title: Diagnosis of crystal-induced arthritis by synovial fluid examination for crystals: lessons from an imperfect test.
abstract: 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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Division of General Internal Medicine, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA.
authors.forename: J B
authors.initials: JB
authors.lastname: Segal
authors.forename: D
authors.initials: D
authors.lastname: Albert
mesh_terms: D000328:Adult; D000368:Aged; D001499:Bayes Theorem; D002805:Chondrocalcinosis / Q000175:diagnosis*; D005188:False Negative Reactions; D005189:False Positive Reactions; D005260:Female; D006073:Gout / Q000175:diagnosis*; D006801:Humans; D008297:Male; D015588:Observer Variation; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013582:Synovial Fluid / Q000166:cytology*