Retrospective study reveals bioassay and mass spectrometry yield discordant itraconazole levels in dogs, indicating the assays are not interchangeable.
Key Points
To compare itraconazole therapeutic drug monitoring results generated by culture-based bioassay versus liquid chromatography-mass spectrometry (LC-MS) in dogs with blastomycosis, and determine how the chosen assay impacts dose adjustments.
Retrospectively evaluated 97 serum samples from 14 client-owned dogs with blastomycosis collected from October 2023 to May 2025.
Quantified drug levels using bioassay alongside three LC-MS metrics: itraconazole alone, hydroxyitraconazole metabolite, and their sum.
Analyzed dosage-concentration relationships using linear mixed models and evaluated clinical agreement for dosage adjustments using a concordance grid.
Itraconazole dosage demonstrated no statistically significant association with measured serum concentrations.
Bioassay concentrations significantly correlated with LC-MS measurements (P < .01), exhibiting weak correlation with itraconazole alone (r2 = 0.40) and moderate correlation with hydroxyitraconazole (r2 = 0.48) and total metabolite sum (r2 = 0.48).
Dosage-adjustment decisions agreed between bioassay and LC-MS in 64% of samples (62/97) and disagreed in 36% of samples (35/97).