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Level of agreement with the microscopic analysis of joint aspirate for the diagnosis of gout in the lower extremity.

BACKGROUND
Although clinical findings, laboratory serum markers, and radiographic images are also utilized, the purported "gold standard" or "standard reference" test for the diagnosis of gout is the microscopic analysis of aspirated joint fluid. The objective of this retrospective, observational investigation was to identify the level of agreement with the microscopic analysis of joint fluid aspirate when evaluated in the setting of suspected lower extremity gout between treating rheumatologists and pathologists within a single institution.


METHODS
A retrospective chart review was performed to identify consecutive subjects seen in consultation by our institution's Department of Rheumatology for suspected gout with performance of a diagnostic joint aspiration. Subjects were included if a synovial fluid sample was obtained from a lower extremity joint and this specimen was independently evaluated by treating physicians from both the Departments of Rheumatology and Pathology. We categorized the documented joint fluid findings into four groups: "no crystals", "sodium urate crystals", "calcium pyrophosphate dihydrate crystals", or "both sodium urate crystals and calcium pyrophosphate dihydrate crystals". We further defined a "crystal disagreement" as when one treating physician observed any type of crystals while the other treating physician observed no crystals.


RESULTS
A rate of disagreement with respect to the four categories of 39.53% was observed with a corresponding intraclass correlation coefficient of 0.670. We observed what we considered to be a crystal disagreement rate of 23.26% with a corresponding intraclass correlation coefficient of 0.496. Rheumatologists were more likely to observe the presence of crystals in a sample when compared to pathologists (88.37% vs. 65.12%; p = 0.02).


CONCLUSIONS
The results of this investigation provides evidence that microscopic analysis of joint fluid aspirate might lack the accuracy and reliability to be considered a gold standard diagnostic test for gout.​.