Delayed examination of synovial fluid by ordinary and polarised light microscopy to detect and identify crystals

Objective: To determine the reliability of a delay in the microscopic examination of synovial fluid (SF) to detect and identify crystals. Methods: Ninety one SF samples were examined, 31 with monosodium urate (MSU) crystals, 30 with crystals of calcium pyrophosphate dihydrate (CPPD), and 30 containing no crystals. The specimens were stored with EDTA, sodium heparin, and without anticoagulant at 4 C before examination at 24 and 72 hours with ordinary and polarised light microscopy. Another aliquot of the same samples was stored in a plastic container without anticoagulant at -80degreesC and examined after two months. Results: When the samples stored at 4degreesC were re-examined after 24 hours, intracellular crystals of MSU were seen in 90/93 (97%) cases where they had been identified previously and 89/93 (96%) cases after 72 hours. Similarly, CPPD crystals were identified in 90/90 and 87/90 (100%) (97%.) cases after 24 and 72 hours. Examination of the samples stored at -80degreesC showed intracellular MSU crystals in 25/31 (81%) of cases and CPPD crystals in 25/30 (83%). No crystals were seen in any sample which had previously been diagnosed as crystal-free. Conclusions: Deferred microscopic examination of refrigerated or deep frozen SF provides a strong probability of detecting MSU or CPPD crystals if these are present initially.