Doctors, medical organizations and the government can claim that healthy population medical screenings for various cancers are of value even if the screening test turns out negative. Of course, the value also depends on how many true positive cases of cancer are turned up and whether early detection plays any role in the overall cure of the cancer. What these medical groups don’t tell the public, because often those advising don’t really know, is what is the frequency of false positive results (detecting a “cancer” when there really is none present)and the harm produced as part of further testing or removal of what was thought to be a cancer but was not.
An advance in the knowledge regarding the frequency of false positives picked up in population screening tests and the subsequent harm is now available as an article “Cumulative Incidence of False-Positive Results in Repeated, Multimodal Cancer Screening” by Jennifer Miller Croswell, M.D. and a host of others published in the Annals of Family Medicine
You must read the entire article to fully understand how the data was obtained and its significance, but to get started, here is the Abstract of the article.
PURPOSE Multiple cancer screening tests have been advocated for the general population; however, clinicians and patients are not always well-informed of screening burdens. We sought to determine the cumulative risk of a false-positive screening result and the resulting risk of a diagnostic procedure for an individual participating in a multimodal cancer screening program.
METHODS Data were analyzed from the intervention arm of the ongoing Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial, a randomized controlled trial to determine the effects of prostate, lung, colorectal, and ovarian cancer screening on disease-specific mortality. The 68,436 participants, aged 55 to 74 years, were randomized to screening or usual care. Women received serial serum tests to detect cancer antigen 125 (CA-125), transvaginal sonograms, posteroanterior-view chest radiographs, and flexible sigmoidoscopies. Men received serial chest radiographs, flexible sigmoidoscopies, digital rectal examinations, and serum prostate-specific antigen tests. Fourteen screening examinations for each sex were possible during the 3-year screening period.
RESULTS After 14 tests, the cumulative risk of having at least 1 false-positive screening test is 60.4% (95% CI, 59.8%–61.0%) for men, and 48.8% (95% CI, 48.1%–49.4%) for women. The cumulative risk after 14 tests of undergoing an invasive diagnostic procedure prompted by a false-positive test is 28.5% (CI, 27.8%–29.3%) for men and 22.1% (95% CI, 21.4%–22.7%) for women.
CONCLUSIONS For an individual in a multimodal cancer screening trial, the risk of a false-positive finding is about 50% or greater by the 14th test. Physicians should educate patients about the likelihood of false positives and resulting diagnostic interventions when counseling about cancer screening.
With more and more publicity regarding healthy population cancer screening, particularly regarding its importance and value in early cancer detection and with more commercial patient requested imaging tests, there has been some concerns in the literature about all this. Specifically there are issues involving the safety of the screening test itself (for example, if X-ray procedures are used) or invasive screening exams (such as sigmoidoscopy or colonoscopy). There is also concern about the results of the screening. Are there a large number of false positives as outcome of the screening which lead to additional testing or more invasive procedures with their own risks? In addition, do these false positives lead to unnecessary financial costs of medical care? Is there a benefit for early detection in terms of cancer cure which trumps all the costs and harms which healthy population screening can develop? Answers to these questions within the education the public receives about the need for screening is essential if the public is to accept the screening under ethically and legally appropriate informed consent. The question for doctors, medical organizations and the government is “do we really know and understand what we are advising the public?” ..Maurice.
Graphic: "False Results" created by me using ArtRage and Picasa 3.