Objective: Diagnosis of the crystal-induced arthritides is primarily based on microscopic identification of crystals in synovial fluid. Therefore, we aimed to estimate the operating characteristics of this test and demonstrate its clinical use.
Methods: Medline was searched for relevant studies. Sensitivity and specificity of identification of crystals were calculated, as were measures of interobserver agreement. Likelihood ratios were calculated and curves constructed using the solutions to the Bayesian equations.
Results: Four studies were identified. The rates of interobserver agreement were low; the false-negative rates in identifying calcium pyrophosphate crystals were particularly high. Only one study allowed calculation of the test operating characteristics, and this was a study that used synthetic crystals and therefore may not be directly useful in a clinical setting.
Conclusion: There is a paucity of data about the accuracy of crystal identification. As it is clearly not a perfectly sensitive and specific test, the most prudent diagnostic strategy, as with essentially all diagnostic tests, is to establish a posterior probability of disease from a prior probability, based on the clinical features of the patient. Determining the operating characteristics of this test in conventional and reference laboratories should be a research priority for high quality clinical research on crystal arthropathies.