Saturday, May 25, 2019

The Test is Not the Truth: One Week in the Lonely Life of a Bayesian Clinician

If there is one thing you should remember about clinical decision making it is this:  the test is not the truth.  A diagnostic test raises or decreases the prior or pre-test probability (PTP) of the disease under consideration.  The amount of increase or decrease in probability with a positive or negative test depends on the starting probability and the likelihood ratio of the test.  (LR+ = sensitivity/1-specificity; LR- = 1-sensitivity/specificity).  If we don't attend to the PTP of disease, serious diagnostic errors and therapeutic misadventures may result.  This is especially true when a low PTP disease is diagnosed on the basis of a test with poor sensitivity and specificity (and a LR not much greater than 1 or 2 or even 4 or 5).  Several examples of this came up a while back.

A woman presented with thunderclap headache and had recurrent seizures during initial evaluation.  A differential diagnosis was formulated and it included PRES (posterior reversible encephalopathy syndrome) with a PTP of about 20%.  Subarachnoid hemorrhage was excluded with CT and LP and the PTP of PRES rose to about 40% (since it occupied some of the probability space previously occupied by SAH once the latter was excluded.)  The subsequent MRI images were consistent with PRES.  Nonetheless, a vascular MRI was ordered to "exclude the possibility of cerebral vasculitis".  The problems are twofold.  First, the probability of PRES is now on the order of 70% if the sensitivity and specificity of MRI are on the order of 80%, and it is 85% if sensitivity and specificity are each 90%.  (Go ahead and plug some numbers into the calculator on the sidebar of the blog.)  This probability meets or exceeds the probability threshold to both consider the diagnosis made, and to take action based on it.  In this case inaction and supportive care are indicated.  Even if a vascular MRI were consistent with cerebral vasculitis, which has a PTP an order of magnitude or more less than PRES, the diagnosis is still PRES.  The truth is not in the test, the truth is in the rationally considered diagnostic process of which the test is one part.