How does the synovial fluid WBC differential help in diagnosing an inflammatory arthritis?
• Neutrophil predominance: most inflammatory synovial fluids. Septic arthritis and crystalline arthropathies have >90%–95% polymorphonuclear cells (PMNs).
Ragocytes are neutrophils that have ingested immune complexes: consider rheumatoid arthritis, septic arthritis, and crystalline arthritides when ragocytes account for more than 50%–70% of all nucleated cells in the synovial fluid.
• Lymphocyte predominance (>70%): consider systemic lupus erythematosus and mycobacterial infections.
• Macrophage predominance (>80%): consider spondyloarthropathies, “Milwaukee shoulder.”
Lipid-laden macrophages: traumatic, pancreatic disease.
• Monocyte predominance (>80%): consider viral arthritis, serum sickness, and spondyloarthropathies.
• Eosinophil predominance: hypereosinophilia syndrome, parasitic arthritides, arthrography (dye), therapeutic radiation, metastatic adenocarcinoma, idiopathic.
• Mast cells present: consider spondyloarthropathies and systemic mastocytosis.