Pleural effusion diagnostic study of choice

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Dushka Riaz, MD


Because the treatment of pleural effusion varies based on the cause it is important to have a good differential diagnosis. This would drive the diagnostic approach and ultimately the diagnostic study of choice based on the presentation. After determining whether the effusion is unilateral or bilateral through chest x-ray, the likely cause should be considered. If the diagnosis is clearly pointing towards nephrotic syndrome or congestive heart failure, then these patients do not necessarily need to have a thoracentesis performed and should be treated. However, a thoracentesis becomes the diagnostic study of choice in the following circumstances:

  • an unclear cause
  • patient experiencing pleuritic chest pain
  • patient experiencing symptoms out of proportion to the size of the effusion
  • no response to treatment

The use of thoracentesis becomes urgent if the patient is decompensating or the pleural effusion is considerably large. [1]

Diagnostic Study of Choice

Study of choice

The diagnostic study of choice is a thoracentesis that should be performed with a current chest x-ray and under ultrasound guidance. The procedure uses a 21 gauge needle with a 50 mL syringe. After the fluid is removed, it is analyzed. Macroscopically, the fluid can point to differentials. If milky, consider a chylothorax, pus can point to empyema and blood can indicate malignancy. LDH and protein are also measured to determine if the fluid is an exudate or transudate as per Light's Criteria. [1]

The Light's Criteria states that one of three of the following criteria must be met for the fluid to be considered an exudate:

Exudates are caused by inflammation or impaired lymphatic drainage whereas transudates are caused by changes in the hydrostatic or oncotic pressures. [2] The initial goal of the thoracentesis is to differentiate between these two and so Light's Criteria remains the guideline of choice for this diagnostic study. [3]

The fluid is further analyzed for pH levels, glucose, amylase, triglycerides, biomarkers and cytology. It is recommended to check pH levels if the cause may be infectious. If the pH levels are less than 7.2, it is advised to drain the fluid immediately to decrease the risk of parapneumonic pleural effusion. Low glucose in the fluid can indicate empyema, tuberculosis, rheumatoid arthritis and malignancy. [1] High amylase content can indicate acute pancreatitis, chronic pancreatitis or esophageal rupture. [4] Elevated levels of triglycerides (greater than 110 mg/dL) would indicate chylothorax. [5]


  1. 1.0 1.1 1.2 Jany B, Welte T (2019). "Pleural Effusion in Adults-Etiology, Diagnosis, and Treatment". Dtsch Arztebl Int. 116 (21): 377–386. doi:10.3238/arztebl.2019.0377. PMC 6647819 Check |pmc= value (help). PMID 31315808.
  2. Light RW, Macgregor MI, Luchsinger PC, Ball WC (1972). "Pleural effusions: the diagnostic separation of transudates and exudates". Ann Intern Med. 77 (4): 507–13. doi:10.7326/0003-4819-77-4-507. PMID 4642731.
  3. Beaudoin S, Gonzalez AV (2018). "Evaluation of the patient with pleural effusion". CMAJ. 190 (10): E291–E295. doi:10.1503/cmaj.170420. PMC 5849448. PMID 29530870.
  4. Joseph J, Viney S, Beck P, Strange C, Sahn SA, Basran GS (1992). "A prospective study of amylase-rich pleural effusions with special reference to amylase isoenzyme analysis". Chest. 102 (5): 1455–9. doi:10.1378/chest.102.5.1455. PMID 1385051.
  5. Staats BA, Ellefson RD, Budahn LL, Dines DE, Prakash UB, Offord K (1980). "The lipoprotein profile of chylous and nonchylous pleural effusions". Mayo Clin Proc. 55 (11): 700–4. PMID 7442324.

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