Counting synovial fluid monosodium urate crystals may be useful in the management of patients with gout

Background: Gout is the most common form of arthritis, and its prevalence is increasing. The mainstay of its diagnosis is the identification of monosodium urate (MSU) crystals in the synovial fluid (SF) (1,2). In a recent study we tested a feasible and reliable technique for counting MSU crystals in the SF (3). This method could provide useful information for the management of patients with gout. Objectives: To investigate correlations between MSU crystals count in SF and clinical characteristics of gout patients. Methods: Forty-four consecutive patients (6 women) affected by gout diagnosed according to the ACR preliminary criteria were studied. Their SFs were examined: for crystal count, 20 ml of fresh SF were placed on a microscope slide and examined by compensated polarized microscopy (400x). The slide was divided into 4 equal parts drawing a cross with a pencil. Crystal count was obtained by continued viewing and in each quadrant the crystals were counted up to a maximum of 400 (maximum number of crystals/patient was 1600). Clinical information included: acute inflammation at the time of SF aspiration, disease duration, number of attacks, number of affected joints, presence of radiographic erosions, presence of tophi, chronic renal failure, type of antiinflammatory and urate lowering treatment (ULT), ESR, CRP, creatinine and serum uric acid (SUA). In five patients, crystal count was repeated after ULT. Results: Mean age was 65.5±11.7 yrs and median disease duration was 6 years (range 0.1-35 years). Aspirated joints were the knee (40 SFs), elbow (1 SF), 1st metatarsophalangeal joint (2 SFs), and ankle (1 SF). Median crystal count was 273 (range 3-1600). Joints with recent (<1 week) inflammation tended to have a higher SF crystal count than the asymptomatic ones (491 [range 3-1600] vs. 100 [range 4-1600]; p=0.1). The number of crystals in the SF correlated with SUA (p=0.04), but not with ESR and CRP. No correlations were found with clinical features. In the 5 patients on ULT, mean MSU crystals count decreased from 871 to 473 (p=0.05). Conclusions: SF MSU crystals count is not associated with the clinical features of gout, correlates with SUA, and decreases with ULT.