Background/Purpose: Calcium pyrophosphate deposition disease (CPPD) has a spectrum of manifestations, of which pseudogout is the most acute inflammatory phenotype. Studies focusing on pseudogout are limited by a paucity of algorithms to identify this condition in large datasets. To our knowledge, there is only one published algorithm for CPPD, developed at a  Veterans’ Administration Medical Center to identify “definite or probable CPPD” per Ryan and McCarty’s diagnostic criteria.¹ The algorithm includes ≥1 ICD-9 code 275.49 (other disorders of calcium metabolism) or 712.1-712.39 (chondrocalcinosis due to dicalcium phosphate crystals, pyrophosphate crystals, or cause unspecified). We examined characteristics of patients fulfilling this algorithm at a tertiary care academic medical center, with a focus on subjects clinically manifesting as pseudogout.

Methods: Following the published methods, we applied the algorithm to patients with ≥1 encounter at our center over 2 years (1/1/15-12/31/16). 100 patients were randomly selected for medical record review from date of 1st qualifying ICD-9 code through present. We evaluated each record for 2 phenotypes: 1) “definite or probable CPPD”¹, defined as joint pain, and either synovial fluid with calcium pyrophosphate crystals or radiographic chondrocalcinosis in any joint, or both; 2) pseudogout, defined as synovitis and synovial fluid aspirate with calcium pyrophosphate crystals. We recorded information on demographics, healthcare utilization, musculoskeletal diagnoses, and evaluation and treatment.

Results: 68% of patients had one or both phenotypes; 32% met neither definition (Table). 18 patients (18%) had pseudogout, all of whom also met the definition of “definite or probable CPPD”. 50 patients (50%) had “definite or probable CPPD” but not pseudogout. Overall 73% had osteoarthritis per x-ray reports or notes, with the highest frequency (90%) among “definite or probable CPPD” only, compared to 72% of pseudogout patients. Synovial fluid aspiration was performed in 25% of patients and was positive in all with crystal-proven pseudogout by definition, but not positive in any others. Among 92 patients with x-rays, chondrocalcinosis was noted in 61% and in 100% of patients with “definite or probable CPPD” only. Chondrocalcinosis was present in <1/3 of patients with both x-rays and crystal-proven pseudogout; among these, it was present in <20% of symptomatic joints.

Conclusion: Among subjects identified using a published CPPD algorithm, 18% had crystal-proven pseudogout, of whom <20% had x-ray chondrocalcinosis in the affected joint. These findings highlight a need for improved understanding of pseudogout and improved approaches to identify this acute phenotype of CPPD.

1. Bartels C, Singh J, Parperis K, Huber K, Rosenthal A. Validation of administrative codes for calcium pyrophosphate deposition. J Clin Rheum 2015;21:189-92