David Harris (1), Douglas White (1,2), Uri Arad (1,2)
Affiliation(s):
1. Rheumatology Department, Waikato Hospital, Hamilton, New Zealand;
2. Waikato Clinical School, University of Auckland, Hamilton, New Zealand.
Objective: Acute calcium pyrophosphate (CPP) crystal arthritis is characterised by the presence of CPP crystals in the synovial fluid of a clinically inflamed joint. CPP crystals can be found in an intracellular or extracellular location; however, no studies have specifically assessed the importance of the CPP crystal location in the clinical setting. The objective of this retrospective cohort study was to assess the relevance of the CPP crystal location in the diagnostic work up of acute CPP crystal arthritis.
Methods: Data was collected from Waikato District Health Board (WDHB) to identify a study population with synovial fluid samples positive for CPP crystals. Two cohorts were identified, one with intracellular and one with extracellular CPP crystals. The proportion of cases diagnosed with acute CPP crystal arthritis according to predefined criteria, were compared between these cohorts. Further analysis was made with respect to demographics, other laboratory results and cartilage calcification.
Results: The study population consisted of 134 patients, 108 with intracellular CPP crystals and 26 with extracellular CPP crystals. Acute CPP crystal arthritis cases made up 84% of the intracellular cohort and 42% of the extracellular cohort (p<0.001). When septic arthritis cases were excluded from the analysis, 96% of cases with intracellular CPP crystals were secondary to acute CPP crystal arthritis (p<0.001).
Conclusion: Our study suggests the presence of an intracellular CPP crystal location is more likely to be associated with acute CPP crystal arthritis. An extracellular CPP crystal location appears less specific and more likely to be an incidental finding.