Myth busted June 2006
Busted Again! August 2006
Sometimes patients schedule annual visits to health professionals even if they don't have any symptoms, because clinicians might discover something with their specialized knowledge and technologies that enable "early detection" of illness. Doctors and advocacy organizations often encourage this screening of healthy people, in the belief it is good practice.
Unfortunately, many widely used tests are not very accurate, or they find conditions for which there is no effective treatment. At their worst, they leave patients worse off than they were before.
No clear answers
Evidence-based guidelines suggest that instead of an annual health check-up, for which there is no evidence, doctors should tailor screening to individual patient health profiles and move to "opportunistic" screening - taking the time to talk about prevention and screening when patients come see them for an acute problem. i- iv
According to some researchers, doctors should also focus screening on people who can benefit the most, provide follow-up treatment, and monitor their patients' compliance with medical recommendations. Finally, they should screen only for conditions that cause serious illness or functional difficulties, and only when an accurate test and effective treatments are available. v
Of course, no test is 100-percent accurate. If a condition is very rare in the population being screened, the false-positive rate will be high. Even with common conditions, prevalence will still be low enough to lead to many false positives. These false results cause stress and anguish for patients who do not actually have the condition. vi, vii A test that provides a false negative result is also problematic, as it can lead to complacency and a false sense of security - for example, a common urine dipstick test to detect diabetes could fail to do so in four of every five patients who have the disease. vi
Another problem with many screening tests is "lead-time bias"- the test could discover a disease before the patient feels ill, but it does not actually extend the patient's life. This early detection can artificially inflate survival time by moving up the diagnosis date, making the test appear to be useful even though mortality doesn't in fact change. viii, ix
Exhibit A: The PSA test
Early detection is often an important strategy in the fight against cancer, particularly with cancers that are aggressive and must be found early to improve the patient's odds of survival. However, one of the more widely used tests - to detect prostate cancer, a relatively slow-growing form of cancer - is quite problematic.
The prostate-specific antigen (PSA) test does not detect cancer itself - only a biopsy can do that - but rather levels of a protein produced by the prostate gland which is associated with prostate cancer. The test leads to treatment for many cases of cancer that, if left alone, would never become life-threatening.
Advocates often claim that since the PSA test was introduced, deaths from prostate cancer have dropped, but mortality rates started falling well before the PSA test could have had an effect. x- xii The test is not recommended for widespread screening of men without symptoms, largely because of its high false-positive rate. Patients receiving a false-positive result can suffer anxiety, and they could have to undergo painful and unnecessary follow-up treatments that can have severe side effects, such as impotence and incontinence. x, xi, xiii, xiv
More importantly, research to date shows that patients with prostate cancer who take the test have no better odds of surviving than patients who don't. This includes a recent study of more than 71,000 men, which found similar mortality among screened patients compared to unscreened patients. xv A Canadian study also estimated only 16 percent of tested men with prostate cancer would have their lives extended by treatment. The rest would have died of another cause before the cancer had a chance to become lethal. xvi
Exhibit B: Prenatal diagnosis of genetic abnormalities
Not all early detection strategies are about prevention. In some cases, they can instead provide advance knowledge about a medical condition that already exists. However, sometimes this information can raise a series of difficult or uncomfortable decisions for some patients.
One example is the practice of examining fetuses early in the pregnancy to provide early knowledge about birth defects and other problems. This can be accomplished through many forms of non-invasive testing, including combinations of blood test and ultrasound.
In the case of genetic abnormalities such as Down's syndrome, women considered by heath professionals to be of advanced age for childbirth (usually over age 35) are often offered invasive tests such as chorionic villus sampling in the first trimester and amniocentesis in the second trimester. xvii
The accuracy of these diagnostic tests is not in question. However, they may often raise a number of difficult decisions for mothers-to-be, including whether or not to terminate the pregnancy. Although many mothers may appreciate this information, for others this early detection may result in increased anxiety and even regret at having consented to the test. xviii
Before any specific test is put into widespread use, patients and practitioners need to consider whether it is worthwhile and accurate, and whether they would be empowered to do something with the results.
i. Prochazka AV et al. 2005. "Support of evidence-based guidelines for the annual physical examination." Archives of Internal Medicine; 165(12): 1347-1352.
ii. Oboler SK et al. 2002. "Public expectations and attitudes for annual physical examinations and testing." Annals of Internal Medicine; 136(9): 652-659.
iii. Laine C. 2002. "The annual physical examination: needless ritual or necessary routine?" Annals of Internal Medicine; 136(9): 701-703.
iv. Gordon PR and J Senf. 1999. "Is the annual complete physical examination necessary?" Archives of Internal Medicine; 159(9): 909-910.
v. Cadman D et al. 1984. "Assessing the effectiveness of community screening programs." Journal of the American Medical Association; 251(12): 1580-1585.
vi. Greenhalgh T. 1997. "How to read a paper: papers that report diagnostic or screening tests." British Medical Journal; 315(7107): 540-543.
vii. Streiner DL. 2003. "Diagnosing tests: using and misusing diagnostic and screening tests." Journal of Personality Assessment; 81(3): 209-219.
viii. Kramer BS. 2004. "The science of early detection." Urologic Oncology; 22(4): 344-347.
ix. B.C. Cancer Agency. 2005. "Screening for cancer." www.bccancer.bc.ca/HPI/CancerManagementGuidelines/ScreeningforCancer.htm#7
x. Gibbons L and C Waters. 2003. "Prostate cancer - testing, incidence, surgery and mortality." Health Reports; 14(3): 9-20.
xi. Coldman AJ et al. 2003. "Trends in prostate cancer incidence and mortality: an analysis of mortality change by screening intensity." Canadian Medical Association Journal; 168(1):31-35.
xii. Perron L et al. 2002. "PSA screening and prostate cancer mortality." Canadian Medical Association Journal; 166(5): 586-591.
xiii. U.S. Preventive Services Task Force. 2002. "Screening for prostate cancer: recommendation and rationale." Annals of Internal Medicine; 137(11): 915-916.
xiv. U.S. Preventive Services Task Force. 2002. "Screening for prostate cancer: an update of the evidence for the U.S. Preventive Services Task Force." Annals of Internal Medicine; 137(11): 917-929.
xv. Concato J et al. 2006. "The effectiveness of screening for prostate cancer: a nested case-control study." Archives of Internal Medicine. 166(1): 38-43.
xvi. McGregor M et al. 1998. "Screening for prostate cancer: estimating the magnitude of overdetection." Canadian Medical Association Journal; 159(11): 1368-1372.
xvii. Alfirevic Z et al. 2003. "Amniocentesis and chorionic villus sampling for prenatal diagnosis." The Cochrane Database of Systematic Reviews; 3(CD003252).
xviii. Green JM et al. 2004. "Psychosocial aspects of genetic screening of pregnant women and newborns: a systematic review." Health Technology Assessment; 8(33): 1-109.
Mythbusters are prepared by staff at the Canadian Health Services Research Foundation and published only after review by experts on the topic. The Foundation is an independent, not-for-profit corporation. Interests and views expressed by those who distribute this document may not reflect those of the Foundation.