Ab1005 association between changes in serum alkaline phosphatase levels and radiographic progression in ankylosing spondylitis

Background The changes in bone metabolism may occur earlier than those that can be identified on radiography. Although studies have evaluated the changes in inflammatory markers that precede radiographic progression, no studies have evaluated the relationship between the timing of changes in bone metabolism and radiographic progression in patients with AS. Objectives To determine the relationship between serum alkaline phosphatase levels and radiographic changes over time in ankylosing spondylitis. Methods This retrospective study evaluated the electronic medical records of patients with ankylosing spondylitis between January 2001 and December 2018. Longitudinal data including serum alkaline phosphatase levels were imputed by linear interpolation at 3-month intervals. Among the serum alkaline phosphatase levels for 8 years prior to measurement of the modified Stoke Ankylosing Spondylitis Spinal Score, the serum alkaline phosphatase level having the highest beta coefficient with the modified Stoke Ankylosing Spondylitis Spinal Score was selected. Linear mixed models with the selected serum alkaline phosphatase levels and the modified Stoke Ankylosing Spondylitis Spinal Score, including clinical variables, were investigated. Results Overall, 1122 patients were included, with a mean follow-up period of 8.20 (standard deviation: 2.85) years. Of the series of serum alkaline phosphatase levels, the level in the previous 5 years and 3 months showed the highest beta coefficient with the modified Stoke Ankylosing Spondylitis Spinal Score (Figure 1). In the linear mixed model including clinical variables, the serum alkaline phosphatase level 5 years and 3 months before radiographic changes was significantly associated with the modified Stoke Ankylosing Spondylitis Spinal Score (β=0.021, 95% confidence interval: 0.017–0.025, p<. 001). Conclusion Serum alkaline phosphatase levels measured at approximately 5 years before may be a surrogate marker for predicting spinal radiographic changes. Long-term prospective clinical and experimental studies of > 5 years are required for biomarker discovery or therapeutic research on the radiographic progression of ankylosing spondylitis. Figure 1. Beta coefficients of ALP levels to calculate the modified Stoke Ankylosing Spondylitis Spinal Score (mSASSS). ALP levels 5 years and 3 months before measured mSASSS had the highest significant beta coefficient (0.020, 95% CI 0.016–0.023) with mSASSS (asterisk). mSASSS, the modified Stoke Ankylosing Spondylitis Spinal Score; ALP, alkaline phosphatase. REFERENCES: NIL. Acknowledgements: NIL. Disclosure of Interests None Declared.