Abstract
Background:
Accurate measurement of plasma phenobarbital concentration is essential to optimize seizure control while minimizing the risk of toxicity. Although both on-site point-of-care (POC) and off-site reference laboratory analyzers are commonly used in clinical practice, notable discrepancies between their results have been observed.

Aim:
The primary objective was to assess the agreement between an on-site POC analyzer (Catalyst One, IDEXX Laboratories) and an off-site reference laboratory analyzer (Cobas 6000 c501, Roche Diagnostics) for quantifying plasma phenobarbital concentrations in dogs and cats. A secondary objective was to investigate whether clinical or biological variables were associated with measurement discrepancies between the two analyzers.

Methods:
This dual-institutional retrospective study included dogs and cats with plasma phenobarbital concentrations measured concomitantly from the same blood sample, both on-site using a POC analyzer and off-site using a reference laboratory analyzer. Data were collected between September 2023 and December 2024. Agreement between analyzers was assessed using Passing-Bablok regression, Lin’s concordance correlation coefficient (CCC), and Bland-Altman analysis. Associations between clinical or biological variables and measurement discrepancies were evaluated using Spearman’s rank correlation and Student’s t-tests.

Results:
Thirty-three paired phenobarbital concentrations were measured using both analyzers. Passing-Bablok regression and Lin’s CCC showed that the two analyzers were not equivalent (systematic proportional bias) and demonstrated poor agreement (CCC = 0.74). Bland-Altman analysis revealed a mean positive bias of +22.50 µmol/L (p < 0.0001) between the POC and reference laboratory analyzers, with limits of agreement ranging from −13.5 to +58.5 µmol/L. Discrepancies in medical interpretation occurred in 30.3% of cases when using manufacturer-specific thresholds. No significant association was found between measurement differences and any assessed clinical or biological variable.

Conclusion:
This study suggests that caution should be exercised when using both analyzers interchangeably in clinical practice. In most cases from this study’s population, the POC analyzer yielded higher phenobarbital concentrations. Clinicians should consider these discrepancies and interpret phenobarbital levels with caution, especially when switching analyzers during therapeutic drug monitoring. Further research is warranted to explore the causes of these differences and assess their clinical relevance in a broader population.

Key words: Method comparison; Phenobarbital; Point-of-care testing; Therapeutic drug monitoring; Veterinary neurology.