Background Epidemiological studies of spondyloarthritis (SpA) are scarce. Using ICD-codes from the Swedish National Patient Register (NPR) offers unique possibilities for such studies. For this purpose, the validity of these ICD-codes needs to be determined.
Objectives To validate the ICD-codes for ankylosing spondylitis (AS) and SpA in the NPR against established classification criteria (modified New York (mNY), ASAS, Amor and ESSG criteria).
Methods All patients with an ICD-code of AS or SpA in the NPR 1966-2009 at a visit to a specialist in rheumatology or internal medicine, or corresponding hospitalization, were identified (n=20074). Following a structured procedure to achieve geographical representativeness, 500 random patients with a registered diagnosis of AS or SpA in 2007-2009 were selected. A structured review of clinical records, with extraction of necessary information for the established classification criteria was performed and positive predictive values (PPV) were calculated.
Results In this cohort 11472 (34% women) patients had received an AS diagnosis and 11004 (56% women) a SpA diagnosis. The overlap group having received both types of diagnoses had similar frequencies for fulfillment of mNY criteria, symptoms and signs of back disease as the group having been coded as AS only.
Of those being coded as AS only, the PPV for fulfilling the mNY, any criteria set and any of the included criteria elements were 70%, 89% and 96% respectively.
Of those with SpA (without AS ever) the corresponding PPV values were 20%, 79% and 99% respectively.
Conclusions A diagnosis of AS or SpA (without AS) had a high validity, suggesting that case identification based on ICD-codes in the Swedish NPR can be used for epidemiological studies of these diseases.
Disclosure of Interest None declared
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