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Table 12.4

Pleural Fluid Analysis: (1) In All Patients; (2) Additional Tests for Exudative Effusions

Pleural fluid analysis (1): In all patients
TestComment

Visual inspection

Protein and lactate dehydrogenase (LDH)

Blood-stained effusion (pleural fluid haematocrit 1–20% of peripheral haematocrit) is likely to be due to malignancy, pulmonary embolism or trauma.

Purulent fluid signifies empyema.

These are the only tests needed if the effusion is likely to be a transudate.

Pleural fluid LDH correlates with the degree of pleural inflammation.

Exudative pleural effusions have a protein concentration >30g/L.

If the pleural fluid protein is around 30 g/L, Light's criteria are helpful in distinguishing between a transudate and exudate. An exudate is identified by one or more of the following:

  • Pleural fluid protein to serum protein ratio >0.5
  • Pleural fluid LDH to serum LDH ratio >0.6
  • Pleural fluid LDH more than two-thirds the upper limit of normal for serum LDH
Pleural fluid analysis (2): Additional tests for exudative pleural effusion
TestComment
Pleural fluid pH and glucose (check these if parapneumonic or malignant pleural effusion is suspected. Send sample in heparinized syringe for measurement of pH in blood gas analyser).Low pH (<7.3)/low glucose (<3.3 mmol/L) pleural fluid may be seen in:
  • Complicated parapneumonic effusion and empyema
  • Malignancy
  • Rheumatoid or lupus pleuritis
  • Tuberculosis
  • Oesophageal rupture
Cytology (total and differential cell count; malignant cells).

Neutrophilia (>50% cells) indicate acute pleural disease.

Lymphocytosis is seen in malignancy, tuberculous pleuritis and in pleural effusions after CABG.

The yield of cytology is influenced by the histological type of malignancy:

>70% positive in adenocarcinoma, 25–50% in lymphoma, 10% in mesothelioma.

Microbiology (Gram stain culture; markers of tuberculosis (TB)).Send fluid for markers of TB if TB is suspected or there is a pleural fluid. lymphocytosis.
Other tests depending on clinical setting (e.g. amylase, triglyceride).

Elevated pleural fluid amylase is seen in the acute pancreatitis and oesophageal rupture.

Check triglyceride level if chylothorax is suspected (opaque white effusion); chylothorax (triglyceride >1.1g/L) is due to disruption of the thoracic duct by trauma or lymphoma.

CABG, coronary artery bypass graft.