Postoperative Recovery Trajectories of C-Reactive Protein, Neutrophil-to-Lymphocyte Ratio, and CRP-to-Albumin Ratio After Open Irrigation and Debridement for Native Knee Septic Arthritis: A Descriptive Single-Center Cohort Study

Background/Objectives: The postoperative course of the serum markers that surgeons follow after open irrigation and debridement (I&D) for native knee septic arthritis is poorly defined. We described recovery trajectories of C-reactive protein (CRP), neutrophil-to-lymphocyte ratio (NLR), and CRP-to-albumin ratio (CAR). Methods: We retrospectively studied 98 adults treated by open I&D at one center between January 2018 and April 2026. Mixed-effects models with natural splines gave population-average geometric-mean curves for postoperative days 0–42. An exploratory group-by-time interaction compared repeat I&D patients after censoring at the first repeat operation. Results: Median age was 77.5 years; median synovial white-cell count was 54,691 cells/µL and median neutrophil percentage 93%; and cultures were positive in 39.8% of patients. Twelve patients (12.2%) underwent repeat I&D, eight during the index admission and four after readmission, and three (3.1%) underwent salvage surgery (minimum estimates). Fitted CRP fell below 5 mg/dL by day 8 and 3 mg/dL by day 13, and CAR fell below 1.0 by day 12; fitted NLR reached 3 by day 22, but its confidence band did not exclude non-attainment within 42 days. Milestones were 1 to 13 days later in culture-confirmed subgroups. In exploratory, underpowered analyses censored at reoperation, neither preoperative levels nor decline slopes clearly distinguished repeat I&D patients (CRP interaction p = 0.78); an uncensored model’s nominal CRP association largely disappeared. Conclusions: CRP, NLR, and CAR showed distinct population-average postoperative trajectories after open I&D. These descriptive curves may assist expectation-setting and study design but should not be used as stand-alone indicators of infection control or joint outcome. Multicenter prospective studies with standardized sampling and functional endpoints are needed.