Comparison of 3 methods of quantification of phenobarbital in canine plasma: high-performance liquid chromatography, a point-of-care immunoassay, and the FDA-approved immunoassay analyzer.

Phenobarbital is a common antiseizure medication that has a relatively narrow therapeutic window. Therapeutic drug monitoring (TDM) is a helpful tool to guide dose adjustments for phenobarbital and avoid its toxicity. We investigated the agreement among 3 methods of quantifying phenobarbital in canine plasma: high-performance liquid chromatography (HPLC), point-of-care (POC) testing, and the FDA-approved immunoassay analyzer. We randomly selected 45 plasma samples obtained by the TDM service (College of Veterinary Medicine, Auburn University, Auburn, AL, USA). Passing-Bablok regression and Lin concordance correlation coefficients (CCCs) were used to determine the agreement of the results obtained for the 3 methods; Bland-Altman plots were used for bias analysis using the results from the HPLC method as a reference. The FDA-approved immunoassay analyzer and POC immunoassay method results agreed with the HPLC. The results from the FDA-approved immunoassay analyzer were better correlated than those from the POC method, with Lin CCCs of 0.96 (95% CI: 0.93-0.98) and 0.94 (95% CI: 0.90-0.97), respectively. The average biases of the FDA-approved and the POC immunoassay analyzers were 0.80 and -0.64 µg/mL, respectively. Based on the CIs of Lin CCCs, the commercial POC phenobarbital test is a good screening tool and agrees with the HPLC method. However, the FDA-approved immunoassay analyzer method allows for more accurate quantification of phenobarbital concentrations, which is required for appropriate dose adjustment of phenobarbital.