Effect of storage duration on carprofen concentration measurements in dog plasma

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.

The objective of this study was to evaluate the stability of carprofen in canine plasma when stored at -80°C for 6 months.

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.

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).

Carprofen-laden canine plasma samples can be stored for up to 6 months before analysis with no degradation in carprofen concentrations expected.