Medically Reviewed by: Dr. Dipak Ladda, M.D.
Expertise: Consultant Pathologist
Last Updated: July 29, 2026
Medical Analysis
Comprehensive Medical Analysis of Pleural Fluid Examination: Pathophysiology, Diagnostic Workup, and Clinical Interpretation
Introduction
The pleural cavity is a potential space lined by mesothelium of the visceral pleura on the inner side and the parietal pleura on the outer side [1]. This cavity contains a small amount of fluid which functions as a plasma filtrate derived from the capillaries of the parietal pleurae [1]. The accumulation of fluid in excess is known as an effusion, resulting from an imbalance between fluid production and reabsorption [1]. Pleural effusion is defined as the pathologic accumulation of excess fluid in the pleural cavity, where the normal quantity is around 50 ml [1]. Classification of this fluid as a transudate or an exudate is very well defined [1]. The biochemical criteria to differentiate between them include protein concentrations where less than 3.0 Gms percent indicates a transudate, and greater than 3.0 Gms percent indicates an exudate [1].
Specimen Collection of Pleural Effusion
Specimen collection of pleural effusion involves thoracocentesis, a process in which a needle is inserted into the pleural space—the area between the lung and the chest wall—to withdraw fluid [6]. The best practice is to perform this procedure under ultrasound guidance [9]. Indwelling Pleural Catheters (IPCs) are utilized for the long-term drainage of recurrent pleural effusions [6]. Pleurodesis is another procedure employed to seal the pleural space to prevent fluid from reaccumulating, which is often performed after thoracocentesis [6]. Furthermore, a needle is put through the chest wall directly into the pleural space, which is the thin gap between the pleura of the lung and of the inner chest wall [6]. Collected fluid is subsequently transferred to an EDTA tube for cell counts and cytology, and to a heparinized tube to avoid clotting for other tests [6].
Causes of Pleural Effusion – Transudate
| Involved | Causes |
| Heart (Cardiac) | CCF (Congestive cardiac failure) [1] |
| Liver (Hepatic) | Cirrhosis [1] |
| Kidney (Renal) | Nephrotic syndrome and others which lead to fluid retention [1] |
| Protein malnutrition | Hypoproteinaemia [1] |
| Peritoneal dialysis | Fluid used in dialysis may leak into pleural space [1] |
| Pulmonary embolism | Blood clot in lung is responsible. Even exudate may be there [1, 2] |
| Myxoedema | Rarely occurs [1] |
| Sarcoidosis | Rarely occurs [1] |
Causes of Pleural Effusion – Exudate
| Involved | Causes |
| Infections | Pneumonia, Parapneumonic effusions (Community acquired or nosocomial), Empyema, Tuberculosis, infarcts & others [1, 5, 12] |
| Cancers | Lung, breast, Lymphoma, GI, Gynaec, Mesothelioma & others [1, 6] |
| Autoimmune | Lupus, Rheumatoid arthritis, etc [1] |
| Pancreas | Pancreatitis [1] |
| Cardiac Injury | Post heart surgery or injury [1] |
| Chylothorax | Collection of lymph [1] |
| Haemothorax | Collection of blood – Trauma, cancer, others [1] |
| Others | Oesophageal rupture, Ovarian hyperstimulation, drug reaction, etc [1] |
| Pulmonary embolism | Transudate also possible [1, 2] |
| GI Diseases | Transudate also possible [1] |
Types of Effusion
| Categories | Transudate | Exudate |
| Cause | Differences between hydrostatic & osmotic pressures more [1, 4] | Increased capillary permeability or decreased lymphatic reabsorption. Eg. Infection, inflammation, malignancy, haemorrhages [1, 4] |
| Colour | Clear or Straw colour [1] | Cloudy, turbid, or bloody fluid [1] |
| Protein | Low (less than 3.0 Gms percent) [1, 4] | High (greater than 3.0 Gms percent) [1, 4] |
| Cell Count | Low [1] | High [1] |
| LDH | Low [1, 2] | High [1, 2] |
| Specific Gravity | Less than 1012 [1] | Greater than 1012 [1] |
Pleural Effusion with Extra-Pleural Sources
Extra-pleural sources contributing to pleural effusions include pancreatitis, ruptured esophagus, urinothorax, and chylothorax [1].
