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

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 all 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 emphasized 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.