The spectrum of patients strongly influences the usefulness of diagnostic tests for pneumonia

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Abstract
Objective - To study the influence of the spectrum of patients on the usefulness of five clinical cues, »very annoying dyspnoea«, »strong lateral chest pain«, crackles, C-reactive protein analysis, and erythrocyte sedimentation rate in the diagnosis of pneumonia. Design - Evaluating the diagnostic properties of the cues against radiographic pneumonia at four steps in the diagnostic process, associated with increasing prevalence of pneumonia: 1. in ail the 581 patients included, 2. in 402 of these patients who underwent physical chest examination, 3. in 188 patients classified by the doctors as having a lower respiratory tract, infection, and 4. in 79 patients referred for radiography by the doctors. Setting - The municipal emergency clinic in Tromsø, Norway. Participants - 581 adult patients with respiratory tract infection. Outcome measures - Sensitivity, specificity, Likelihood Ratio, and Positive predictive value. Results - A tendency of decreasing specificity and Likelihood Ratio with increasing prevalence of pneumonia was demonstrated for all test, except for C-reactive protein analysis. This tendency may be explained either by the emphasis laid on the tests by the doctors when selecting patients for the diagnostic steps, or by an association between the evaluated tests and those emphasiced by the doctors. Conclusion - As the diagnostic value of symptoms and signs are strongly influenced by selection, caution should be shown when transferring diagnostic values from one clinical setting to another.