Evidence based diagnosis: does the language reflect the theory?

Abstract

Innocent generalisations? As trainees, we can all recall hearing pearls of wisdom conveyed in the form of: “Any patient presenting with this sign/symptom is assumed to have disease X until proved otherwise.” The common mnemonic “SPin/SNout” is used to indicate that positive results from specific tests rule in disease, while negative results from sensitive tests rule out disease. One may hear sensitivity or specificity discussed in isolation (“that test is so sensitive that a negative result rules out disease”) or, more commonly, of a test having good positive or negative predictive value. Certain findings are called “non-specific” because they manifest in multiple diseases. Although this language seems to capture simple diagnostic generalisations, does it actually reflect the bayesian logic that underlies diagnostic reasoning? The accuracy of such language is easily overlooked because in common practice test results agree with clinical suspicion and the details of sensitivity, specificity, and predictive value become arguably less important.