Pleural fluid tests and sample collection guidance
| Test | Notes |
| Recommended tests for all sampled pleural effusions | |
| Biochemistry: LDH and protein | 2–5 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 count | Maximum 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 | |
| pH | In 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 |
| Glucose | Occasionally useful in diagnosis of rheumatoid effusion. 1–2 ml in fluoride oxalate tube sent to biochemistry |
| Acid-fast bacilli and TB culture | When there is clinical suspicion of TB pleuritis. Request with MC and S. 5 ml sample in plain container |
| Triglycerides and cholesterol | To distinguish chylothorax from pseudochylothorax in milky effusions. Can usually be requested with routine biochemistry (LDH, protein) using the same sample |
| Amylase | Occasionally useful in suspected pancreatitis-related effusion. Can usually be requested with routine biochemistry |
| Haematocrit | Diagnosis of haemothorax. 1–2 ml sample in EDTA container sent to haematology |
LDH, lactate dehydrogenase; PH, pulmonary hypertension; TB, tuberculosis