A resident asks you, the attending physician in an emergency department, to see a 68 year old woman with severe pain in her chest. The pain’s location (retrosternal, radiating through to her back), quality (“tearing”), and onset (sudden, not crescendo) prompted the resident to think of acute aortic dissection along with other causes of chest pain; however, because the examination shows symmetric pulses in her arms, the resident dismisses dissection and plans no test to exclude it. You recall that pulse asymmetry may not occur in some patients with dissection, but you cannot recall the proportion. Nevertheless, you suggest that the absence of this finding should not be used to exclude dissection, and you decide with the resident to order further testing for this condition. To refresh your memory and teach the resident when to pursue this diagnosis, you seek information on how frequently patients with dissection have asymmetric pulses. In a textbook description of aortic dissection, you find only one sentence about this: “A pulse discrepancy between arms may indicate compromise of the aortic arch vessels”.¹

Clinicians who practise and teach evidence-based medicine are by now familiar with some limitations of textbooks. The usual beef with textbooks is that their treatment recommendations are out of date.² The above scenario illustrates another complaint — that textbook descriptions of disease often omit information about the frequency and temporal characteristics of clinical manifestations in patients with the disorder, even when good evidence from clinical care research exists.^3–5 When one textbook came up short, we looked at several others; the table …