Diagnosing primary aldosteronism with frusemide stimulation test in hypertensive patients with raised aldosterone to renin ratio

The frusemide stimulation test (FST) has been advocated as a cheap and convenient outpatient screening test for mineralocorticoid excess. We have explored the possibility of using this test to diagnose primary aldosteronism (PA) in a hypertension clinic population. We assessed the sensitivity and specificity of this test by comparing it with the salt loading and fludrocortisone suppression test. Patients with possible PA were pre-screened using a raised aldosterone to renin ratio (ARR) of ≥ 750. Each of these patients then underwent a FST. Oral frusemide 40 mg was given at 1800 h on the day before and at 0800 h on the day of the test. Following 4 hours of ambulation, bloods were sampled for plasma renin activity (PRA). A PRA of ≤ 1.5 mg/ml/hr was deemed to indicate a failure to stimulate renin and thus suggestive of the diagnosis of PA. All patients also underwent a standard salt loading and fludrocortisone suppression test over 3 days prior to measuring plasma aldosterone. Non-suppression of plasma aldosterone (i.e. ≥ 140 pmol/l) was deemed diagnostic of PA. 36 patients (16 male, age range 33 to 76 years) with elevated ARR were assessed. There were 24 positive and 12 negative results following FST. The salt loading and fludrocortisone suppression test confirmed PA in all those with positive FST and 7 out of the 12 patients with negative FST. FST therefore had a sensitivity of 77.4%; a specificity of 100%; a positive predictive value of 100%; and a negative predictive value of 41.7% in detecting PA. Thus, the frusemide stimulation test was found to be specific in detecting PA but a negative test did not rule out the diagnosis. About two thirds of patients with a negative test result proved to have PA in this study. The implication is that FST lacks sensitivity and will miss possible cases of PA.