Association Between Changes in Serum Alkaline Phosphatase Levels and Radiographic Progression in Ankylosing Spondylitis

Abstract

Abstract This retrospective study evaluated the electronic medical records of patients with ankylosing spondylitis (AS) between January 2001 and December 2018 to determine the relationship between serum alkaline phosphatase (ALP) levels and radiographic changes over time. Longitudinal data including serum ALP levels were imputed by linear interpolation at 3-month intervals. Among the serum ALP levels for 8 years prior to modified Stoke Ankylosing Spondylitis Spinal Score (mSASSS) measurement, the serum ALP level having the highest beta coefficient with the mSASSS was selected. Linear mixed models with the selected serum ALP levels, mSASSS, and clinical variables were investigated. We included 1122 patients, with a mean follow-up period of 8.20 (standard deviation: 2.85) years. The serum ALP level from 5 years and 3 months prior showed the highest beta coefficient with the mSASSS. In the linear mixed model, the serum ALP level 5 years and 3 months before radiographic changes was significantly associated with the mSASSS (β = 0.021, 95% confidence interval: 0.017–0.025, p < 0.001). Serum ALP levels measured 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 AS radiographic progression.