captopril suppression test captopril suppression test the captopril suppression test (cst) is a non-invasive medical test that measures plasma levels of aldosterone. aldosterone production is suppressed by captopril through the renin–angiotensin–aldosterone system. cst results are used to assist in the diagnososis of primary aldosteronism (conn syndrome). captopril suppression testaldosterone (chemical structure) is measured in the cst.purposemeasures plasma levels of aldosterone.