exudate

Exudate forms during acute or chronic inflammation when endothelial cell junctions loosen (mediated by histamine, bradykinin, leukotrienes) or vascular injury occurs, allowing plasma proteins (especially fibrinogen and immunoglobulins) and leukocytes to escape into the interstitium or serous cavities. Because vascular permeability increases, exudates are protein-rich (>3 g/dL), have a specific gravity >1.020, and often contain inflammatory cells (neutrophils, lymphocytes) and sometimes fibrin, giving them a cloudy or purulent appearance. Common clinical contexts include pleural effusions from pneumonia, empyema, malignancy, or tuberculosis; peritoneal exudates from peritonitis; and pericardial exudates from pericarditis. Light's criteria are used clinically to distinguish exudative from transudative pleural effusions: exudate is suggested by pleural fluid protein/serum protein ratio >0.5, pleural fluid LDH/serum LDH ratio >0.6, or pleural fluid LDH greater than two-thirds the upper limit of normal serum LDH. In contrast, transudates (e.g., from CHF, cirrhosis, nephrotic syndrome) result from increased hydrostatic pressure or decreased oncotic pressure without primary increased vascular permeability, and are low in protein and cells. Exudates can be further classified by predominant component: serous (clear, low cellularity), fibrinous (fibrin-rich, seen in pericarditis producing a 'bread and butter' appearance), purulent/suppurative (neutrophil-rich, as in abscess or bacterial infection), or hemorrhagic (blood-rich, suggesting malignancy or severe tissue damage). Recognizing exudate vs. transudate is a classic clinical reasoning exercise on Step 1 and Step 2, and analysis of effusion fluid characteristics is a key diagnostic tool for identifying underlying etiology (infectious, inflammatory, malignant, or mechanical).