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
AppearanceNext TestInterpretation
BloodyHaematocrit<1% = not significant
1–20% = cancer, PE, trauma
>50% = haemothorax
Cloudy or turbidTriglycerides>110 mg/dL = chylothorax
Putrid odourMCSSuggests 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

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 contrastVentriculomegaly,
OTHER: Pseudofracture on X-Ray

LABS

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

Compendium

Dr Caitlin Rigler LITFL Author
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 |

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