Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (881 page)

Source: Judkins SW, Cornbleet PJ. Synovial fluid crystal analysis.
Lab Med.
1997;28:774. With permission from American Society for Clinical Pathology and ASCP Press.

   Use

   According to the American College of Radiology, synovial fluid analysis should be undertaken in the febrile patient with an acute flare of established arthritis (e.g., RA, osteoarthritis) to rule out superimposed septic arthritis.

   Repeated aspiration and synovial fluid analysis may be used to monitor the response of septic arthritis to treatment and may also be valuable for diagnosis of some cases of gout in which the initial aspirate does not have detectable crystals.

   Interpretation

   Positive identification of crystals provides a definitive diagnosis of joint disease.

   Limitations

   Powdered anticoagulants such as oxalate are themselves crystalline; their use may cause confusion, masking the presence of synovial fluid crystals definitive for the disease.

   Substantial variability has been noted among hospital laboratories in the ability to properly identify the presence or absence of MSU and CPPD crystals in synovial fluids. Studies of the performance of different hospital laboratories on the same synovial fluids suggest that MSU crystals are more easily detected than CPPD crystals.

   MSU crystals: reported sensitivity ranges from 63 to 78%; specificity from 93 to 100% (positive likelihood ratio of 14 for a diagnosis of gout).

   CPPD crystals: reported sensitivity ranges from 12 to 83%; specificity from 78 to 96% (positive likelihood ratio of 2.9 for a diagnosis of CPPD-associated arthritis).

   The stability of crystals in synovial fluids is studied by many at different temperatures. CPPD crystals dissolved significantly, and MSU crystals were detectable up to weeks but became smaller and less numerous. As storage time increased, new artificial crystals developed in the form of star-shaped arrays, plate-like structures, and positive birefringent Maltese crosses. Synovial fluid should be evaluated within 1 hour of collection.

CYCLIC CITRULLINATED PEPTIDE ANTIBODY, IgG

   Definition