Evaluation of the Oral Fludrocortisone Suppression Test for Diagnosing Primary Hyperaldosteronism in Cats

Background Primary hyperaldosteronism ( PHA ) in cats is suggested by clinical signs and an elevated plasma aldosterone‐to‐renin ratio ( ARR ), but a test to confirm the diagnosis is lacking. Hypothesis Fludrocortisone does not suppress urinary aldosterone excretion in cats with PHA , but does so in cats with arterial hypertension because of other causes. Animals Nineteen client‐owned cats with arterial hypertension because of PHA (n = 9) or other causes (n = 10). Methods Prospective clinical study. The urinary aldosterone‐to‐creatinine ratio ( UACR ) was determined in morning urine before, during, and after 4 days of oral fludrocortisone administration in a dose of 0.05 mg/kg q12h. Arterial blood pressure and plasma potassium concentration were measured before and after fludrocortisone administration. Results A basal UACR above 46.5 × 10 −9 , the upper limit of the reference range, was found in 3 cats with PHA . All PHA cats had basal UACR s >7.5 × 10 −9 . In all non‐ PHA cats with a basal UACR >7.5 × 10 −9 , fludrocortisone administration induced >50% suppression. In contrast, fludrocortisone administration resulted in <50% suppression in 6 of the 9 PHA cats. Neither basal UACR , nor UACR after suppression testing, correlated with the etiology of PHA (adenoma, adenocarcinoma, or suspected bilateral hyperplasia of the zona glomerulosa). Fludrocortisone induced hypokalemia in 7 cats, but did not induce or exacerbate arterial hypertension. Conclusions and Clinical Importance Measuring the UACR before and after 4 days of administering fludrocortisone is a practical method of confirming most cases of PHA in cats, and of substantiating the absence of PHA in cats having an ARR within the reference range.