Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (597 page)

   Gross appearance

   Clear, straw-colored fluid is typical of transudate.

   Turbidity (cloudy, opaque appearance) may be caused by lipids or increased WBCs; after centrifugation, a clear supernatant indicates WBCs or debris as the cause; clear or white supernatant is caused by chylomicrons.

   Red indicates blood; brown indicates blood has been present for a longer time. RBC count of 5,000–10,000/μL causes a blood-tinged color. If grossly bloody, Hct >50% of peripheral Hct indicates a hemothorax.

   Bloody fluid suggests malignancy, pulmonary infarct, trauma, postcardiotomy syndrome; also uremia, asbestos, pleural endometriosis. Bloody fluid from traumatic thoracentesis should clot within several minutes, but blood present more than several hours will have become defibrinated and does not form a good clot. Nonuniform color during aspiration and absence of hemosiderin-laden macrophages also suggest traumatic aspiration. Absent of platelets suggests that the condition is not caused by traumatic thoracentesis.

   White fluid suggests chylothorax, cholesterol effusion, or empyema.

   Chylous (milky) is usually due to trauma (e.g., auto accident, postoperative) but may be obstruction of duct (e.g., especially lymphoma; metastatic carcinoma, granulomas) or parenteral nutrition via a central line with perforation of superior vena cava.

   After centrifugation, supernatant is clear in empyema but cloudy or turbid in chylous effusion caused by chylomicrons, which also stain with Sudan III.

   Pleural fluid triglycerides >110 mg/dL or triglyceride pleural fluid-toserum ratio >2 occurs only in chylous effusion (seen especially within a few hours after eating). Triglycerides <50 mg/dL excludes chylothorax. Equivocal triglyceride levels (50–10 mg/dL) may require a lipoprotein electrophoresis of fluid to demonstrate chylomicrons, which are diagnostic of chylothorax.

   Pseudochylous (may have lustrous sheen) appearance in chronic inflammatory conditions (e.g., rheumatoid pleurisy, TB, chronic pneumothorax therapy) is caused by either cholesterol crystals (rhomboid-shaped) in sediment or lipid-containing inclusions in leukocytes. Distinguish from chylous effusions by microscopy. Chylomicrons ≤50 mg/dL with cholesterol >250 mg/dL occurs in pseudochylous effusions.

   Black fluid suggests
Aspergillus niger
infection.

   Greenish fluid suggests biliopleural fistula.