Like everyone else, I've been trying to digest this news. The fuss is based on a limitation that arises with all screening tests: Whenever you have a low incidence of disease in a population (as you have for breast cancer in younger women), it's hard to come up with a good screening tool that will catch all of the positive cases (high sensitivity), and pass on all the negative cases (high specificity). As you get a higher and higher natural incidence of disease in a population (as we have for breast cancer in older women), screening outcomes look better. The extreme example would be a disease that occurs in nearly 100% of the population. If you had a remarkably dumb screening test that called everyone positive, it would seldom be wrong for a population in which just about everyone has the disease.
When your screening test is flawed (as most are), it's always tough to draw a line in the population between those who benefit from the test and those who are harmed by the test.
The problem with mammographic screening is especially difficult because mammography is a complex, interpreted test. I'll explain what this means further on in this blog, but the upshot is that some labs can do mammographic breast cancer screeing much better than other labs. The high-performing labs may produce results that would prove highly beneficial to women in the 40-50 age range. The low-performing labs skew the national data and lead statisticians to think that screening is bad for this group of women, when the truth may be that only "bad" screening is bad.
A complex test is a test where lots of things can go wrong in the preparation of the test output. Was the patient positioned properly? Was the mammogram machine working properly and was it well-calibrated? Did the lab use the best possible mammographic equipment? Were the prior tests on the same patient made available for review and comparison with the current test? Was a proper history taken, to ensure that that radiologist had all the information needed to render the best possible diagnosis for the patient?
An interpreted test is one in which the output (the mammogram) needs to be rendered into a diagnosis. All interpreted tests can be misinterpreted. Some laboratories do a much better job at interpretation than others. The best labs have radiologists who are highly trained to diagnose mammograms, and who look at many mammograms, routinely. The radiologists should review the patient's prior mammograms, when relevant, and should read the relevant sections of the patient's history and physical examination. When a radiologist has a tough case, he/she should have a way of getting help from another radiologist. A good lab has records of these kinds of consultations, and can prove that they seek consultation on a reasonable number of cases.
Good labs have a system of quality controls over every aspect of the mammographic tests, and have a way of reviewing outcomes, so that a false negative or a false positive finding from the lab can be discussed by all of the laboratory personnel. In other words, does the lab have a method of knowing when they have made a mistake, and does the lab have a way of learning from the mistake?
Most importantly, a lab must be able to prove that it is a good lab. It should have a way of conducting quality checks on the diagnoses that come from the lab, comparing the different radiologists in the lab, and comparing their lab against other labs.
This is just a generalization, but my experience has been that there are vast differences in quality among screening laboratories. In the realm of my field (pathology), it has been shown again and again that large, high-volume labs tend to do much better with complex tests than labs that do only occasional testing.
So, the question that I have about mammographic screening is: has anyone determined whether there are ANY labs for which screening in the 40-50 year range is beneficial?
- © 2009 Jules J. Berman, Ph.D., M.D.
key words: mammographic, laboratory testing, screening test, mammography, breast examination, breast cancer, breast cancer screening, new recommendations, adverse effects of breast cancer screening, early cancer screening, cancer death rate, screening mammogram, screening mammography, new guidelines, new recommendations, task force recommendations
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