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

The present study aimed to investigate the relationship between delirium and tryptophan and its metabolites in critically ill older patients.

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.

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).

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.