Effect of phenobarbitone on the low-dose dexamethasone suppression test and the urinary corticoid: creatinine ratio in dogs.

OBJECTIVES: To investigate potential effects of phenobarbitone on the low-dose dexamethasone suppression (LDDS) test and urinary corticoid to creatinine ratio in dogs in a controlled prospective study and in a clinical setting. ANIMALS: Ten crossbreed experimental dogs and 10 client-owned dogs of mixed breeds treated chronically with phenobarbitone to control seizures. PROCEDURES: Experimental dogs were allocated to treatment (6 mg/kg oral phenobarbitone, n = 6) and control (n = 4) groups. LDDS tests (dexamethasone 0.01 mg/kg intravenously, cortisol concentration determined at 0, 2, 4, 6 and 8 h) were conducted repeatedly over a 3-month period. Urinary corticoid to creatinine ratios were measured before LDDS tests. A single LDDS test was performed on 10 epileptic dogs. RESULTS: LDDS and urinary corticoid to creatinine ratios in dogs were not affected by treatment with phenobarbitone. CONCLUSIONS: Phenobarbitone does not interfere with LDDS testing regardless of dosage or treatment time. Urinary corticoid to creatinine ratios are also unaffected.