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Letter: Conventional wisdom vs. truth

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A recent health news report stated that a new blood test had been developed and had a better than 90 percent accuracy rate in determining if one has Alzheimer’s disease. Current diagnostic tests are complex and costly, so this appears to be good news. Conventional wisdom would lead most of us to believe that 90 percent or better accuracy is a good test. But conventional wisdom is not necessarily truth. 
Mathematical reasoning (math is not just numbers and equations, it’s just usually taught that way) needs to be applied to the data. The national Alzheimer’s organization states that there are 5.2 million persons in the U.S. who have the disease, which is 1.6 percent of our 316 million population. From the foregoing data it can be calculated that if you do have a positive blood test, the chances that you really have the disease is 23.6 percent, or less than one in four. It is not 95 percent.One can empathize with medical professionals who are faced with making decisions on the basis of a single test outcome. The rest of us must deal with the anxiety and the cost.As a final thought, if the test is really only 90 percent accurate, the likelihood you have Alzheimer’s based on this blood test being positive is 12.8 percent or about one in eight.Roy FlatenAtwater
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