Intravenous Saline Suppression Test

Assay for Aldosterone:

Enzymatic radioimmunoassay (RIA).

Precautions:

Do not perform this test in patients with severe uncontrolled hypertension, renal insufficiency, cardiac insufficiency, cardiacarrhythmia, or severe hypokalemia.

Interpretation:

Postinfusion plasma aldosterone levels < 5 ng/dL make the diagnosis of primary aldosteronism (PA) unlikely, and levels > 10 ng/dL are a very probable sign of PA. Values between 5 and 10 ng/dL are indeterminate [1–3].

Caveats:

Stop all potassium sparing for at least 4 weeks and stop potassium wasting diuretics, beta blockers, beta-adrenergic blockers, central alpha-2 agonists (e.g., clonidine, alpha-methyldopa), nonsteroidal antiinflammatory drugs, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, renin inhibitors, and dihydropyridine calcium channel antagonists for 2 weeks [1]. Blood pressure can be controlled with verapamil, prazosin, doxazosin, terazosin, or hydralazine in the meantime[1].
Potassium levels should be corrected prior to the test [1].
When aldosterone levels are in the indeterminate category, other confirmatory tests should be performed, such as oral salt loading test or captopril challenge test [3]. Although Fludrocortisone suppression test has more diagnostic accuracy, it requires hospitalization.
Measuring plasma renin activity (PRA) and/or cortisol does not improve the diagnostic accuracy performance of the test [4].
In patients with severe uncontrolled hypertension (BP > 180/100 mmHg), delaying the test till better BP control is achieved, or proceeding with the Captopril challenge test may be considered.