Association of the CALLY Index with Short-Term Treatment Response in Ankylosing Spondylitis

Background/Objectives: We examined the association between the pretreatment C-reactive protein–albumin–lymphocyte (CALLY) index and three-month clinical response to advanced therapy in patients with ankylosing spondylitis. Methods: We retrospectively analyzed 209 patients with ankylosing spondylitis who received biologic or targeted synthetic disease-modifying antirheumatic drugs. Laboratory measurements and Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) scores were recorded at baseline; BASDAI was reassessed at three months. Response was defined as a reduction of at least 50% in BASDAI (BASDAI50). Analyses included Spearman’s rank correlation, multivariable logistic regression, nonlinear spline modeling, and receiver operating characteristic (ROC) analysis. Results: Among 209 patients, 167 (79.9%) achieved BASDAI50 response. Following recalculation from the recorded laboratory measurements, baseline CALLY was associated with treatment response, with a nonlinear exposure–response relationship (spline versus linear likelihood-ratio χ2 = 26.27, df = 2; p < 0.001; overall association p < 0.001). The CALLY AUC was 0.763 (95% bootstrap CI: 0.671–0.852), compared with 0.773 for CRP (paired bootstrap p = 0.485). The apparent and optimism-corrected AUCs of the adjusted spline model were 0.786 and 0.753, respectively. Conclusions: Baseline CALLY was nonlinearly associated with three-month BASDAI50 response and showed moderate discrimination between responders and nonresponders in this cohort. It did not demonstrate incremental value beyond CRP. Its clinical predictive utility remains unestablished and requires external validation.