A new diagnostic index for bacterial conjunctivitis in primary care. A re-derivation study.

BACKGROUND: Eighty per cent of primary care patients with infectious conjunctivitis are treated with antibiotics, although in only 30%, there is a bacterial cause. An accurate diagnostic index to distinguish bacterial from viral conjunctivitis may help reduce unnecessary antibiotics.

OBJECTIVES: To validate and, if necessary, improve an existing diagnostic index for bacterial conjunctivitis.

METHODS: Non-experimental validation and updating study of an existing diagnostic index in Dutch General Practice. We collected 210 adult patients with incident symptoms suggestive for acute infectious conjunctivitis. GPs completed a standardized questionnaire and a physical examination of the eye(s) and took a conjunctival sample for culture. Cultures were analysed masked for the GPs' findings. On bad performance of the existing index on the new patients, we developed a new index combining the dataset on which the original model had been developed (n = 176) and the new dataset (n = 210). Bootstrapped backward variable selection and shrinkage of regression coefficients was used to protect the new index against bad performance in future patients.

RESULTS: The bacterial culture was positive in 36.3%. The items age and number of glued eyes at awakening were consistent predictors. This model classified 48% (107/386) of patients at a low (< 25%) chance of having a positive culture and 2% as at high (> 70%) chance.

CONCLUSION: Correction of a previously derived diagnostic index for bacterial conjunctivitis yielded a simple index, based on history only. The index is potentially useful to rule out bacterial conjunctivitis in patients below 50 years of age with no history of glued eyes at awakening. This study underscores the importance of external validation of diagnostic indices.