Alfaxalone increases measured progesterone concentration in neutered male cats when determined by immunoreactivity.

Objective: Alfaxalone is a commonly used anesthetic agent in small animals. In cats, alfaxalone can be administered as an IM agent to achieve clinically useful sedation or anesthesia, negating the need for IV injection in difficult patients. The molecular structure of alfaxalone is similar to the hormone progesterone (P4). It is hypothesized that alfaxalone would cross-react with the assay measuring progesterone causing a false elevation.

Animals: 8 healthy neutered male, domestic shorthair cats that were privately owned were enrolled in the study.

Methods: Male neutered cats were administered 3 mg/kg of alfaxalone IM. Blood samples were collected at set time points (baseline, 30 minutes, 60 minutes, 3 hours, 6 hours, and 10 hours after administration), and serum concentrations of progesterone immunoreactivity (IR) were determined using the Siemens Immulite 1000 automated immunoassay system. Statistical analysis was performed with repeated measures ANOVA and a Tukey-Cramer multiple comparisons test. A P value of < .05 was used for significance.

Results: Serum progesterone IR was significantly elevated at 30 minutes, 1 hour, and 3 hours (P < .05) when compared to baseline progesterone immunoreactivity. Progesterone immunoreactivity had returned to baseline by 6 hours.

Clinical Relevance: This study suggests that alfaxalone administered IM in cats may interfere with immunoassay measurement of serum progesterone for up to 6 hours. Caution should be used when interpreting serum progesterone immunoreactivity results in cats within 4 hours of alfaxalone.