Table 1

Pleural fluid tests and sample collection guidance

Recommended tests for all sampled pleural effusions
Biochemistry: LDH and protein2–5 ml in plain container or serum blood collection tube depending on local policy. Blood should be sent simultaneously to biochemistry for total protein and LDH so that Light's criteria can be applied
Microscopy and culture (MC and S)5 ml in plain container. If pleural infection is particularly suspected, a further 5 ml in both anaerobic and aerobic blood culture bottles should be sent
Cytological examination and differential cell countMaximum volume from remaining available sample in a plain universal container. Refrigerate if delay in processing anticipated (eg, out of hours)
Other tests sent only in selected cases as described in the text
pHIn non-purulent effusions when pleural infection is suspected. 0.5–1 ml drawn up into a heparinised blood gas syringe immediately after aspiration. The syringe should be capped to avoid exposure to air. Processed using a ward arterial blood gas machine
GlucoseOccasionally useful in diagnosis of rheumatoid effusion. 1–2 ml in fluoride oxalate tube sent to biochemistry
Acid-fast bacilli and TB cultureWhen there is clinical suspicion of TB pleuritis. Request with MC and S. 5 ml sample in plain container
Triglycerides and cholesterolTo distinguish chylothorax from pseudochylothorax in milky effusions. Can usually be requested with routine biochemistry (LDH, protein) using the same sample
AmylaseOccasionally useful in suspected pancreatitis-related effusion. Can usually be requested with routine biochemistry
HaematocritDiagnosis of haemothorax. 1–2 ml sample in EDTA container sent to haematology
  • LDH, lactate dehydrogenase; PH, pulmonary hypertension; TB, tuberculosis