Oral fludrocortisone test for diagnosing primary hyperaldosteronism

Abstract

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 &#xd7; 10(-9), the upper limit of the reference range, was found in 3 cats with PHA. All PHA cats had basal UACRs >7.5 &#xd7; 10(-9). In all non-PHA cats with a basal UACR >7.5 &#xd7; 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.