Patient preparation prior to day of testing:
(1) The patient should have a normal salt intake.
(2) All diuretics should be stopped.
(3) Antihypertensive medications stopped, if possible, for 3 weeks prior to testing.
Patient preparation on day of testing:
(1) The patient should be seated for at least 30 minutes prior to testing.
Drug Dosage: Captopril, 50 mg diluted in 10 mL water immediately prior to testing, for po administration.
Testing Protocol and Specimen Collection:
(1) After the patient has been seated sufficiently long, blood pressure readings are taken at 5 minute intervals and averaged.
(2) A baseline blood sample is collected for plasma renin activity.
(3) The dose of captopril is administered orally.
(4) The patient's blood pressure is measured at 15, 30, 40, 45, 50, 55 and 60 minutes after the dose has been administered.
(5) At 60 minutes after the drug was given, a second ("stimulated") blood sample is collected.
Interpretation
A positive captopril test must have all 3 of the following:
(1) a stimulated (1-hour post dosage) plasma renin level >= 12 ng AI/mL/hour;
(2) an increase in plasma renin level >= 10 ng AI/mL/hour;
(3) one of the following:
(3a) if the baseline plasma renin level is >= 3 ng AI/mL/hour, then the increase in plasma renin activity (1-hour post renin level minus baseline level) must be > 150%;
(3b) if the baseline level is < 3 ng AI/mL/hour, then the increase must be > 400%
Patients with a positive captopril test are likely to have renovascular hypertension, rather than essential hypertension.
Limitations:
• Renal insufficiency can significantly reduce the sensitivity and specificity of the test:
(1) patients with renovascular hypertension may have negative tests
(2) patients with secondary hypertension may have positive test results
