The Truth Doesn't Always Need A Test: Thresholds for Medical Decisions

Jason Carr, MD didn't need a test to know what this is

Kassirer (and Pauker) got the idea for the Threshold Approach to Medical Decisions in part (as he tells it in his memoir Unanticipated Outcomes) because he had observed a pattern of irrational behavior among nephrologists (he was one) in the 1960s.  Evaluating a patient with nephrotic syndrome and knowing the diagnosis was very likely to be membranous glomerulonephritis, most nephrologists still insisted on a biopsy prior to starting corticosteroids.  Pauker and Kassirer realized that we never get to 100% certainty in medicine and that tests only modify the pre-test probability of disease.  They reasoned that there ought to be a threshold of pre-test probability of disease that is high enough to justify treatment without testing.  That is, sometimes, the pre-test probability of disease is so high as that it obviates testing, and we can just skip to treatment and avoid risky and invasive testing like a kidney biopsy.  Only if the disease fails to respond to therapy (thereby reducing the clinical pre-test probability below the treatment threshold) should we resort to the biopsy.

Almost 40 years after the publication of their article, we still are not consistently following the axioms of the threshold approach.  Here are several recent examples:

A 20-something black woman presents with cough, weight loss, massive hilar and mediastinal lymphadenopathy and interstitial parenchymal changes.  She is scheduled for a bronchoscopy and EBUS to confirm the diagnosis of sarcoidosis prior to initiating corticosteroids.

Underperforming the Market: Why Researchers are Worse than Professional Stock Pickers and A Way Out

Here's an occasional cross post from the Medical Evidence Blog.  It's a bit more technical than what is usually posted on Status Iatrogenicus, but it gives a summary description of a pervasive problem with "evidence based medicine" that is so widely applicable to practice that I think it deserves some recognition here.

Underperforming the Market: Why Researchers are Worse than Professional Stock Pickers and A Way Out

The Hits are Recorded, the Misses are Not: How the Culture of Medicine and the Third Party Payer System Foment Waste and Inefficiency

It doesn't make sense, but that's how we do it.  Usually, if you want to judge the calibration of a decision maker, you tally hits and misses both, not just the hits.  If you fail to record misses, a high hit rate might just signify a lot of swings.  But in medicine, we are taught to ignore misses.  It's only hits that count.  This doesn't make sense because it encourages overtesting and waste.  If surgeons practiced this way, all heck would break loose, everybody would be cut open and organs wantonly plucked from them.

There is a physician at a Midwestern institution who is known to come on service and order an echo bubble study on several (all?) patients the first day he is on service.  He has doubtless impressed a generation of medical students when the occasional PFO (patent foramen ovale) is found and he gets to wax prolific about shunts and other mechanisms of hypoxemia for a few minutes as they listen with fawning and rapt attention.  What they do not know is that he finds PFOs about 10% of the time he orders his echos, and this is just above the rate of PFOs in the general population.  He's basically finding PFOs at random.