Background: Which diagnostic procedures should be used to differentiate between idiopathic and malignant pleural effusions, is not well established.

Aim: To identify which parameters allow differentiation between idiopathic and malignant pleural effusions.

Design: Case-note review.

Methods: Over a 12-year period, we treated 1014 consecutive pleural effusion patients, of whom 346 were diagnosed as having an idiopathic or malignant aetiology. We analysed medical history, chest X-ray, pleural fluid analysis (biochemical, microbiological and cytological), chest CT scan and additional examinations that were used according to clinical findings, and compared them with the eventual diagnosis and outcome.

Results: Eighty-three patients with idiopathic effusions and 263 with malignant effusions were included. Idiopathic pleural effusion resolved in 47 patients, improved in 20 and persisted in 16. Biochemical pleural fluid analysis did not predict these outcomes. A history of neoplasm, chest X-ray and CT features, as well as additional examinations according to clinical findings, established a diagnosis or suspicion of malignancy in 256 (97.7%) of the 263 patients who received a diagnosis of malignant effusion. Diagnostic thoracoscopy was helpful in seven patients in whom malignant disease was strongly suspected, despite the absence of other pathological findings.

Discussion: Non-invasive complementary examinations generally allowed accurate differentiation between malignant and idiopathic effusions. Patients with idiopathic pleural effusions generally had favourable outcomes.