Serum amyloid A on postoperative day 3 for early prediction of deep surgical site infection after degenerative spine surgery

Deep surgical site infection (dSSI) is a serious complication after degenerative spine surgery, and early postoperative risk identification remains challenging. This prospective single-center cohort study serially measured C-reactive protein (CRP), procalcitonin (PCT), and serum amyloid A (SAA) before surgery and on postoperative days 1–3 (POD1–3) to characterize their early trajectories and identify a potential biomarker for subsequent dSSI. Associations and discriminative performance were assessed using multivariable logistic regression and ROC analysis with bootstrap internal validation. Among 136 patients, 19 developed dSSI, all diagnosed after POD3 sampling. POD3 SAA showed the strongest discrimination and remained associated with subsequent dSSI after adjustment for intraoperative blood loss (adjusted odds ratio 1.549 per 100 mg/L increase, 95% confidence interval 1.259–1.907; P < 0.001). Its apparent AUC was 0.907 and the optimism-corrected AUC was 0.904. The data-derived cutoff was 390.55 mg/L; bootstrap resampling yielded a median cutoff of 390.55 mg/L and a 2.5th–97.5th percentile interval of 390.55–449.74 mg/L. A combined biomarker model showed no clear incremental benefit over SAA alone. POD3 SAA may provide an early warning signal for dSSI, but the proposed cutoff requires validation in larger multicenter cohorts before clinical application.