Polarised light microscopy of synovial fluid is an established diagnostic technique widely regarded as reliable for the detection of crystals. The threshold concentration of crystals which can be detected has been investigated and the sensitivity and specificity of six observers compared. Various concentrations of laboratory manufactured crystals of monosodium urate monohydrate (MSUM), calcium pyrophosphate dihydrate (CPPD), and basic calcium phosphates (BCP) were added to the synovial fluid. The threshold for reliable identification of MSUM and CPPD was in the range of 10-100 micrograms/ml. False positives were frequent. The mean sensitivity of the six observers for MSUM was 69% and for CPPD was 82%. The mean specificity for MSUM was 97% and for CPPD was 78%. There was much discrepancy in the results of the slides stained with alizarin red S. Thus the value of alizarin red S as a screening test for BCP is questioned. In view of the variable sensitivity and specificity of different observers for MSUM and CPPD and the concentration threshold for reliable crystal identification, greater caution in the interpretation of synovial fluid analysis is advised, and recommendations for increased quality control are supported.
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