Determination of biological variation components of serum carnitine in healthy individuals

Introduction and Aim: The analysis of carnitine (L-3-hydroxy-4-trimethylaminobutyric acid) in biological specimens is critical for the diagnosis and clinical management of patients with carnitine deficiency syndromes and certain inborn errors of metabolism. In this study, we aimed to provide up-to-date biological variation (BV) data for serum free carnitine, acylcarnitine, and the acylcarnitine/free carnitine ratio to support their safe and reliable use in clinical practice. Methods: This prospective BV study included a group of 30 healthy individuals (14 males and 16 females; age range: 23–56 years), from whom blood samples were collected weekly for six consecutive weeks. All serum samples were analyzed in a single run using liquid chromatography-tandem mass spectrometry (LC-MS/MS), with each sample tested in duplicate. The results were evaluated for outliers, homogeneity of variance, normality, and steady-state conditions. Subsequently, nested ANOVA was applied to estimate analytical (CVA), within-subject (CVI), and between-subject (CVG) biological variation, along with 95% confidence intervals. BV calculations were also performed separately by sex. Results: The within-subject biological variation (CVI) values for serum free carnitine, acylcarnitine, and the acylcarnitine/free carnitine ratio were 13,8 (%95 GA; 12,3-15,6), 16,7 (%95 GA; 15,0-18,8), 23,1 (%95 GA ; 20,7-26,1) and 26,5 (%95 GA 23,9-29,9), respectively. The between-subject biological variation (CVG) values for these analytes were 14,9 (%95 GA; 12,4-18,4), 15,5 (%95 GA ; 13,7-17,8), 22,0 (%95 GA ;19,5-25,2) and 24,4 (%95 GA; 21,6-28,0), respectively. No significant sex-based differences were observed in CVI and CVG values for serum carnitine. The index of individuality for the different types of carnitine was found to range between 0.6 and 1.4. The analytical performance specifications for desirable imprecision and bias were 6.9% and 5.1% for total carnitine, 8.3% and 5.7% for free carnitine, 11.6% and 7.9% for acylcarnitine, and 13.2% and 9.0% for the acylcarnitine/free carnitine ratio, respectively. Conclusion: This study aimed to provide well-characterized BV estimates for serum carnitine concentrations. These data can support objective determination of analytical performance specifications. Serum carnitine exhibited low individuality; thus, the use of population-based reference intervals is considered more appropriate for its interpretation in clinical settings. Keywords: Biological variation, carnitine, individuality index, reference change value