Plasma renin activity (PRA) is higher in the renal vein of a kidney showing renovascular hypertension than in one that is not. If renovascular disease involves one kidney but not the other, a comparison of the PRA levels can indicate which kidney is affected, since the affected side will be high and the unaffected side low.
Patient Preparation: The intent is to have maximum renin stimulation, in order to accentuate any differences between the two kidneys.
(1) Stop medications that affect renin release for as long as possible prior to testing (prefer 1 month, at least 1 week); these include oral contraceptives, antihypertensives (propranolol, methyldopa, guanethidine, clonidine) and diuretics.
(2) Take a low-salt, high potassium diet for 7 days prior to testing.
(3) Take a thiazide diuretic for 1-3 days prior to testing.
(4) Sit upright for at least 2 hours prior to testing.
Specimen Collection: Samples are collected from:
(1) each renal vein after catheterizing the inferior vena cava under fluoroscopic guidance
(2) arterial blood, sometimes from the aorta; blood from the inferior vena cava taken below the kidneys can be used to approximate an arterial sample
Testing: The capacity of each sample to generate angiotensin I is an indirect measure of the renin activity. This is expressed in ng angiotensin I per mL per hour.
renal vein plasma renin activity ratio =
= (MAX(PRA left vein, PRA right vein)) / (MIN(PRA left vein, PRA right vein))
renal vein PRA relative to arterial PRA =
= ((PRA renal vein) - (PRA arterial blood)) / (PRA arterial blood)
estimated renal renin secretion per minute, as ng AI per mL per minute =
= 144 * (PRA arterial blood)
Interpretation
The cutoff points for renin activity ratio vary; a ratio of 1.5:1 is often recommended although some use a ratio of 2:1.
A renin activity ratio less than the cutoff does not exclude the diagnosis of renovascular hypertension, particularly in the presence of bilateral disease.
A renin activity ratio >= 1.5 in a patient with unilateral renal artery stenosis is associated with good chance of cure for hypertension (almost 90%), especially if the venous renin activity from the opposite kidney is suppressed (less than or equal to the arterial renin activity).
The "arterial" PRA serves for reference and is useful in unilateral disease:
(1) a kidney with renovascular disease will be higher
(2) the contralateral kidney less
The renal vein PRA relative to arterial PRA is:
(1) normally 24%
(2) in an ischemic kidney 48%
(3) in the contralateral kidney approximately 0%
Limitations - The following conditions may give misleading results:
(1) Bilateral disease (both kidneys affected by renovascular disease)
(2) Segmental lesions
(3) Poor catheter placement or sample aspiration, permitting sample mixing
(4) Improper or inadequate patient preparation
Purpose: To attempt to identify a kidney affected by renovascular disease and to whether surgery is likely to be effective.
Specialty: Endocrinology, Clinical Laboratory, Nephrology, Cardiology
Objective: laboratory tests
ICD-10: I10, I15.0, I12.0, I12.9,
