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BACKGROUND: The majority of patients with lower respiratory tract infections (LRTIs) are treated with antibiotics; some of them are unnecessary because of a viral cause. Information on prediction of the aetiology, especially in a general practice setting, is missing. AIM: To differentiate between viral and bacterial LRTI on simple clinical criteria, easily obtained at the bedside. DESIGN OF STUDY: Prospective observational study. SETTING: General practices in the Leiden region of The Netherlands. METHOD: Adult patients with LRTI were included. Standard medical history and physical examination were performed. Sputum, blood and throat swabs were collected for diagnostic tests. According to microbiological findings, patients were classified as bacterial, viral, dual infection and unknown cause. In a logistic regression model independent predictors were determined. Scoring systems were developed. The accuracies of the diagnostic rules were tested by using receiver operating characteristic (ROC) curves. RESULTS: One-hundred and forty-five patients were classified as having bacterial (n = 35), viral (n = 49), or dual infection (n = 8), or infection of unknown cause (n = 53), respectively. Independent predictors for bacterial infection were fever (odds ratio [OR] = 8.0; 95% confidence interval [CI] = 0.9 to 71.0), headache (OR = 4.3; 95% CI = 1.0 to 19.1) cervical painful lymph nodes (OR = 8.7; 95% CI = 1.1 to 68.0), diarrhoea (OR = 0.3; 95% CI = 0.1 to 1.0) and rhinitis (OR = 0.3; 95% CI = 0.1 to 0.9). As an additional independent predictor, an infiltrate on chest X-ray (OR = 5.0; 95% CI = 1.2 to 20.5) was found. The diagnostic rules developed from these variables classified the aetiology of LRTI with a ROC curve area of 0.79 (clinical score), 0.77 (simplified score) and 0.83 (extended score). CONCLUSIONS: A diagnostic rule was developed, based on information that is easy to obtain at the bedside, to predict a bacterial infection. This diagnostic rule may be a tool for general practitioners in their management of patients with LRTI.