Probable vs. Possible

This is for my premed and med student friends in reaction to the following article After a verdict: Doctors need to be taught a lesson. Really?.

Diagnosis is dependent on testing, and every test has some mix of sensitivity, specificity, positive predictive value and negative predictive value. Each of the following are tests, and none of them are perfect:

Verbal – “Does your sore elbow suggest fracture?”
Visual – “Does that lesion look infected?”
Digital – “Might that bump on your prostate suggest cancer?”
Blood – “Does your elevated troponin suggest heart attack?”
Radiographic – “Is there infection on chest X-ray?”

Sensitivity: Of everyone who has a problem like a pulmonary embolus (PE), how many actually test positive with the test? For example, the d-dimer is a blood test that is super sensitive for the presence of PE. If you have a PE, you’ll probably test positive.

Specificity: Of everyone who doesn’t have a problem like a PE, how many actually test negative? The d-dimer is not very specific. In other words, many patients with no PE have elevated d-dimers. These are called “false positives.” False positive d-dimers point us toward more sensitive and specific chest CTs. However, these deliver radiation and are costly. In addition, they are highly sensitive for “incidentalomas.”

ProbableVsPossiblePositive predictive value: Of everyone who tests positive, how many actually have the problem or disease? (Read closely because this is not sensitivity.) The positive predictive value of the d-dimer isn’t very good, because many without PEs test positive.

Negative predictive value: Of everyone who tests negative, how many actually have the problem or disease? This is what makes the d-dimer nice. If it comes back negative, we can rest easy that the possibility for PE is very low.

We think of every test in this way, consciously or not. Looping back to the delayed MRI mentioned in the above article, why wouldn’t Kaiser make MRIs available to any patient whenever they may request? The answer relates to both expense and hypersensitivity of the test. If expense is the only factor, then that’s worthy of outrage.

But because MRIs are so sensitive, they unearth incidentalomas like crazy, which lead to further evaluation, additional follow up, risk, anxiety and expense. As such, for any group like Kaiser, decisions have to be made for how long to make people wait.

Kaiser’s doctor made a choice to postpone an MRI for 3 months in part to save expense, but also to participate within a system that hopefully strives to protect other Kaiser patients from needless workups. Kaiser may move their bar down to 2 months now, which will lead to more false positives, additional workups and expense.

For all tests, choices are made for when to perform them, and where to draw lines between positive and negative.

… Or maybe instead of economics and science at Kaiser, it’s truly good vs. evil as such a verdict and penalty would suggest. I’ve never worked for them.

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