Sex-Based Estimation of Biological Variation in Plasma-Free Amino Acid Concentrations Among Healthy Adults

Abstract

Introduction: Free amino acid (FAA) analysis plays a crucial role in diagnosing and monitoring inborn errors of metabolism, assessing nutritional status, and identifying metabolic imbalances associated with various diseases. This study aimed to provide updated biological variation (BV) data to support the reliable clinical application of FAA concentrations in plasma samples, utilizing LC-MS/MS. Materials and methods: Venous blood was collected from 22 healthy Turkish adults (9 men and 13 women) over approximately nine weeks. Plasma FAAs were measured in duplicate. BV estimates with 95 % confidence intervals were determined using nested ANOVA for the entire study group and sex-stratified subgroups, following analysis of outliers, normality, steady-state conditions, and variance homogeneity. Results: Within-subject variation (CVI) and between-subject variation (CVG) estimates ranged from 9.5 % to 32.5 % and 8.6 % to 50.0 %, respectively. The estimated CVI values for essential amino acids were significantly lower than those for non-essential amino acids (P = 0.03). For most plasma FAAs, no significant differences in CVI (except for alanine, arginine, glutamic acid, and threonine) or CVG were observed between sexes. However, differences in the indices of individuality were noted between men and women for some plasma FAAs. Conclusions: This Biological Variation Data Critical Appraisal Checklist-compliant study provides the first updated BV data for plasma FAAs. The significant variation observed in CVI estimates is hypothesized to result from differences in the metabolic regulation of essential versus non-essential amino acids. The sex-stratified indices obtained in this study will aid in the appropriate application of population-based reference intervals for plasma FAA assessment.