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Medicine Math Stats

Another Dementia Test Oversold 24

An anonymous reader writes: Many prominent news organizations, including the BBC, are reporting on a study (PDF) that claims a new blood test is 87% accurate in predicting which patients will develop cognitive impairment. It's hailed as a major step forward in efforts to fight dementia and Alzheimer's disease. Unfortunately, reality isn't quite so impressive. An article at MedPage Today explains all of the statistical facts that the mainstream press glosses over: "Only about 10% of patients of patients with MCI convert to clinical dementia per year. With nearly 30% of positive results false (remember, the specificity was 71%) as well as 15% of negative results false, most of the positive results in such a group will be false. Yes, it's time once again for a tutorial in positive predictive values. If we have 100 MCI patients and a 10% conversion rate, then 10 of them will develop dementia. These are the true positives. There will be 90 true negatives — the ones who don't convert. But with a specificity of 71%, the test will falsely identify 29% of the 90 true negatives, or 26, as positive. Meanwhile, with a false negative rate of 15%, only nine (rounding up from 8.5) of the 10 true positives will be correctly identified. ... It's easy to get a high negative predictive value when the annual event rate is 10%. If I simply predict that no one will convert, I'll be right 90% of the time."
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Another Dementia Test Oversold

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  • by Anonymous Coward

    All the mainstream articles I've read, whilst as usual glossing over the stats, are presenting this as a vast improvement, not as something that should be rolled out clinically. Didn't read it as hype.

    Is it a major step towards a viable test, or is that just not true?

    • by sjames ( 1099 )

      The test looks for a set of 10 proteins in the blood. I suspect that part is fairly reliable which means the theory that those proteins are markers for Alzheimer's is probably incorrect or incomplete.

      That leans towards just not true.

      • The test looks for a set of 10 proteins in the blood. I suspect that part is fairly reliable which means the theory that those proteins are markers for Alzheimer's is probably incorrect or incomplete.

        Whether or not those 10 proteins are markers for Alzheimer's is not the issue at all. When you look for markers, the search goes this way: disease => chemicals. But when you start testing patients for markers, you're actually using the inverse of that relationship: chemicals => disease. If the disease is rare, false positives will vastly outnumber the number of patients really afflicted by the disease even though you test for the right markers.

        There are 3 important percentages for disease tests:
        - How

  • This is a generic problem with media reports of medical results. No understanding of the statistics.

    Add in the lack of repeatability of many studies (for example just about anything claiming bad effects from low doses of bisphenol-A has proven to not be repeatable) and outright fraud (Wakefield) or attempts to push the data using questionable statistics (Séralini) and you have a giant problem with media and medical research results.

  • "Thank God!" (Score:1, Flamebait)

    Said Thad Cochran [washingtonexaminer.com].
  • Hey, the condescending article manages to not know what positive predictive values are!

  • These sales people are demented, and I have the tests to prove it.

  • A while back, a friend of mine went to the doctor complaining of fatigue. He was just constantly tired and unsure why. They checked him into the hospital for tests, and shortly declared he had Leukemia and needed to start chemotherapy right away. After several weeks, including a long stay in the ICU, the doctors declared the treatment a "success". He remained in the ICU and died yesterday.

    Fucking quacks "practicing" medicine and performing "treatments" that have a proven low survival rate - but, hey, ev

  • There are three kind of lies: lies, damn lies and statistics.

God made the integers; all else is the work of Man. -- Kronecker