Pleural and Ascitic Fluid Analysis
On this page
Direct answer
Fluid analysis classifies an effusion first as transudate or exudate, then pursues the cause within that half. For pleural fluid, Light's criteria — pleural-to-serum protein ratio above 0.5, pleural-to-serum LDH ratio above 0.6, or pleural LDH above two-thirds of the upper normal serum limit — label an exudate if any one is met. For ascites, the serum-ascites albumin gradient (SAAG) replaces them: a gradient of 1.1 g/dL or more indicates portal hypertension with over 95 per cent accuracy, while a low gradient points to tuberculous peritonitis, malignancy or pancreatic disease. In Indian practice the lymphocytic exudate with high adenosine deaminase is the tuberculous signature, and a polymorph count of 250 per microlitre or more in cirrhotic ascites diagnoses spontaneous bacterial peritonitis.
What you must remember
- Light's criteria: protein ratio over 0.5, LDH ratio over 0.6, or absolute pleural LDH over two-thirds of the serum upper limit — any single criterion makes an exudate.
- Diuretic caveat: diuretic-treated cardiac failure can masquerade as an exudate; a serum-pleural albumin gradient above 1.2 g/dL then unmasks the transudate.
- Pleural infection rules: a parapneumonic effusion with pH below 7.2, glucose under 60 mg/dL or LDH above 1000 U/L needs tube drainage; frank pus is an empyema.
- Tuberculous pleuritis: lymphocyte-predominant exudate with ADA commonly above 40 U/L (pericardial and peritoneal cutoffs near 35-40), confirmed by cartridge-based molecular testing or biopsy; India's bread-and-butter effusion.
- Malignant effusion: haemorrhagic or straw-coloured exudate, cytology positive in roughly half on first sample, closed pleural biopsy (Abrams needle) raises the yield.
- SAAG rule: 1.1 g/dL or above — portal hypertension (cirrhosis, heart failure, Budd-Chiari); below 1.1 — tuberculosis, peritoneal carcinomatosis, nephrotic syndrome, pancreatitis.
- SBP definition: ascitic neutrophil count 250 per microlitre or more without a surgical source, in a cirrhotic — treat empirically, no polymorph excuse; a chylous effusion (triglycerides above 110 mg/dL) suggests lymphatic disruption by malignancy, tuberculosis or trauma.
Two taps that diverge on one gradient
A man with cirrhosis and increasing abdominal girth undergoes diagnostic paracentesis: the fluid is straw-coloured, protein 2.0 g/dL, and SAAG 1.7 — portal hypertensive ascites, and the cell count matters more than the chemistry: 300 neutrophils per microlitre makes this spontaneous bacterial peritonitis, starting third-generation cephalosporins without waiting for culture. Now a 25-year-old woman with low-grade fever and vague abdominal pain: her ascites is lymphocytic with protein 4.5 g/dL, SAAG 0.8 and ADA 52 — a low gradient excludes portal hypertension and the high ADA with lymphocytosis argues strongly for tuberculous peritonitis, to be confirmed by peritoneal biopsy or molecular testing before antitubercular therapy. One number, the gradient, sends the two patients to different departments entirely.
Where students slip
The Light's criteria are quoted as "all three" when it is any one; and the absolute LDH criterion (two-thirds of the upper limit of normal, often written as about two-thirds ULN) is confused with a fixed 200 U/L. SAAG is computed with serum minus ascitic albumin — candidates subtract in reverse. The tuberculous question in Indian exams is answered by cytology alone; ADA (with its cutoffs) plus molecular confirmation is the expected complete answer. Finally, remember that transudate-versus-exudate is a first fork, not a diagnosis — the cell count, cytology and chemistry within each half carry the real answer.
Frequently asked questions
What are Light's criteria for an exudative pleural effusion?
Pleural/serum protein above 0.5, pleural/serum LDH above 0.6, or pleural LDH above two-thirds of the serum upper limit — any one criterion suffices.
What does a SAAG of 1.1 g/dL or more indicate?
Portal hypertension as the mechanism of ascites — cirrhosis, cardiac failure or hepatic venous obstruction — with high diagnostic accuracy.
Which pleural fluid values mandate chest drainage?
A parapneumonic effusion with pH below 7.2, glucose under 60 mg/dL, LDH above 1000 U/L or frankly purulent fluid.
What defines spontaneous bacterial peritonitis?
An ascitic neutrophil count of 250 per microlitre or more in cirrhosis without a surgical source, treated with empirical antibiotics after culture.
How does tuberculous effusion profile read?
A lymphocyte-rich exudate with adenosine deaminase commonly above 40 U/L (around 35-40 for peritoneal fluid), confirmed by molecular testing or biopsy.