Assessment and Diagnosis
What the exam tests
Assessment questions test clinical reasoning more than recall. You must generate a prioritized differential, identify red flags, choose the test that most changes management, and interpret results in the context of pretest probability. Know sensitivity (a highly sensitive test, when negative, helps rule out disease) and specificity (a highly specific test, when positive, helps rule in disease). Positive predictive value falls as prevalence falls.
Concepts to master
A focused history and physical should precede testing. Use validated decision rules-Wells, PERC, CURB-65, Centor, Ottawa ankle-to move from vague concern to probability. Understand likelihood ratios: LR+ above 10 and LR− below 0.1 meaningfully shift probability; values near 1 do not. Interpret common labs with attention to confounders: a normal white count does not exclude infection, and anemia in an older adult warrants iron studies and GI evaluation. Imaging and biopsy answer different questions; biopsy is not a screening test.
Traps
Anchoring on the first diagnosis; ordering a shotgun panel without a plan for abnormal results; treating a lab value rather than a patient; ignoring atypical presentations in older adults (delirium, falls, poor intake rather than fever); and using rule out as a diagnosis. A negative highly sensitive test is useful only when pretest probability was not already extremely high.
Worked example
A 34-year-old with pleuritic chest pain and tachycardia but no leg swelling, surgery, or estrogen use has a negative PERC, so pulmonary embolism is effectively excluded without a D-dimer. By contrast, a 68-year-old with recent hip surgery and dyspnea has high pretest probability; proceed directly to CT angiography rather than relying on D-dimer.
Rule of thumb
Test only when the result will change management, and interpret every result against pretest probability.