So when a patient known to have a large abdominal aortic aneurysm presented to hospital with severe abdominal pain, everyone thought he had a leaking aneurysm. When his emergency CT scan showed no leak the doctors stopped worrying, but this is in fact what the patient had. When a patient in atrial fibrillation presented with severe central abdominal pain, everyone thought he had an ischaemic bowel. When his lactate was normal, the surgical resident decided that could not be the diagnosis, but it was.
Teaching the correct use and interpretation of diagnostic tests, the fundamental starting point of pre-test probability (our judgement based on the history and physical examination findings) and how tests shift our thinking, but sometimes by less than we think, is a vital aspect of clinical reasoning and fundamental to safe clinical care.