Association between circulating tryptophan metabolites and postoperative pain intensity in lumbar disc herniation surgery: An observational study.

Gut microbiota is increasingly recognized for its involvement in pain modulation mechanisms, including visceral, inflammatory, neuropathic pain, and opioid tolerance. Although alterations in microbiota composition may affect host neuroimmune interactions, direct evidence in surgical pain contexts remains limited. This observational study explored associations between circulating tryptophan-derived metabolites and postoperative acute pain intensity in patients undergoing lumbar disc herniation surgery. Patients aged ≥ 18 years scheduled for lumbar disc herniation surgery were enrolled. Blood samples were obtained preoperatively and at 8 and 24 hours postoperatively to quantify tryptophan metabolites (picolinic acid, 3-OH kynurenine, anthranilic acid, kynurenine, quinolinic acid, kynurenic acid, and xanthurenic acid) via LC-MS/MS. Concurrent visual analog scale scores were recorded. Spearman correlation analysis was performed to evaluate associations between metabolite levels and pain scores. Thirty-seven patients (51.4% male, mean age 50.2 ± 11.8 years, body mass index 27.6 ± 2.9 kg/m2) were included. Significant perioperative changes were observed in picolinic acid (P = .001), kynurenic acid (P = .006), and xanthurenic acid (P < .001). Quinolinic acid levels correlated with 24-hour postoperative visual analog scale scores (r = 0.422; P = .009). Additional associations were identified between age and changes in picolinic acid (r = 0.357; P = .030), and between operation time and changes in 3-OH kynurenine (r = -0.359; P = .029). This exploratory analysis suggests that specific tryptophan metabolites, particularly quinolinic acid, may be linked to postoperative pain intensity and could serve as potential biomarkers in pain assessment. Future studies with direct microbiota profiling are needed to clarify the mechanistic pathways involved.