Prevalence of calcium pyrophosphate and monosodium urate crystals in synovial fluid of patients with previously diagnosed joint diseases.

OBJECTIVE: The main aim of this study was to investigate the frequency of monosodium urate (MSU) and calcium pyrophosphate (CPP) crystals in synovial fluid (SF) obtained from patients with previously diagnosed joint diseases. METHODS: We reviewed the results of SF analysis of 5020 samples identifying those collected from patients with a previously definite diagnosis (2370 samples). SF analysis results, age, sex, diagnosis and disease duration were recorded from computerized records of patients' archives. RESULTS: The prevalence of CPP crystals in SF was 22.28% in osteoarthritis (OA), 8.28% in rheumatoid arthritis (RA), 3.82% in psoriatic arthritis (PsA), 2.79% in other spondyloarthropathies (SpA), 10% in septic arthritis (SeA), 0.66% in gout and 9.18% in the miscellanea of joint diseases, respectively. The prevalence of MSU crystals in SF was 0.30% in RA, 3.34% in PsA, 0.70% in other SpA, 0.80% in acute CPP crystal arthritis (CPP-CA), 0% in OA, reactive arthritis (ReA), SeA, juvenile idiopathic arthritis (JIA) and miscellanea. In OA group, we observed that age and SF inflammatory indices were higher in SF positive to CPP crystals with respect to those without crystals (P<0.0001). In RA, we found that the group of patients with CPP crystals was significantly older (P=0.001) and had a SF less inflammatory (P=0.022) with respect to that without crystals but with a higher disease duration than those without crystals. CONCLUSION: Crystals can be detected more frequently than expected in SF from joint diseases with a previous established diagnosis. This highlights the importance of SF analysis for the diagnosis of possible comorbidities linked to the presence of crystals.