Pleural Fluid Analysis

A pleural effusion is a collection of fluid in the pleural space, resulting from:

Increased fluid accumulation
Decreased lymphatic clearance
Obstruction to drainage
Increased venous pressure

Most common causes: CCF, infection (e.g. pneumonia), and malignancy. Diagnostic sampling is performed via pleural tap (thoracocentesis).

Exudate vs Transudate

Exudates (local disease; high protein)

Local factors influence accumulation/clearance. Common causes:

Malignancy: lung, breast, pleura
Infection: pneumonia, empyema, viral pleuritis
Autoimmune: RA, SLE
Vascular: PE
Cardiac: pericarditis, post-CABG
Respiratory: haemothorax, chylothorax
Abdominal: subphrenic abscess

Transudates (systemic illness; low protein <30 g/L)

Caused by imbalance in oncotic/hydrostatic pressure. Common causes:

Cardiac: CCF, PE
Liver: cirrhosis, ascites
Renal: glomerulonephritis, nephrotic syndrome
Ovarian: Meigs syndrome
Autoimmune: sarcoidosis
Thyroid: myxoedema

Differentiation: Light’s Criteria

Used to differentiate transudates from exudates. Exudate if any of the following are met:

Pleural fluid protein/serum protein ratio > 0.5
Pleural fluid LDH/serum LDH ratio > 0.6
Pleural fluid LDH > 2/3 of upper limit of normal serum LDH

Additional criteria (if equivocal)

Serum albumin – pleural fluid albumin gradient < 1.2 g/dL suggests exudate

Pleural Fluid Characteristics

Appearance	Next Test	Interpretation
Bloody	Haematocrit	<1% = not significant 1–20% = cancer, PE, trauma >50% = haemothorax
Cloudy or turbid	Triglycerides	>110 mg/dL = chylothorax
Putrid odour	MCS	Suggests anaerobic infection

Pleural Fluid Lab Tests

Differential Cell Count

Neutrophils: pneumonia, PE, pancreatitis, empyema
Lymphocytes: TB, cancer
Eosinophils: pneumothorax, haemothorax, asbestosis, Churg-Strauss
Mononuclear cells: chronic inflammatory states

Cytology and Microbiology

Send in blood culture bottles and sterile containers
Gram stain, culture, and cytology if TB, fungal infection, or malignancy suspected

Glucose

Low in: pneumonia, malignancy
Rarely low in: TB, haemothorax, Churg-Strauss

LDH

Typically high in exudates
Trends help monitor disease activity:
- Rising = ongoing inflammation
- Falling = resolving process

pH

Assessed in suspected pneumonia or malignancy
<7.20 with pneumonia: indicates need for drainage
<7.20 with malignancy: poor prognosis (30-day survival)

Amylase

Elevated in pleural effusion secondary to pancreatitis

References and Links

CCC – Pleural effusion
CCC – Pleural tap

CCC Differential Diagnosis Series
NEURO	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
RESP	Bronchial breath sounds, Bronchiectasis, High airway pressures, Massive haemoptysis, Sore throat, Tracheal displacement
CVS	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
GIT	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
GUT	Genital ulcers, Groin lump, Scrotal mass, Urine colour, Urine Odour, Urine transparency,
MSK	Arthritis, Shoulder pain, Wasting of the small muscles of the hand
DERM	Palmar erythema, Serious skin signs in sick patients, Thickened Tethered Skin, Leg ulcers, Skin Tumour, Acanthosis Nigricans
ENDO	Diabetes Insipidus, Diffuse Goitre, Gynaecomastia, Hirsutism, Hypoglycaemia, SIADH, Weight Loss
HAEM	Splenomegaly
PAEDS	Floppy infant
MISC	Anaphylaxis, Autoimmune associated diseases, Clubbing, Parotid Swelling, Splinter haemorrhages, Toxic agents and abnormal vitals, Toxicological causes of cardiac arrest
IMAGING	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, ABDO: Gas on abdominal X-ray, Kidney mass, BRAIN: Intracranial calcification, Intracranial structures with contrast, Ventriculomegaly, OTHER: Pseudofracture on X-Ray,
LABS	LOW: Anaemia, Hypocalcaemia, hypochloraemia, Hypomagnesaemia, HIGH: Bilirubin and Jaundice, Hyperammonaemia, Hypercalcaemia, 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

Critical Care

Compendium

…more CCC

Caitlin Rigler

BA, BM BCh University of Oxford, PgCert (Medical Education)University of Dundee. Aspiring medical physician with a particular interest in respiratory medicine, public health and medical education

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 | On Call: Principles and Protocol 4e| Eponyms | Books |