Light’s Criteria in Pleural Effusion
| Light’s Criteria | Exudate | Transudate |
| Pleural Fluid : Serum Albumin Ratio | Greater than 0.5 [2] | Less than 0.5 [2] |
| Pleural Fluid : Serum LDH Ratio | Greater than 0.6 [2] | Less than 0.6 [2] |
| Pleural Fluid LDH to that of Serum LDH | Greater than two-thirds of Serum LDH [2] | Less than two-thirds of Serum LDH [2] |
Indications
Thoracocentesis is indicated for an undiagnosed effusion in cases of empyema, malignancy, tuberculosis, and chylothorax [5, 9, 12]. It is also performed for therapeutic purposes in patients presenting with massive symptomatic effusions [5, 9]. Additional indications include the preparation of cell blocks and further histopathology and immunohistochemistry (IHC) studies for the confirmation and differentiation of primary malignant lesions, as well as monitoring treatment response during follow-up [6]. All previously stated causes under transudate and exudate fluid natures must be thoroughly considered [1, 4].
Parts of Pleural Fluid Examination
The examination of pleural fluid is systematically divided into gross examination, chemical examination, and microscopic examination [2, 7].
Gross Examination – Appearance
| Colour | Comments |
| Pale yellow or colourless | Normal [1] |
| Milky (Turbid) | Chylothorax or Pseudochylous [1] |
| Food particles | Oesophageal perforation [1] |
| Putrid odour | Anaerobic empyema [1] |
| Anchovy Brown | Amoebic abscess [1] |
| Bile stained | Chylothorax (Biliary fistula) [1] |
| Black | Aspergillus infection [1] |
| Reddish | Malignancy, Pneumonia, Trauma, Surgery, Pulmonary embolism [1, 2] |
| Purulent (Pus like) | Empyema [1, 5] |
Differentiation of Causes
| Parameters | Characteristics | Differential Diagnosis |
| Blood | Hematocrit less than 1.0 percent [1] | Trauma, malignancy, infarction [1, 6] |
| Turbid: Centrifuge it | Hematocrit greater than 50.0 percent (Clear supernatant) [1] | Haemothorax [1] |
| Cellular Debris | Turbidity persists [1] | Chylothorax or pseudochylous [1] |
Chemical Examination
| Biochemical Markers | Interpretation |
| Proteins | Low – Transudate and High – Exudate [1, 4] |
| Sugar | Low – Infection or malignancy [2, 8] |
| pH | Less than 7.3 suggests infection or malignancy [2, 9] |
| High triglyceride levels | Greater than 110 mgs percent with a cholesterol level below a certain threshold [1] |
| High cholesterol levels | Greater than 200 mgs indicates Pseudochylous; Cholesterol crystals seen [1] |
| TSA (Tuberculostearic Acid) | Low sensitivity and specificity. Limited value for diagnosis of tuberculosis [8] |
| ADA (Adenosine Deaminase) | Greater than 50 U/L suggests tuberculous effusion [8, 12] |
| Amylase | Pancreatitis, Oesophageal rupture, Malignant effusion [1] |
| LDH | Elevated especially in inflammation or tissue damage; greater than 90 mgs percent infectious pleuritis [2] |
| CRP | Greater than 90 mg/l: Parapneumonic effusion; 26 mg/l: TB; 23 mg/l: Malignant effusion [8] |
Chylous and Pseudochylous Effusion
| Parameters | Chylous | Pseudochylous |
| Aetiology | Leakage from thoracic duct [1] | Breakdown of cellular lipids in long standing effusions [1] |
| Onset | Sudden [1] | Gradual [1] |
| Appearance | Milky white or yellow to bloody [1] | Milky or greenish metallic shin [1] |
| DC | Lymphocytosis [1] | Mixed cellular reaction, cholesterol crystals [1] |
| Triglycerides | Greater than 110 mg/dl [1] | Less than 50 mg/dl [1] |
| Lipoprotein electrophoresis | Chylomicron present [1] | Absent [1] |
