Axial spondyloarthritis (axSpA) and ankylosing spondylitis (AS) belong to a group of inflammatory rheumatological conditions defined by inflammation in the sacroiliac joints (SIJ), spine and, in
some cases, the peripheral joints and enthesis. The diagnosis of early-stage axSpA can be challenging, as there are no specific biomarkers or clinical tests that can differentiate between
inflammatory disease and unspecified low back pain. To address the significant delays in diagnosing axSpA patients, the Assessment of SpondyloArthritis international Society (ASAS)
introduced classification criteria for axSpA, including magnetic resonance imaging (MRI) of the SIJ, in 2009.
The overall purpose of this PhD project is to investigate possible predictors of axSpA for early diagnosis. The first study evaluates the trends in axSpA and AS diagnoses before and after the
ASAS criteria were implemented. We found a significant increase in the national and regional incidences of axSpA and AS, especially after 2009.
Study II evaluates the incidence of inflammatory findings suggestive of inflammatory back disease on SIJ MRI in a mixed cohort of patients. The incidence of inflammatory changes in the
SIJ correlated well with the findings of previous studies. Furthermore, patient-reported outcome measures (PROMs) were evaluated as predictors of inflammatory changes on SIJ MRI. However, PROMs were not effective in predicting inflammatory changes in this cohort.
Studies III, IV and V are based on a cohort of patients with suspected early-stage axSpA after the implementation of the ASAS criteria in 2009 (the Molecular Biology of Infectious Agents in the Early Diagnosis of Spondyloarthritis [MICSA] cohort). Study III evaluates a possible association between the biomarker complement C3d and inflammatory MRI changes in the MICSA cohort.
Studies IV and V are register-based studies that use different Danish patient registries. We found no association between the biomarker C3d and MRI findings. In Study IV exploring long-term
diagnoses at one- and five-year follow-ups in the MICSA cohort, a temporary overestimation of axSpA diagnoses was found at baseline and in subsequent years. Study V evaluates the extent to
which a possible overdiagnosis of axSpA early in the process could influence a patient's socioeconomic prognosis at long-term follow-up. Patients with sustained inflammatory diagnosis at five-year follow-up had a higher employment rate than patients with no registered inflammatory diagnosis.
In conclusion, our evaluations of specific biomarkers, PROMs and MRI data revealed no significant predictors of early-stage axSpA. We also found that the implementation of the ASAS classification criteria contributed to a temporary increase in and overestimation of axSpA patients in the Danish population.