Under normal conditions, pleural fluid is secreted by the parietal pleural capillaries at a rate of 0.6 millilitre per kilogram weight per hour, and is cleared by lymphatic absorption leaving behind only 5–15 millilitres of fluid, which helps to maintain a functional vacuum between the parietal and visceral pleurae.

[3] The mechanism for the exudative pleural effusion in pulmonary thromboembolism is probably related to increased permeability of the capillaries in the lung, which results from the release of cytokines or inflammatory mediators (e.g. vascular endothelial growth factor) from the platelet-rich blood clots.

[7] Less common causes include esophageal rupture or pancreatic disease, intra-abdominal abscesses, rheumatoid arthritis, asbestos pleural effusion, mesothelioma, Meigs's syndrome (ascites and pleural effusion due to a benign ovarian tumor), and ovarian hyperstimulation syndrome.

[7] Pleural effusions may also occur through medical or surgical interventions, including the use of medications (pleural fluid is usually eosinophilic), coronary artery bypass surgery, abdominal surgery, endoscopic variceal sclerotherapy, radiation therapy, liver or lung transplantation, insertion of ventricular shunt as a treatment method of hydrocephalus,[8][9] and intra- or extravascular insertion of central lines.

[12] A pleural effusion is usually diagnosed on the basis of medical history and physical exam, and confirmed by a chest X-ray.

Once accumulated fluid is more than 300 mL, there are usually detectable clinical signs, such as decreased movement of the chest on the affected side, dullness to percussion over the fluid, diminished breath sounds on the affected side, decreased vocal resonance and fremitus (though this is an inconsistent and unreliable sign), and pleural friction rub.

A systematic review (2009) published as part of the Rational Clinical Examination Series in the Journal of the American Medical Association showed that dullness to conventional percussion was most accurate for diagnosing pleural effusion (summary positive likelihood ratio, 8.7; 95% confidence interval, 2.2–33.8), while the absence of reduced tactile vocal fremitus made pleural effusion less likely (negative likelihood ratio, 0.21; 95% confidence interval, 0.12–0.37).

[15] Chest radiographs in the lateral decubitus position (with the patient lying on the side of the pleural effusion) are more sensitive and can detect as little as 50 mL of fluid.

Between 250 and 600mL of fluid must be present before upright chest X-rays can detect a pleural effusion (e.g., blunted costophrenic angles).

[16] Chest computed tomography is more accurate for diagnosis and may be obtained to better characterize the presence, size, and characteristics of a pleural effusion.

[17] To increase diagnostic accuracy of detection of pleural effusion sonographically, markers such as boomerang and VIP signs can be utilized.

Briefly, transudate is produced through pressure filtration without capillary injury while exudate is "inflammatory fluid" leaking between cells.

[30] An accurate diagnosis of the cause of the effusion, transudate versus exudate, relies on a comparison of the chemistries in the pleural fluid to those in the blood, using Light's criteria.

Previous to Light's landmark study, which was based on work by Chandrasekhar, investigators unsuccessfully attempted to use other criteria, such as specific gravity, pH, and protein content of the fluid, to differentiate between transudates and exudates.

More recent studies have examined other characteristics of pleural fluid that may help to determine whether the process producing the effusion is local (exudate) or systemic (transudate).

Repeated effusions may require chemical (talc, bleomycin, tetracycline/doxycycline), or surgical pleurodesis, in which the two pleural surfaces are scarred to each other so that no fluid can accumulate between them.

Each day the patient or caregivers connect it to a simple vacuum tube and remove from 600 to 1000 mL of fluid, and can be repeated daily.