Many consider routine screening almost a duty, not unlike voting. But there are disadvantages as well as advantages.
Screening can detect medical conditions before symptoms present themselves. The best-case scenario is that treatment is more effective in a disease’s early stages and perhaps lives are saved.
But there are several downsides:
- Screening people with low disease risk is expensive and needlessly clogs the medical system.
- Screening can produce anxiety, discomfort, and exposure to radiation and chemicals.
- A false positive can be stressful and lead to additional expensive tests and possibly unnecessary treatment.
- A false negative, on the other hand, creates a misleading sense of security that could delay a definitive diagnosis.
False-positive screening results can be unsettlingly high. One in 5 falsely tested positive for syphilis, according to a recent CDC study of five U.S. laboratories. This no doubt led to unnecessary treatment and untold anguish.
In a national survey, nearly 9 out of 10 adults said they believed that cancer screening is almost always a good thing, and 3 out of 4 said it saves lives. But more than one out of 3 had received a false-positive screening test, and more than 40 percent of those called it “very scary” or the “scariest time of my life.” Remarkably, 3 out of 4 said they would choose to get a full-body CT screening instead of $1,000 in cash – despite no scientific evidence of the screening’s benefit or safety.
Many credit widespread cancer screening for an increase in the five-year survival rate of cancer patients. But a comprehensive review of the 20 most common tumors from 1950 to 1995 showed that screening simply identified the cancer earlier and patients were aware of it longer. There was little effect on the death rate.
The sad truth is that the deadliest cancer sprouts and kills so quickly that routine screening rarely catches it. Cancer screening finds lots of what Dr. Gilbert Welch of Dartmouth Medical School calls pseudo-disease. That is an abnormality that meets the criteria of being cancerous but will never grow to become harmful. High-resolution MRIs and CT scans are especially good at discovering these. However, the doctor and patient often feel compelled to do something. This often leads to unnecessary treatment that can create harmful side effects.
Dr. Laura Esserman is director of the breast cancer center at the University of California, San Francisco. She was co-author of an essay in the Journal of the American Medical Association reflecting on her experience. She wrote: “After (25 years) of screening for breast and prostate cancer, conclusions are troubling: Overall cancer rates are higher, many more patients are being treated, and the absolute incidence of aggressive or later-stage disease has not been significantly decreased. Screening has had some effect, but it comes at a significant cost, including overdiagnosis, overtreatment, and complications of therapy.”
Oddly, people with advanced cancer continue to be screened long after there is any useful purpose. Up to 1 in 5 Medicare patients with poor-prognosis cancer were screened for cholesterol, prostate cancer and breast cancer. Researchers speculated that screening is such an ingrained habit that the terminal patients believed they should continue to do what they have always done regardless of their circumstances. Why health-care providers indulged them is another issue.
The number needed to treat (NNT) is a useful health statistic that measures the effectiveness of a preventive service to avoid one additional bad outcome, such as death or a heart attack. The idea is that one person would benefit by a screening or medication while everyone else would incur the expense, anxiety and potential harm of the intervention.
Welch calculated NNT for mammography to save the life of a 50-year-old woman. Five women out of 1,000 would die of breast cancer without mammography. To lower that rate to four women, 1,000 women would be screened annually for 10 years. What happens to the other 999 screened women?
- 2 to 10 women will be overdiagnosed and treated needlessly.
- 10 to 15 will learn they have breast cancer earlier than they would have otherwise, but their prognosis will not change.
- 100 to 500 women will have at least one “false alarm,” and about half of these will have a biopsy.
In another example, 53,000 men ages 55 to 74 with a history of heavy smoking received CT scans to detect early lung cancer. News reports hailed the results, saying the risk of lung cancer was reduced by 20 percent. But according to the NNT in this study, 300 would need to be screened to extend the life of one person. What about the other 299? One out of 4 had a false positive, resulting in a cascade of more tests, radiation, anxiety and biopsies. The cost of each CT scan is about $300.
An earlier study showed a false-positive rate of 33 percent for those who had two lung CT scans. Among those, 7 percent had either a biopsy or surgery, or both.