Pleural effusion (the emergency department workup and the Light criteria)

Also known as Pleural fluid · Water on the lung · Parapneumonic effusion · Empyema

Pleural effusion — the transudate-versus-exudate distinction by the Light criteria (the pleural fluid to serum protein ratio over 0.5, the LDH ratio over 0.6, the pleural fluid LDH over two-thirds the upper limit of normal), the causes (the heart failure, the pneumonia, the malignancy, the PE), the diagnostic approach (the chest radiograph, the ultrasound, the pleural fluid analysis), the management (the treat-the-cause, the therapeutic drainage, the chest drain for the empyema), and the parapneumonic effusion and the empyema. ACEM-primary, globally tagged.

medium8 referencesUpdated 26 July 202618 min readSource-verified · Sept 2026

Checked against its sources on 3 Sept 2026

The one-line answer

A pleural effusion is fluid in the pleural space, and the only decision that matters at the bedside is transudate or exudate — settled by the Light criteria (pleural-to-serum protein ratio over 0.5, pleural-to-serum LDH ratio over 0.6, or pleural LDH over two-thirds the upper limit of normal; any one makes an exudate). A transudate is a systemic problem (heart failure, cirrhosis, nephrotic) you treat medically and almost never tap; an exudate is a local problem (parapneumonic, malignant, PE, TB) you investigate with fluid analysis and drain when the pH falls below 7.2, the glucose below 3.3 mmol per litre, the culture turns positive, or frank pus appears. The empyema gets a chest drain and the intrapleural tPA plus DNase; the heart-failure effusion gets furosemide.^[1]^[2]^[3]

Meet the patient

A 68-year-old smoker, three weeks of progressive dyspnoea, six kilograms of weight loss, a right-sided pleuritic pain. The chest radiograph shows a large right effusion pushing the mediastinum away from the side. Your registrar wants to give furosemide and see what happens. The move that wins the viva is the opposite — a diagnostic tap, the Light criteria, and a fluid pH.^[3]

The two questions that govern every pleural effusion are the two every Fellowship stem turns on: is this a transudate or an exudate? (the paired serum and fluid answer it) and does this fluid need to come out now? (the pH, the glucose, the culture, and the size of the effusion answer it). Hold those two and every section below slots into place.^[1]^[3]

References8Show

[1]Roberts ME, Rahman NM, Maskell NA, et al. British Thoracic Society Guideline for pleural disease Thorax, 2023.PMID 37433578
[2]Bhatnagar M, Monsch GM, Kette S, et al. Pleural Infection in Adults: Integrating Medical and Surgical Management for Optimal Outcomes Thorac Surg Clin, 2026.PMID 42336508
[3]Porcel JM, Light RW. Pleural Fluid Analysis: Are Light's Criteria Still Relevant After Half a Century? Clin Chest Med, 2021.PMID 34774168
[4]Maskell NA, Davies CWH, Nunn AJ, et al. U.K. Controlled trial of intrapleural streptokinase for pleural infection N Engl J Med, 2005.PMID 15745977
[5]Rahman NM, Maskell NA, West A, et al. Intrapleural use of tissue plasminogen activator and DNase in pleural infection N Engl J Med, 2011.PMID 21830966
[6]Psallidas I, Corcoran JP, Rahman NM. Management of parapneumonic effusions and empyema Semin Respir Crit Care Med, 2014.PMID 25463162
[7]Gordon CE, Feller-Kopman D, Balk EM, et al. Pneumothorax following thoracentesis: a systematic review and meta-analysis Arch Intern Med, 2010.PMID 20177035
[8]Feller-Kopman D, Berkowitz D, Boiselle P, et al. Large-volume thoracentesis and the risk of reexpansion pulmonary edema Ann Thorac Surg, 2007.PMID 17954079