Criteria for Screening Tests
Steps Necessary for Widespread Disease Detection
Certain diseases do not have screening tests in place, even those that should. The explanation for this requires an understanding of what makes a good screening test.
In October of 2006, the media provided coverage of what sounded like a significant clinical study. A trial called ELCAP presented results showing that screening for lung cancer with CT scans of the chest could save lives. It is a big deal when one considers that lung cancer is a widespread killer. If screening for cervical cancer is possible, why not lung cancer?
There is more to it than just how many people have the disease. Other factors have to be considered. In fact, in order for any screening test to be widely available, a series of criteria called the Wilson and Jungner criteria have to be met.
The Disease
In order to catch a disease early and treat it sooner, the disease must have a period of being detectable before the symptoms start. For example, colon cancer has no symptoms early on, yet there is a tumor that one cannot see until the patient has a flexible sigmoidoscopy or colonoscopy. One cannot screen for a disease that appears quickly, like a cold. Even if the disease is screenable, it must also be common enough to be worth screening. Lung cancer is a great example of this. Screening for a less common condition, like stomach cancer in the United States, would feel less purposeful.
The Test
The screening test itself must be a good one. It must be sensitive so that people with the disease would correctly test positive, or else those with the disease are incorrectly tested as not having it (false negatives). It must also be specific so that those without the disease would correctly test negative, or else individuals without the disease would test positive because of some other cause (false positive). Ideally, the test would pick up all diseased individuals and be positive because of that disease only, but in reality, screening tests are acceptable if they do the job well enough. They must also be cheap, easy, reliable, and as painless as possible.
The Treatment
Even if one can detect the disease well, the ultimate purpose is to treat it. If a disease can be screened but its treatment is hard to accept, like a life-changing surgery for prostate cancer, then it makes screening for it less acceptable. At the same time, the disease may be worth screening if there is evidence of benefit outweighing the risks. In the medical community, an endpoint that is typically considered is mortality benefit. If a group of people who are screened and then treated live longer than a group of people who are not screened, then the screening test may be great for the public. Otherwise, if the two groups of people die at the same rate, there is no point in screening for the disease.
The Policy
The last step for implementing a screening test is health policy. Operating a population screening program is costly. There must be enough funding for testing materials and treatment of the disease. Other issues, including the necessary facilities, whom to screen, and how often to screen, need to be worked out as well. If a cost-effective plan can be developed, then the screening test can become widely available.
Examples of Screening
Going back to lung cancer, it is true that this disease is widespread and worth screening. The problem is the test itself. The CT scan study has been criticized for not having a control group to compare the results with. Basically, there is still no clear evidence of any lung cancer screening test that can ultimately provide a survival benefit. That includes chest x-rays, a modality that had been studied decades ago. Therefore, the search for a good lung cancer screening test continues.
Meanwhile, other screening tests have come along because they fit most of the above criteria. There is the Pap smear for cervical cancer, blood tests for diabetes and cholesterol that cause atherosclerosis, and breast exam plus mammogram for breast cancer, just to name a few. As medicine advances, new means for disease prevention may become available, but only time will tell.
References
- Henschke, Claudia I., et al. "Survival of Patients with Stage I Lung Cancer Detected on CT Screening." The New England Journal of Medicine 355.17 (October 26, 2006): 1763-71.
- World Health Organization - Principles and Practice of Screening for Disease
