Diagnostic Tests — Sensitivity, Specificity, Predictive Values, Likelihood Ratios & ROC

Meet the patient — the D-dimer that came back positive

A 55-year-old smoker presents to the emergency department with pleuritic chest pain and dyspnoea. The Wells score puts her at moderate probability of pulmonary embolism. The D-dimer comes back positive at 850 micrograms per litre. The registrar reaches for the CT pulmonary angiography request form — but is the positive D-dimer really diagnosing PE?^[1][7]

It is not. The D-dimer is a rule-out test — its power lies in the negative result, not the positive. A positive D-dimer barely shifts probability upward, because its specificity is only about 40 per cent. This single vignette contains every concept in diagnostic testing: the two-by-two table, the prevalence dependence of predictive values, and the likelihood ratio that combines the test with the clinical picture. A test result only means something when you know where you started.^[1][7]

References11ShowHide

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