Joint aspiration findings can help narrow the differential diagnosis of joint pain. A synovial fluid leukocyte count of less than 2,000 cells/mL is seen in patients with noninflammatory conditions such as osteoarthritis. Leukocyte counts between 2,000 and 75,000 cells/mL are seen in inflammatory conditions like rheumatoid arthritis and gout. Counts above 100,000 cells/mL suggest infectious arthritis.
| **Normal ** | Noninflammatory (eg, OA) | Inflammatory (eg, crystals, RA) | Septic joint | |
| Appearance | Clear | Clear | Translucent or Opaque | Opaque |
| **WBCs **(mm3) | <200 | 200-2,000 | 2,000-100,000 | 50,000-150,000 |
| PMNs | <25% | 25% | Often >50% | >80%-90% |