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 degrees 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 -80 degrees C and examined after two months. RESULTS: When the samples stored at 4 degrees C 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 (100%) and 87/90 (97%) cases after 24 and 72 hours. Examination of the samples stored at -80 degrees C 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.     

Detection of crystals in synovial fluids by light microscopy: sensitivity and reliability.

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.     

The ordinary light microscope: an appropriate tool for provisional detection and identification of crystals in synovial fluid.

To determine if the ordinary light microscope is a useful tool in the detection and identification of crystals in synovial fluid 63 fluids (13 with monosodium urate (MSU), 14 with calcium pyrophosphate dihydrate (CPPD), one with both types of crystal, and 35 without crystals) were examined blindly by two observers, first with an ordinary light microscope, then with a polarising light microscope. Tentative identification of the crystals by their shape was attempted. The sensitivity of the ordinary light microscope for the detection of crystals was 96.2% and 100%, with specificity of 100% and 97.1% for observers 1 and 2, both with respect to the polarising light microscope. The concordance between observers for the ordinary light microscope was chi = 0.90 (0.897, 0.902) and for the polarising light microscope chi = 0.96 (0.958, 0.961). The specificity for identification of MSU and CPPD crystals by their shape with respect to the polarising light microscope was 92.3% and 85.7% for observers 1 and 2. When a polarising light microscope is not available examination of synovial fluid by an ordinary light microscope may allow a provisional diagnosis; definitive identification of the crystals requires a polarising light microscope.