Pleural Fluid Analysis


The gross appearance of pleural fluid should be noted as this may suggest a specific diagnosis (see Table 1).

Appearance of Aspirate Suspected Disease
Putrid odour Anaerobic empyema
Food particles Ruptured oesophagus
Bile Stained Biliary Fistula (cholothorax)
Milky Chylothorax
‘Anchovy sauce’ Ruptured amoebic abscess


When pleural fluid is aspirated it should be sent for the following:

  • Protein
  • LDH
  • Cytology, cell count and differential
  • Fluid pH
  • Fluid glucose
  • Gram staining and culture +/- Acid fast bacilli
  • Haematocrit (if there is uncertainty about the presence of a haemothorax)

Learning bite

A transudate contains less than 25 g/l of protein.
An exudate contains more than 35 g/l of protein.

Light’s Criteria

If the pleural fluid contains protein at levels between 25 g/l and 35 g/l then Lights Criteria should be used to decide whether the effusion is a transudate or an exudate.

Lights criteria state that the fluid is an exudate if one or more of the following criteria are met:

  • Pleural fluid : Serum protein ratio is greater than 0.5
  • Pleural fluid LDH : Serum LDH is greater than 0.6
  • Pleural fluid LDH is greater than two thirds the upper limit of normal serum LDH


Malignant effusions can be diagnosed by pleural fluid cytology in about 60% of cases.

Cell count/differential

Pleural fluid cell count and differential may give important clues as to the underlying diagnosis [10] (Table 2)

Fluid cell differential Common causes
High red cell count (>100,000 mm3) Malignancy
Pulmonary Infarction
Neutrophilia Acute inflammatory effusion
Lymphocytosis (>85% lymphocytes) Malignancy (e.g. lymphoma)
Eosinophilia (>10% eosinophils) Malignancy
Some infections e.g. fungal, TB
Drug induced (e.g. sulfasalazine)
Post CABG surgery

Pleural fluid pH

Pleural fluid pH should be measured on all non-purulent effusions:

  • It can be performed using a heparinised sample of fluid in a standard blood gas analyser
  • Low pH will be found in effusions caused by infection, malignancy, oesophageal rupture and rheumatoid disease.
  • A fluid pH of <7.2 in the setting of infection indicates that tube drainage is required and differentiates between simple and complex para-pneumonic effusion [8,10]

Learning bite

Frank pus should not be put in a blood gas analyser since it is an indication for pleural drainage regardless of fluid pH and it may also damage the machine.

Pleural fluid glucose

Low pleural fluid glucose is found in the same conditions which give a low pH. The lowest levels of fluid glucose are found in rheumatoid arthritis and empyema.


This is useful in suspected oesophageal rupture or pancreatitis where levels will be higher than normal serum amylase.

Triglyceride and cholesterol levels

These may be helpful when a chylothorax is suspected.