CRP in Pleural Effusion
| Values | Inference |
| Less than 20 mgs/dl | Transudate: Heart failure or Cirrhosis [1] |
| Greater than 20 mgs/dl | Exudate: Inflammation or Infection [1, 2] |
| High CRP Levels | Parapneumonic effusions, Empyema [5] |
| Lower CRP Levels | Tuberculous or malignant effusion [8, 12] |
| Cut off value of 47.4 mgs/dl | Differentiates tuberculous from parapneumonic effusion [8, 12] |
| Cut off value of 49.2 mgs/dl | Differentiates malignant effusion from parapneumonic effusion [6, 8] |
| Procalcitonin cut off value of 0.345 ng/ml | Between transudate (less than) and exudate (greater than) [2, 7] |
pH
Pleural fluid pH yields the highest diagnostic accuracy in assessing the prognosis of parapneumonic effusion [2, 9]. A parapneumonic exudate with a pH greater than 7.3 resolves with medical treatment, whereas values less than 7.2 require surgical drainage [5, 9].
Immunological Studies
Approximately 5 percent of rheumatoid arthritis (RA) and 50 percent of systemic lupus erythematosus (SLE) patients develop pleural effusions [1]. A pleural fluid titer of RA factor of 1:320 is suggestive of rheumatic pleuritis, while an antinuclear antibody (ANA) titer of 1:160 is suggestive of lupus pleuritis [1].
Exudate versus Transudate Additional Tests
| Tests | Exudate | Transudate |
| Pl. Fluid Cholesterol | Greater than 45.0 mgs/dl [7] | Less than 45.0 mgs/dl [7] |
| Pl. / Serum Cholesterol Ratio | Greater than 0.3 [7] | Less than 0.3 [7] |
| Serum / Pleural Albumin Gradient | Greater than 1.2 [7] | Less than 1.2 [7] |
| Pl. / Serum Bilirubin Ratio | Greater than 0.6 [7] | Less than 0.6 [7] |
Cell Count and Differential Count
| Parameters | Inference |
| Total cell count | As such not that much reliable; greater than 1000/c.mm. indicates exudate [1, 2] |
| RBCs greater than 100000/c.mm. | Malignancy, Trauma, and Infarct [1, 6] |
| Mesothelial Cells | Inflammatory processes, e.g., Tuberculous pleurisy, Empyema, Rheumatoid pleuritis [1, 12] |
| Abnormal Cells / Tumour cells | Needs cytological study and Cell block preparation [6] |
| Neutrophils greater than 50 percent | Exudate, Pyogenic Effusion, Pulmonary infarction, Pancreatitis, Abscess, Early Tuberculosis [1, 5] |
| Lymphocytosis greater than 50 percent | TB, Viral, Malignancy, RA, SLE, Chylothorax, In 30 percent transudate cases with no clinical significance [1, 12] |
| Eosinophilia greater than 10 percent | Pneumothorax, Hypersensitivity reaction, Trauma, Pulmonary Infarction, CCF and Drug reaction [1, 2] |
Reversal of Differential Count
| Proteins | Differential Count | Impression |
| Transudate | Lymphocytes greater than Polymorphs | Partially treated bacterial pleural effusion [1] |
| Exudate | Polymorphs greater than Lymphocytes | Partially treated tuberculous pleural effusion [1, 12] |
Cytological Examination
Every pleural fluid sample must be submitted for cytological examination [6]. The fluid is centrifuged, and smears are prepared from the sediment and stained with Papanicolaou, H&E, or Toluidine Blue stains [6]. Evaluations must screen for differential counts, abnormal cells, parasites, food particles (vegetative material), and tumor cells (primary or metastatic) [6]. Lymphatic obstruction by tumor cells leads to the development of an effusion which is blood-tinged or hemorrhagic [6]. Examination of three separately obtained pleural fluid samples increases the sensitivity of cancer cell detection to 80 percent [6].
