Adverse Effects of Screening
At first glance screening would seem to be a good thing to do, but there are consequences to screening that carry a cost, and the potential benefits of screening need to be weighed against the risks, especially in subsets of the population that have low prevalence of disease!
There are two important down sides to screening:
- false positives, people who test positive even though they really don't have disease, an
- false negatives, people who test negative even though they really have disease.
Specifically, one needs to consider what happens to the people who had a positive screening test but turned out not to have the disease (false positives). Women between 20-30 years old can get breast cancer, but the probability is extremely low (and the sensitivity of mammography is low because younger women have denser breast tissue). Not only will the yield be low, but many of the false positives will be subjected to extreme anxiety and worry. They may also undergo invasive diagnostic tests such as needle biopsy and surgical biopsy unnecessarily. In the case of fecal blood testing for colorectal cancer, patients with positive screening tests will undergo colonoscopy, which is expensive, inconvenient, and uncomfortable, and it carries its own risks such as accidental perforation of the colon. Such complications are uncommon, but they do occur. The other problem is false negatives, who will be reassured that they don't have disease, when they really do. These hazards of screening must be considered before a screening program is undertaken.
For a very relevant look at this, see the following brief article from the New York Times on the potential harms of screening for prostate cancer. Link to the article
Over-Diagnosis
There is concern among some that there is an inordinate emphasis on early diagnosis of disease and that the increasingly aggressive pursuit of abnormalities among people without symptoms is leading to actually harm and great cost without reaping any benefits. For an interesting perspective, see the following essay, Link to "What's Making Us Sick Is an Epidemic of Diagnoses," in the New York Times by Gilbert Welch, Lisa Schwartz, and Steven Woloshin.
This is an article in the New York Times (Tara Parker-Pope: Link to "Scientists Seek to Rein In Diagnoses of Cancer") in which the problem of over-diagnosis is discussed.