Pleural & ascitic fluid — transudate vs exudate
What it means
Pleural: protein >30 g/L = exudate (if borderline 25–35 use Light's criteria: fluid:serum protein >0.5, fluid:serum LDH >0.6, or fluid LDH >⅔ upper serum limit). Exudate = leaky capillaries (infection/parapneumonic, malignancy, TB, PE, pancreatitis, RA/SLE). Transudate (<30 g/L) = pressure imbalance (heart failure, cirrhosis, hypoalbuminaemia/nephrotic, hypothyroidism). Low pleural glucose/pH → empyema/RA/TB/malignancy; high amylase → pancreatitis/oesophageal rupture. Ascites uses SAAG (serum − ascites albumin): ≥11 g/L = portal hypertension (cirrhosis, HF, Budd–Chiari); <11 = exudative (malignancy, TB, pancreatitis).
Worked example
An ex-smoker with haemoptysis and a pleural aspirate protein of 56 g/L → EXUDATE → malignancy, NOT heart failure (which gives a transudate).
In the exam
A 62-year-old ex-smoker with haemoptysis has a moderate pleural effusion; the aspirate protein is 56 g/L.
What settles it
Exudate (protein >30 / Light's) = inflammation/malignancy/infection; transudate = HF/cirrhosis/hypoalbuminaemia. SAAG ≥11 does the same job for ascites (portal hypertension).
Source: BTS pleural disease guideline; EASL ascites