Pleural Fluid Analysis

A pleural effusion is a collection of fluid in the pleural space. Pleural effusion are the result of :

  • Increased fluid accumulation
  • Decreased lymphatic clearance of fluid
    • Obstruction to drainage
    • Increased venous pressure
  • Pleural effusions are most commonly caused by CCF, Infection (pneumonia) and Malignancy
  • Pleural Fluid may be examined by a pleural tap or thoracocentesis

Common causes

  • Exudate (local disease) (High protein). Local factors influence the accumulation or clearance of fluid.
    • Malignancy – Lung, breast, pleural.
    • Infection – Pneumonia, empyema, pleuritis, viral disease
    • Autoimmune – Rheumatoid, SLE
    • Vascular – PTE
    • Cardiac – Pericarditis, CABG
    • Respiratory – Haemothorax, Chylothorax
    • Abdominal – Subphrenic abscess
  • Transudate (systemic illness) (Low protein <30g). Imbalance between oncotic and hydrostatic pressures
    • Cardiac – CCF, PTE
    • Liver – Ascites, Cirrhosis
    • Renal – Glomerulonephritis, Nephrotic syndrome
    • Ovarian – Meigs syndrome
    • Autoimmune – Sarcoid
    • Thyroid – Myxoedema

Differentiation of exudate and transudate fluid

  • Aims to identify local from systemic illness. Common causes can then be actively sought and treated
  • Use Light’s criteria is moderately sensitive for differentiation, further tests are then required to further define the exudate

Pleural fluid from thoracocentesis

Pleural fluidTest indicatedInterpretation

Comparison to serum Haematocrit

  • <1% - non-significant
  • 1-20% - Cancer, PTE, trauma, pneumonia
  • >50% - Haemothorax
Cloudy or turbidTriglycerides>110mg/dL-chylothorax
Putrid odourMCSPossible anaerobic infection

Pleural fluid laboratory findings

  • Lights criteria (High protein and LDH = exudate), determines presence of exudate with protein and LDH levels
    • Pleural fluid protein to serum protein ratio >0.5
    • Pleural fluid LDH to serum LDH ratio >0.6
    • Pleural fluid level >2/3 of upper value for serum LDH
  • Additional criteria – Confirm exudate if results equivocal
    • Serum albumin – pleural fluid albumin <1.2g/dL

Further tests

If exudate is confirmed, further testing required to evaluate cause of exudate

  • Differential cell count (predominance of white cells)
    • Neutrophils – PTE, pancreatitis, pneumonia, empyema
    • Lymphocytes – Cancer, TB pleuritis
    • Eosinophila – Pneumothorax, haemothorax, asbestosis, Churg-Strauss
    • Mononuclear cells – Chronic inflammatory process
  • Gram stain and culture and cytology
    • Use blood culture bottles and specimen jars – especially if chronic illness or suspect TB or fungus
    • Cytology useful in cases of suspected malignancy
  • Glucose
    • Low
      • Common: Infection (pneumonia) and malignancy
      • Rare: TB, haemothorax, Churg-Strauss
  • LDH level – This is classically high in exudates
    • Repeated testing confirms continuation or cessation of process
      • Increasing LDH (ongoing inflammation)
      • Decreasing LDH (cessation of process)
  • Pleural fluid pH (Low glucose and pH = infection or malignancy)
    • Taken if suspect pneumonic or malignant process (Low glucose)
    • <7.20 with pneumonia…Drain the fluid
    • <7.20 with malignancy …Life expectancy 30 days
  • Amylase
    • Useful if suspect pancreatitis as cause

CCC Differential Diagnosis Series


Anosmia, Ataxia, Blepharospasm, Bulbar and Pseudobulbar palsy, Central Pontine Myelinosis, Cerebellar Disease, Chorea, Cranial nerve lesions, Dementia, Dystonia, Exophthalmos, Eye trauma, Facial twitches, Fixed dilated pupil, Horner syndrome, Loss of vision, Meningism, Movement disorders, Optic disc abnormality, Parkinsonism, Peripheral neuropathy, Radiculopathy, Red eye, Retinal Haemorrhage, Seizures, Sudden severe headache, Tremor, Tunnel vision


Bronchial breath sounds, Bronchiectasis, High airway pressures, Massive haemoptysis, Sore throat, Tracheal displacement


Atrial Fibrillation, Bradycardia, Cardiac Failure, Chest Pain, Murmurs, Post-resuscitation syndrome, Pulseless Electrical Activity (PEA), Pulsus Paradoxus, Shock, Supraventricular tachycardia (SVT), Tachycardia, VT and VF, SVC Obstruction


Abdominal distension, Abdominal mass, Abdominal pain, Asterixis, Dysphagia, Hepatomegaly, Hepatosplenomegaly, Large bowel obstruction, Liver palpation abnormalities, Lower GI haemorrhage, Malabsorption, Medical causes of abdominal pain, Rectal mass, Small bowel obstruction, Upper GI Haemorrhage


Genital ulcers, Groin lump, Scrotal mass, Urine colour, Urine Odour, Urine transparency


Arthritis, Shoulder pain, Wasting of the small muscles of the hand


Palmar erythema, Serious skin signs in sick patients, Thickened Tethered Skin, Leg ulcers, Skin Tumour, Acanthosis Nigricans


Diabetes Insipidus, Diffuse Goitre, Gynaecomastia, Hirsutism, Hypoglycaemia, SIADH, Weight Loss




Floppy infant 


Anaphylaxis, Autoimmune associated diseases, Clubbing, Parotid Swelling, Splinter haemorrhages, Toxic agents and abnormal vitals, Toxicological causes of cardiac arrest


CHEST: Atelectasis, Hilar adenopathy, Hilar enlargement on CXR, Honeycomb lung, Increased interstitial markings, Mediastinal widening on mobile CXR, Pulmonary fibrosis, Pseudoinfiltrates on CXR, Pulmonary opacities on CXR,
Gas on abdominal X-ray, Kidney mass,
Intracranial calcification, Intracranial structures with contrastVentriculomegaly,
OTHER: Pseudofracture on X-Ray


LOW: Anaemia, Hypocalcaemia, hypochloraemia, Hypomagnesaemia

HIGH: Bilirubin and Jaundice, HyperammonaemiaHypercalcaemia, Hyperchloraemia, Hyperkalaemia, Hypermagnesaemia

ACID BASE: Acid base disorders, Resp. acidosis, Resp. alkalosis,

Creatinine, CRP, Dipstick Urinalysis, Laboratory Urinalysis, Liver function tests (LFTs), Pleural fluid analysis, Urea, Urea Creatinine Ratio, Uric acid, Urinalysis, Urine Electrolytes


CCC 700 6

Critical Care


BA MA (Oxon) MBChB (Edin) FACEM FFSEM. Emergency physician, Sir Charles Gairdner Hospital.  Passion for rugby; medical history; medical education; and asynchronous learning #FOAMed evangelist. Co-founder and CTO of Life in the Fast lane | Eponyms | Books | Twitter |

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