Effect of storage duration on carprofen concentration measurements in dog plasma

BACKGROUND: Storage of samples may be necessary prior to testing drug levels in certain study designs; however, the effect of storage duration on measured drug levels is not known for all drugs. OBJECTIVES: The objective of this study was to evaluate the stability of carprofen in canine plasma when stored at −80°C for 6 months. METHODS: Six healthy dogs were enrolled (1–10 years old, 17–35 kg) and received compounded carprofen at 2.2 mg/kg orally every 12 h for 2 days. On the third day, blood was collected immediately before the morning dose (trough), then 1 and 6 h after the dose (sampling timepoint). Whole blood was immediately centrifuged, and plasma was stored at −80°C. Plasma carprofen concentration was measured at day 2, week 2 and then monthly for 6 months using reversed‐phase high‐performance liquid chromatography. The measured carprofen concentrations were analysed statistically using a linear mixed effects model. RESULTS: There was no effect of storage time over 6 months (p = 0.891) on measured carprofen levels. Although there was an effect of sampling timepoint (0, 1 and 6 h) (p < 0.001), the interaction between storage timepoint and sampling timepoint was not statistically significant (p = 1). CONCLUSIONS: Carprofen‐laden canine plasma samples can be stored for up to 6 months before analysis with no degradation in carprofen concentrations expected.