This guide provides a structured approach to the interpretation of pleural fluid results for specific diseases. Reference ranges vary between labs, so always consult your local medical school or hospital guidelines.
Normal Pleural Fluid
- Clear fluid
- pH: 7.60-7.64
- Protein: < 2% (1-2 g/dL)
- White blood cells (WBC): < 1000/mm³
- Glucose: similar to that of plasma
- LDH: <50% plasma concentration
- Amylase: 30-110 U/L
- Triglycerides: <2 mmol/l
- Cholesterol: 3.5–6.5 mmol/l
A pleural effusion is usually diagnosed on the basis of a chest X-ray. At least 300mL of fluid must be present before chest X-rays can detect a pleural effusion.
Once the accumulated fluid is more than 300 mL, clinical signs such as decreased chest-wall movement, dullness to percussion and diminished breath sounds on the affected side of the chest become evident. If there is a large effusion then tracheal deviation may occur away from the effusion. CT Thorax is more accurate for diagnosis and is better at characterising the size and location of a pleural effusion.
Transudate vs Exudate
Transudative pleural effusions are defined as effusions that are caused by factors that alter hydrostatic pressure, pleural permeability, and oncotic pressure.
Conditions associated with transudative pleural effusions include:
- Congestive heart failure
- Liver cirrhosis
- Severe hypoalbuminemia
- Nephrotic syndrome
Exudative pleural effusions are caused by changes to the local factors that influence the formation and absorption of pleural fluid.
Conditions associated with exudative pleural effusions include:
- Infection (empyema due to bacterial pneumonia)
- Pulmonary infarction
- Pulmonary embolism
Diagnostic criteria for pleural effusion
Protein <30 g/L (in patients with a normal serum protein level)
Protein >30 g/L (in patients with a normal serum protein level)
Light’s Criteria are more accurate for the diagnosis of exudative effusions.
The fluid is considered an exudate if any of the following are found:
- Ratio of pleural fluid to serum protein > 0.5
- Ratio of pleural fluid to serum LDH > 0.6
- Pleural fluid LDH > two-thirds of the upper limits of normal serum value
If a patient is thought to have a transudative pleural effusion but the Light’s Criteria suggest an exudate, then the serum–pleural fluid protein gradient should be examined.
- Frankly purulent fluid indicates an empyema (an anaerobic empyema is indicated if the fluid has a putrid odour).
- A milky fluid suggests chylothorax or pseudochylothorax. This is most often caused by lymphatic obstruction secondary to malignancy, chronic inflammation or thoracic duct injury by trauma or a surgical procedure.
- Grossly bloody fluid is usually associated with trauma. Less common causes include TB, aortic dissection, ruptured aortic aneurysm or malignancy.
- Straw coloured fluid with the distinctive smell of ammonia is indicative of urinothorax.
- The presence of food particles suggests oesophageal rupture.
- Black pleural fluid is extremely rare, indicative of only a few diseases, including:
- Aspergillus niger infection
- Malignant melanoma (black colour caused by cells containing melanin pigment)
- Haemorrhage and haemolysis associated with non-small cell lung cancer
Pleural fluid LDH
- Levels > 1000 IU/L are suggestive of empyema, malignancy or rheumatoid effusion.
- A low pleural fluid glucose level (<3.4 mmol/l) may be found in empyema, rheumatoid pleuritis and pleural effusions associated with TB, malignancy and oesophageal rupture.
- A very low pleural glucose concentration (<1.6 mmol/l) is indicative of empyema and rheumatoid disease.
- A pleural fluid pH of < 7.3 is associated with the same pathologies that cause a low pleural fluid glucose.
- In malignant effusions, a pleural fluid pH of < 7.3 has been associated with more extensive pleural involvement and shorter life expectancies.
- Pleural fluid amylase levels > 110U/L are indicative of pancreatitis, malignancy or a ruptured oesophagus.
White Blood Cells
- Results generally are not diagnostic, but most transudates have WBC counts <1000 cells/µL.
- Exudates generally have WBC counts > 50,000 cells/µL.
- Pleural fluid lymphocytosis suggests TB, sarcoidosis or malignancy.
- Neutrophil dominant effusions are associated with empyema or pulmonary embolism.
- Pleural fluid eosinophilia (PFE) is usually caused by the presence of air or blood in the pleural space.
- Blood in the pleural space may be the result of pulmonary embolism or benign asbestosis. The presence of PFE does not exclude a malignancy.
Cholesterol and Triglycerides
- Pleural fluid triglyceride levels >1.24 mmol/l with a cholesterol <5.18 mmol/l is diagnostic of chylothorax.
- A triglyceride level <0.56 mmol/l with a cholesterol >5.18 mmol/l is found in pseudochylothorax.
A 52-year-old male presents with a cough, shortness of breath and fever. CXR shows a right-sided pleural effusion. A thoracentesis is performed and the results of the pleural fluid analysis are below.
- Colour – purulent
- Pleural/serum total protein ratio – >0.5
- pH – 7.1
- WBC Count – 67,000 cells/µL
- Glucose – 1.5 mmol/l
- LDH – 1430 IU/L
This is empyema. This gentleman has presented with, fever, shortness of breath and cough. The pleural fluid is purulent on inspection, the white cell count is significantly raised and glucose levels are low. The history and pleural fluid results are strongly suggestive of empyema, and therefore he should be treated empirically while culture results are awaited.
A 56 -year-old, previously well woman was admitted with a 4-week history of a cough, night sweats and 1-week history of progressive breathlessness. She had never smoked and has no history of heart disease or rheumatological disease. Pleural fluid results are shown below.
- Colour – milky white, odourless
- Triglyceride – 0.5 mmol/l
- Cholesterol – 12.4 mmol/l
This is pseudochylothorax. This woman has presented with a cough, night sweats and progressive breathlessness. A triglyceride level <0.56 mmol/l with a cholesterol >5.18 mmol/l is indicative of pseudochylothorax. A pleural biopsy revealed chronic inflammation, and Mycobacterium tuberculosis was isolated on a pleural fluid culture.
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