Gout Crystal Type
The crystals found in synovial fluid of patients with gout are monosodium urate (MSU) crystals, which demonstrate negative birefringence under polarized light microscopy and serve as the gold standard for definitive diagnosis. 1
Diagnostic Characteristics of MSU Crystals
Detection of MSU crystals has excellent diagnostic value with a sensitivity of 0.84 (95% CI, 0.77 to 0.92) and extremely high specificity of 1.00 (0.99 to 1.00) during acute gout attacks. 1 The likelihood ratio is remarkably high at 566.60 (35% CI, 35.46 to 9053.50), making crystal identification virtually pathognomonic for gout when present. 1
Key Crystal Properties
MSU crystals appear as needle-shaped structures that are negatively birefringent under polarized light microscopy, distinguishing them from calcium pyrophosphate crystals seen in pseudogout. 2, 3
The crystals form when monosodium urate precipitates in joint spaces due to hyperuricemia, though the presence of hyperuricemia alone does not confirm gout, as many hyperuricemic patients never develop crystal deposition. 1
The 3D refractive index of MSU crystals ranges between 1.383 and 1.440, which can be measured using advanced optical diffraction tomography. 4
Clinical Application and Timing
MSU crystals can be identified in both symptomatic and asymptomatic joints. During intercritical periods (between acute attacks), synovial fluid from asymptomatic joints shows MSU crystals in approximately 70% of patients with proven gout, even in joints that have never experienced an acute attack. 1
Diagnostic Recommendations
Routine search for MSU crystals is recommended in all synovial fluid samples from undiagnosed inflamed joints (strength of recommendation: 90,95% CI 83-97), as gout is a prevalent cause of joint inflammation and may present atypically. 1
Joint aspiration with crystal identification remains the gold standard for diagnosis, though clinical algorithms can achieve sensitivities and specificities greater than 80% when synovial fluid analysis is not feasible. 5
Critical Pitfall to Avoid
Always perform synovial fluid Gram stain and culture when septic arthritis is suspected, even if MSU crystals are identified. Coexistent septic arthritis and gout can occur in the same joint—among 30 reported cases, 73% had positive synovial fluid cultures, and outcomes included one above-knee amputation and two deaths. 1 The discriminative value of synovial fluid culture between septic arthritis and gout shows sensitivity of 0.76 and specificity of 0.96, making it essential not to miss this treatable cause of rapid joint destruction. 1
Observer Variability Consideration
Crystal identification results may vary between observers and laboratories (level Ib evidence), which has important implications for training and quality control. 1 This variability underscores the importance of experienced personnel performing the microscopic examination.