Pleural effusions can be divided into transudates and exudates. Light’s criteria is used to distinguish between them.
Light’s criteria:
Pleural fluid is an exudate if one or more of the following criteria are met:
- Pleural fluid protein divided by serum protein is >0.5
- Pleural fluid LDH divided by serum LDH is >0.6
- Pleural fluid LDH is >2/3 the upper limits of laboratory normal value for LDH
Diagnostic pleural taps should ideally be done under US guidance and the fluid sent for:
- All cases:
- Protein
- LDH
- Gram stain
- Cytology and cell count
- Microbiological culture
- Specific cases:
- pH – if pleural infection suspected
- Amylase – only if oesophageal rupture or pancreatic-associated effusion suspected
- Glucose – useful in diagnosis of rheumatoid effusion
- Triglycerides and cholesterol – if effusion milky and chylothorax suspected
- Haematocrit – if haemothorax suspected
Causes of exudative pleural effusions:
- Common
- Malignancy
- Parapneumonic
- TB
- Less common
- PE
- RA
- Benign asbestos effusion
- Pancreatitis
- Post MI
- Post CABG
- Rare
- Yellow nail syndrome
- Drugs
- Methotrexate
- Amiodarone
- Phenytoin
- Nitrofurantoin
- Beta blockers
- Penicillamine
- Cyclophosphamide
- Fungal infections
Causes of transudative pleural effusions
- Very common
- LVF
- Liver cirrhosis
- Less common
- Hypoalbuminaemia
- Peritoneal dialysis
- Hypothyroidism
- Nephrotic syndrome
- Mitral stenosis
- Rare causes
- Constrictive pericarditis
- Meigs’ syndrome
- Superior vena cava obstruction
- Causes of lymphocytic pleural effusion:
- Very high (>80%) lymphocytes
- TB
- Lymphoma
- RA
- Post CABG
- Sarcoidosis
- Less high (>50%):
- Any long-standing pleural effusion
- Malignancy –mesothelioma, metastatic adenocarcinoma
- Cardiac failure
- Yellow nail syndrome
- Uraemia
- pH
- pH <7.2 indicates the need for drainage
- pH <7.3
- malignant effusions
- pleural infection
- connective tissue diseases
- TB
- Oesophageal rupture
- Glucose
- Low (<3.4mmol/l)
- Parapneumonic
- RA
- TB
- Malignancy
- Oesophageal rupture
- Very low (<1.6mmol/l)
- RA
- Empyema
When PE's cause effusions, the dyspneoa is often disproportionate to the size of the effusion.
Post CABG effusions: left >right
Management
- If clinically a transudate is suspected, treat cause
- If cause unclear, perform a diagnostic tap and then treat cause as appropriate
- If pleural aspiration (thoracocentesis) is indicated:
- Stop when patient develops chest discomfort or cough or when 1.5l has been drained
- Malignant pleural effusion (symptomatic)
- Unless very short life expectancy, chest drain as high probability of recurrence if theraupetic aspiration alone
- Consider pleurodesis once effusion drained and lung re-expanded
- Drain large effusions in a controlled fashion to decrease risk of re-expansion pulmonary oedema
Small print gem: there needs to be around 200mls of fluid for it to be detectable on a PA CXR.
