In reply: Many factors potentially contribute to error in generating a diagnostic test result, and include random pre-analytical errors arising from patient preparation and specimen collection, random errors associated with the act of measurement, and systematic errors caused by, for example, drug interference. Tested individuals may also harbour an interfering substance, such as a drug or immunoglobulin. The theoretical and practical description of these components of test error is generally well understood and documented by laboratories, and the basics of test error and diagnostic sensitivity and specificity are taught in medical schools. However, I think trying to apply probability data to a test result for a specific patient is of limited value to the treating doctor.
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