Background/Purpose:

Gout is the most common form of arthritis, and its prevalence is increasing. The identification of monosodium urate (MSU) crystals in synovial fluid (SF) or in an aspirate of a tophus is considered to be the gold standard for the definitive diagnosis of gout (1,2). MSU crystal disappearance from SF depends on the reduction of serum uric acid concentrations and is associated with a decrease of joint inflammatory episodes (3). In addition to diagnostic purposes, SF examination, in particular MSU crystals counting, could be used to evaluate the efficacy of therapy.

Aim of the study is to test an objective method for counting MSU crystals in the SF.

Methods: The SFs of 44 consecutive patients affected by gout diagnosed according to the ACR preliminary criteria were studied. Six of the patients underwent multiple SF aspirations for a total of 52 SFs. SF aspirated was divided into two test tubes, for cytological evaluation and crystal detection, respectively. Cytological evaluation included leukocyte and differential count. For crystal detection, 20 μl of fresh SF were placed on a microscope slide and examined by compensated polarized microscopy (400x). For crystal count, the slide was divided into 4 equal parts drawing a cross with a pencil. The method of count was performed by continued viewing and in each quadrant the crystals were counted up to a maximum of 400 (maximum number of crystals/patient was 1600). Two observers evaluated separately 27 SFs and repeated the count in 21 SFs after 24 hours.  

Results: Forty of forty-four patients (90.9%) were men; mean age was 65.2±11.8 yrs. Aspirated joints were the knee (48 SFs), elbow (1 SF), 1^st metatarsophalangeal joint (2 SFs), and ankle (1 SF). The volume of SF ranged between 0.1 and 45 ml (median 3 ml). Median leukocyte count was 400 cells/μl (range 50-14.000 cells/μl), median percentage of PMN was 9% (range 0%-98%), median crystal count was 179.5 (range 3-1600). Inter-reader agreement was high, with a weighted k of 0.89 (95% CI 0.85-0.94) for the first examination and 0.86 (95% CI 0.80-0.92) for the second one. Intra-reader concordance was also high with a k of 0.89 (95% CI 0.84-0.93) for the first observer, and of 0.85 (95% CI 0.78-0.93) for the second. The intraclass correlation coefficient for the 4 readings was 0.998 (95% CI 0.996-0.999). Maximum time needed for the count was 30 minutes. Number of crystals in the SF did not correlate with the amount of SF (p=0.15), nor with leukocyte count (p=0.52), percentage of PMN (p=0.69), patients’ gender (p=0.46), or age (p=0.89). Interestingly, joints with recent (< 1 week) inflammatory reaction had a higher SF crystal count than the asymptomatic ones (460.5 [range 3-1600] vs. 48.5 [range 4-1600], p=0.03).

Conclusion: MSU crystal count in the SF of gout patients is a feasible and highly reliable technique. It could be used to follow the efficacy of urate lowering treatments within the joint, the location where the noxius effect of MSU crystals is more evident.

References: 1. Doherty M. Rheumatology 2009; 48:ii2-ii8. 2. Pascual E et al. Current Opinion in Rheumatology 2011; 23:161-69. 3. Pascual E et al. Ann Rheum Dis 2007; 66: 1056-58.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/a-method-for-counting-monosodium-urate-crystals-in-the-synovial-fluid-of-gout-patients/