The accuracy of methods for urate crystal detection in synovial fluid and the effect of sample handling: a systematic review

This study aims to compare different methods of monosodium urate crystal (MSU) detection in synovial fluid (SF) and the effect of sample storage and handling on crystal detection. A systematic literature search was performed in MEDLINE, EMBASE, the Cochrane Library and the American College of Rheumatology/European League Against Rheumatism conference abstracts of 2010 and 2011. Studies that compared a method for detecting MSU crystals in SF with polarised light microscopy (PLM) or compared various SF storage and handling factors with the detection of MSU crystals as an outcome were included. Twelve studies out of 247 identified references were included in the review. Seven studies compared different methods of MSU crystal detection in SF with PLM. Due to study heterogeneity, methodological limitations and risk of bias, no firm conclusions could be drawn from the available data. Five studies examining SF storage and handling factors were identified. A reduction in MSU crystal concentration was observed over time at room temperature that was not seen in refrigerated samples. The use of anticoagulation as a storage medium provided no benefit. Dried cytospin preparations appeared to be a suitable medium for long-term storage and delayed crystal analysis for at least 12 months. The existing data do not provide a compelling argument for the replacement of PLM as the current standard. SF sample storage and handling have an effect on MSU crystals and may impact on the reliability of analysis.