The possible interaction between tryptophan and its metabolites with delirium in older patients with critical illnesses

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Abstract
Aim: The present study aimed to investigate the relationship between delirium and tryptophan and its metabolites in critically ill older patients. Methods: This prospective and observational study was conducted on patients who were > 60 years of age and hospitalized for at least 24 h at the internal medicine ICU in the tertiary health care unit (n = 120). All consecutively selected patients were evaluated for delirium at the baseline and follow-up period at the bedside by an intensive care specialist. At the end of the 24 h follow up, the patients were divided into two groups (with and without delirium). Clinical properties and tryptophan (TRP) and its metabolites [kynurenine (KYN), kynurenic acid (KYNA), quinolinic acid (QA), 3-hydroxykynurenine (3-HK), 3-hydroxyanthranilic acid (3HAA)] were compared between groups. Results: The median age of the patients was 79.5 (62–95) years and 53.3% were female. The median age and CCI score were significantly higher among patients with delirium than in those without delirium (P = 0.001 and 0.031, respectively). The level of TRP was significantly (borderline) decreased among patients with delirium (P = 0.056). The KYN/TRP and QA/TRP ratios were statistically and significantly higher in patients with delirium than those without (P < 0.001 and P = 0.016, respectively). The best predictive values for detecting delirium were calculated as ≤ 14,100 ng/mL for TRP (AUC: 0.601, P = 0.052), > 1.12 for KYN/TRP ratio (AUC: 0.704, P < 0.001), and > 0.75 for QA/TRP ratio (AUC: 0.627, P = 0.013). The QA/TRP ratio showed independent and borderline significant association with being delirium in multivariable regression analysis (Odds ratio: 2.007, P = 0.066). Conclusion: This study demonstrated that tryptophan and its metabolites obtained within the first 24 h of ICU admission might have predictive value for determining high-risk older patients for delirium.

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