causes. So would you want to know about this cancer? I wouldn’t.
A new Canadian study compared women who got yearly mammograms for five years to women who didn’t, and followed them over a 25-year period. The overall death rate was the same in both groups! How could that be? There are some problems with this study, but one possibility is that mammography does save some women but the unnecessary biopsies and surgeries also kill an equal number so it balances. Women who had a mammogram and subsequent successful treatment will always say that the mammogram saved their lives, but those who died of complications from surgery or from a cancer that was not curable never get to tell their stories. Stories are not science. And this one study is not the be-all and endall either. The process of science is to test theories, evaluate all the facts, and look for reproducibility. “Science,” said Thomas Henry Huxley, “is the assassination of beautiful theories by ugly facts.” We haven’t assassinated this one yet… mammograms are still recommended.
Any screening interval that is long enough to be practical detects mostly cancers that are slow- growing and miss bad ones that come up quickly between the screens. The only effective antidote is to look for the root causes of cancers and work on prevention, for example, smoking in lung cancer.
Thus, early detection is more complex than it seems at first glance. When I want to know whether a screening procedure is actually valid, I listen to the doctors in the system who have no bone to pick, no agenda to push, and who do not benefit financially from the tests and treatments: the epidemiologists. Keep talking with your doctor about being appropriately screened based on your own family history and risk profile, and based on the best available evidence.
Dr. Bob Bernstein, Bridgepoint Family Health Team, is Professor of Family Medicine, University of Toronto