Pleural effusions involve a build-up of fluid within the pleural cavity and can be found in patients with or without a malignancy. With malignant causes, pleural effusions can either indicate metastatic disease or primary malignancies (mesothelioma or hematological).

Patients with pleural effusions can present with shortness of breath, cough, chest pain, and pleurisy. On pulmonary exam, patients will have decreased breath sounds on auscultation and increased dullness to percussion over the affected area. Pleural effusions can be detected on plain chest X-ray and computed tomography (CT).

If malignancy is suspected or known and the etiology of the pleural effusion is unclear, the patient can undergo a thoracentesis. It is crucial, however, to determine if the removed fluid is transudative or exudative, as this will help determine the underlying etiology. This determination is made using the Lights Criteria.1

At the time of thoracentesis, it is valuable to have a set of recent bloodwork to help calculate Lights criteria. If the pleural fluid meets one of the following, it is considered exudative: pleural fluid protein/serum protein ratio greater than 0.5, pleural fluid lactate dehydrogenase (LDH)/serum LDH greater than 0.6, or pleural fluid LDH greater than 2/3 of the upper limit of normal for serum LDH.

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The most common causes of transudative effusions include congestive heart failure, nephrotic syndrome, hepatic hydrothorax, and atelectasis.2 The most common causes of exudative pleural effusions include infections, inflammatory conditions (eg, sarcoid, pancreatitis) and malignancy.

Although malignant pleural effusions are typically exudative, they can also be transudative in up to 10% of patients.3,4 If malignancy is on the differential diagnosis of a pleural effusion, it is also important to send the pleural fluid for additional study. The pleural fluid should be sent for cytology, which may identify malignant cells. The glucose concentration of the pleural fluid should also be checked, as concentrations below 60 mg/dL are consistent with malignant pleural effusions.