Responding to Roy Poses et al. about pre-test probabilities... (Hi Roy...long time, no email!)

How much precision do we need in this?  Acknowledging that there is a lot of cognitive bias work that we're ignoring here, I teach something akin to what Huw was getting at - a principal utility of this knowledge is to decide whether a test will change your management of the patient. And for that, maybe we just need broad ranges of pretest probability..."pretty sure disease present", "completely unsure" and "pretty sure disease absent".  (Corresponding PTP numbers would be 80-90%, 50%, and 10-20%).  Then if the test results would move you across those ranges, you should consider ordering it.  I'm not sure clinicians (or patients) need much more precision than that.  I know that's not terribly intellectually satisfying for a bunch of people that like statistics, but an honest guess like this would be better than misapplied attempts at precision.  The question then remains whether physicians are any good at these broad brushes of prediction.  We probably aren't...

John Epling, MD, MSEd

Professor and Chair, Department of Family Medicine

SUNY Upstate Medical University 

Syracuse, NY

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