Cell Block Preparation
| Step / Parameter | Details |
| Volume Needed | 10 to 20 mL pleural fluid [6] |
| Initial Procedure | Centrifuge to concentrate cells; discard or save supernatant [6] |
| Fixation | Resuspend pellet in 10 percent formalin or suitable fixative [6] |
| Embedding | Process pellet like tissue to prepare paraffin block [6] |
| Sectioning | Cut 3 to 5 micrometer sections for H&E staining [6] |
| H&E Staining | Assess cellularity, architecture, cytology [6] |
| HPR Reporting | Will get cells in concentrated forms [6] |
| IHC Use | Confirm tumor origin, differentiation, or specific markers [6] |
Cell Block: The Magic
The cell block can be prepared from the pleural fluid sample and submitted for IHC study for the early detection of recurrence, response to therapy, and situations where cytology shows negative results but remains highly suspicious for malignancy [6]. Examples include Prostate-Specific Antigen (PSA) for prostate cancer, Carcinoembryonic Antigen (CEA) for colonic carcinoma, CA-125 for ovarian tumors, and other markers determined by clinical history and primary diagnosis [6]. DNA ploidy can also be evaluated to diagnose malignancy [6].
Microbiological Examination
Gram’s smear and culture should be carried out on exudates, as well as on purulent, bloodstained, and cloudy samples [5]. The isolation of organisms is increased if pleural fluid is inoculated into blood culture bottles at the bedside [5]. A Ziehl-Neelsen (ZN) stained smear is positive in less than 20 percent of tuberculous pleural effusions, and culture is positive in less than 40 percent of cases [12]. If tuberculosis is suspected and the culture is negative, PCR for mycobacterial DNA or an increased level of adenosine deaminase can aid in diagnosis [8, 12].
Special Studies on Pleural Fluid
| Pleural Effusion Disease | Tests Advocated |
| Tuberculous | Adenosine deaminase, gamma interferon, polymerase chain reaction [8, 12] |
| Rheumatoid Effusion | Rheumatoid Factor [1] |
| Lupus Erythematosus | Antinuclear Antibody (ANA) [1] |
| Pancreatic Disease, Oesophageal | Amylase [1] |
| Malignancy | CEA, CA15.4, CA125, PSA, etc [6] |
| Chylothorax | Triglycerides [1] |
Other Tests
For the diagnosis of pulmonary embolism, a D-dimer test can be performed from a peripheral blood sample [2]. Flow cytometric analysis of the pleural fluid sample is indicated if lymphoma is suspected [6]. In parapneumonic effusions, if the pleural fluid pH is less than 7.20, drainage is indicated, whereas if the pH is greater than 7.30, complete resolution will occur with medical treatment [5, 9]. In malignant pleural effusions, a low pH indicates a poor prognosis and a poor response to tetracycline pleurodesis [6]. The presence of LE cells in pleural fluid establishes the diagnosis of lupus pleuritis [1].
Remember
Even if an effusion is of the exudative type, an elevated NT-proBNP level makes it very likely that congestive heart failure is the cause [1]. Conversely, a negative NT-proBNP finding in the blood rules out congestive heart failure as the cause of a pleural effusion, and a blood NT-proBNP level is sufficient in most cases [1]. Regarding acidosis, pleural fluid acidosis is found in complicated pleural infections, tuberculosis, rheumatoid arthritis, and malignant effusions [1, 5, 12].
For Non-Medicos
What Is Pleural Fluid and Why Is It Tested?
