Purulent fluid indicates infection.
Anchovy (dark red-brown) color is seen in amoebiasis, old blood. Anchovy paste in ruptured amebic liver abscess; amebas found in <10%.
Turbid and greenish-yellow fluid is classic for rheumatoid effusion.
Very viscous (clear or bloody) is characteristic of mesothelioma; also in pyothorax.
Debris in fluid suggests rheumatoid pleurisy; food particles indicate esophageal rupture.
Color of enteral tube food or central venous line infusion due to tube or catheter entering pleural space.
Odor
Putrid due to anaerobic empyema.
Ammonia due to urinothorax.
Protein, Albumin, Lactate Dehydrogenase
When exudate criteria are met by LD but not by protein, consider malignancy and parapneumonic effusions.
Very high pleural fluid LD (>1,000 IU/L) occurs in empyema, rheumatoid pleurisy, paragonimiasis; sometimes with malignancy; rarely with TB. Level indicates degree of pleural inflammation; increasing values suggest need for more aggressive therapy. Measurement of LD isoenzymes is said to have limited value.
Glucose
Transudate has same concentration as serum.
Usually normal but 30–55 mg/dL or pleural fluid-to-serum ratio <0.5 and pH <7.30 may be found in TB, malignancy, SLE; also esophageal rupture; lowest levels may occur in empyema and RA. Therefore, only helpful if very low level (e.g., <30). A level of 0–10 mg/dL is highly suspicious for RA. Poor prognostic sign in pneumonia. In neoplasm, lower glucose indicates greater tumor burden. Rarely found in SLE, Churg-Strauss, urinothorax, hemothorax, or paragonimiasis.
pH
Normal pleural fluid pH is alkaline (7.60–7.66). Transudative effusions have a pH range of about 7.45–7.55, and most exudates have a pH of 7.30–7.45.
