Objectives: Epidemiological studies of spondyloarthritis (SpA), using ICD codes from the Swedish National Patient Register (NPR), offer unique possibilities but hinge upon an understanding of the validity of the codes. The aim of this study was to validate the ICD codes for ankylosing spondylitis (AS) and undifferentiated SpA (uSpA) in the NPR against the established classification criteria [modified New York (mNY), Assessment of SpondyloArthritis international Society (ASAS), Amor, and European Spondyloarthropathy Study Group (ESSG) criteria].
Method: All patients with an ICD-8/9/10 code of AS or uSpA in the NPR 1966-2009 at a visit to a specialist in rheumatology or internal medicine or corresponding hospitalization, alive and living in Sweden 2009, were identified (n=20,089). Following a structured procedure to achieve geographical representativeness, 500 random patients with a diagnosis of AS or uSpA in 2007-2009 were selected. Based on a structured review of clinical records, positive predictive values (PPVs) for fulfilling the criteria sets were calculated.
Results: For those having received an ICD code for AS, the PPVs for fulfilling the mNY criteria or any set of SpA criteria were 70% and 89%, respectively. For those with an uSpA diagnosis (and never an AS diagnosis), the corresponding PPVs were 20% and 79%. The subset with both AS and uSpA diagnoses (overlap=12%) were as likely to fulfil the mNY criteria as the group that had been coded as AS only.
Conclusions: The diagnosis codes for AS or uSpA had high PPVs, suggesting that our case identification in the Swedish NPR can be used for nationwide, population-based, epidemiological studies of these diseases.