Captopril Challenge Test

50 mg of captopril.

Blood tubes for plasma renin activity (PRA) and aldosterone.

Assay for PRA and Aldosterone:

Radioimmunoassay.

Precautions:

Monitor BP hourly.

Interpretation:

Plasma aldosterone is normally suppressed by captopril (> 30 %). In patients with PA, aldosterone remains elevated (less than 30 % suppression) and PRA remains suppressed [1–3]. In diagnosing APA, Post captopril A/R ratio > 35 has sensitivity and specificity of 100 % and 67.9 % compared with 95.4 % and 28.3 % at baseline A/R ratio [3].

Caveats:

There are reports of a substantial number of false negative or equivocal results [4, 5].
In our institution we use this test when saline suppression or oral salt loading is not feasible or contraindicated such as in patients with uncontrolled blood pressure above 180/100.
At low Na intake (< 3 g/day), the saline suppression test (SST) has a higher negative predictive value than the Captopril challenge test. At a high sodium intake, SST offers no advantage over the easier-to-perform Captopril challenge test for the diagnosis of APA [3].
Stop all potassium sparing diuretics for at least 4 weeks and stop potassium wasting diuretics, beta blockers, beta-adrenergic blockers, central alpha-2 agonists (e.g., clonidine, alpha-methyldopa), non-steroidal anti-inflammatory drugs, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, renin inhibitors, and dihydropyridine calcium channel antagonists for at least 2 weeks. Blood pressure can be controlled with verapamil, selective alpha 1 antagonists (e.g. prazosin, doxazosin, terazosin) or hydralazine in the meantime. Potassium levels should be corrected prior to the test [6].
In patients with severe hypokalemia, a small dose of amiloride (≤ 5 mg) may be used to control hypokalemia while the patient undergoes confirmatory testing.