Pleural fluid is a small amount of liquid located in the space between your lungs and chest wall that helps them move smoothly [1]. Doctors test this fluid to find out why extra liquid builds up due to infections, heart problems, or other medical issues [1, 2].
Common Signs of Fluid Buildup Around the Lungs
When too much fluid collects around the lungs, patients typically experience shortness of breath, chest discomfort, and persistent coughing [1].
Understanding Test Results and Treatment Goals
Laboratory testing examines the fluid’s clarity, protein levels, and cell counts to help doctors pinpoint whether the fluid buildup stems from heart failure, infection, or cancer, allowing for targeted medical treatment [1, 2].
References:
Light RW. Pleural Diseases. 6th ed. Philadelphia: Wolters Kluwer; 2013.
Heffner JE, Brown LK, Barbieri CA. Diagnostic value of tests in evaluating pleural effusions. A systematic review and meta-analysis. Chest. 1997;111(6):1704-1714.
Porcel JM, Light RW. Diagnostic approach to pleural effusion in adults. Am Fam Physician. 2006;73(7):1211-1220.
Villena V, Lopez-Encuentra A, Echave-Sustaeta J, et al. Prospective study of 1,004 consecutive patients with pleural effusion. Etiology of effusion and criteria for determining its transudative or exudative nature. Chest. 2002;122(4):1074-1079.
Maskell NA, Butland RJ. Pleural Diseases Group, Standards of Care Committee, British Thoracic Society. BTS guidelines for the investigation of a unilateral pleural effusion in adults. Thorax. 2003;58(suppl 2):ii8-ii17.
Feller-Kopman D, Light RW. Pleural disease. N Engl J Med. 2018;378(8):740-751.
Ferreiro L, San José E, Valdés L. Pleural fluid analysis: beyond Light’s criteria. Med Clin (Barc). 2015;145(8):357-363.
Metintas M, Ak G, Dundar E, et al. Diagnostic value of pleural fluid biochemical parameters in malignant and tuberculous pleural effusions. Respiration. 2007;74(5):544-550.
Hooper C, Lee YC, Maskell N. Investigation of a unilateral pleural effusion in adults: British Thoracic Society Pleural Disease Guideline 2010. Thorax. 2010;65(suppl 2):ii4-ii17.
Porcel JM. Biomarkers in the diagnosis of pleural diseases: a 2010 update. Embe Med. 2012;2(3):289-299.
Rahman NM, Chapman SJ, Davies RJ. Pleural effusion: a structured approach to care. Br Med Bull. 2005;72:31-47.
Loddenkemper R, Antony VB. Pleural Tuberculosis. Eur Respir J. 2005;25(3):566-574.
FAQ’s:
- What is pleural fluid?
Physiological liquid present within the pleural cavity between the parietal and visceral pleura. - Why examine pleural fluid?
To differentiate transudative and exudative processes, identify underlying diseases, and guide therapeutic interventions. - How are effusions classified?
Using established biochemical criteria, most notably Light’s criteria based on protein and LDH ratios. - What causes transudative effusions?
Systemic factors altering hydrostatic or oncotic pressures like congestive heart failure and cirrhosis. - What causes exudative effusions?
Local pathological processes affecting pleural surfaces including infections, malignancies, and inflammatory conditions. - How is fluid acquired?
Through diagnostic thoracentesis performed under strict aseptic precautions and local anesthesia with ultrasound guidance. - What indicates an exudate protein?
Protein concentrations exceeding 3.0 g/dL point toward an exudate rather than a transudate process. - What does neutrophil predominance suggest?
Acute processes such as bacterial pneumonia, acute pulmonary infarction, or early pancreatitis. - What does lymphocyte predominance indicate?
Chronic inflammatory conditions such as tuberculosis, lymphoma, or sarcoidosis.
What is cytology used for?
Detecting malignant cells in suspected carcinomatous effusions using concentrated cell smears and stains.
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