Synovial fluid lipid abnormalities in various disease states: review and classification

Although lipids are not usually present in large quantities in normal synovial fluids or in the usual synovial fluid seen in most rheumatologic conditions, their presence in synovial fluid may have diagnostic importance. As summarized in Table 2, analysis of synovial fluids for lipid constituents is relatively simple. On standing or after centrifugation, significant amounts of lipids may layer out and be visible as a supernatant. On microscopic examination, lipid droplets are usually easily seen and should be stainable with appropriate dyes (oil red O or Sudan III or IV), or may occasionally be visualized as intracellular or extracellular inclusions by polarized microscopy. Small (0.5 to 2.0 microns) intracellular inclusions containing triglycerides may be seen in neutrophils from most synovial fluids, and are of no diagnostic importance. Cholesterol crystals may be readily recognized microscopically by their characteristic flat, plate-like appearance and notched corners Synovial fluid may also be analyzed for cholesterol and triglycerides in routine clinical laboratories, and free fatty acids and lipolytic enzymes in special lipid laboratories. The presence of massive increases in cholesterol associated with microscopically visible cholesterol crystals is usually associated with chronic RA, occasionally in the setting of super-imposed bacterial infection. The presence of gross or microscopic lipid droplets is usually associated with trauma and hemorrhagic effusions. When present in this setting, the clinician should entertain a high suspicion for a significant intraarticular injury, such as fracture, meniscal tear, or severe ligamentous injury. In addition, however, several instances of non-traumatic inflammatory effusions associated with intracellular and extracellular lipid droplets have been reported.(ABSTRACT TRUNCATED AT 250 WORDS)