Functional testing: renin studies

Static tests of plasma renin activity (PRA) or renal vein renins, even when assessed in relationship to dietary salt intake by renin-sodium profiling, are insufficiently sensitive for reliable screening for renovascular hypertension. Angiotensin converting enzyme inhibitors (ACEIs) stimulate PRA selectively in patients with renovascular hypertension. In large series, the captopril-stimulated PRA test has a high sensitivity in excess of 90% where it is validated against a functional response of an improvement or cure in blood pressure after an intervention to correct renal artery stenosis. However, there are wide variations in the reported accuracy of this test between studies. In part, this may reflect an inappropriate use of a functional test to screen for an anatomical abnormality of renal artery stenosis. Both ACEI-stimulated PRA and ACEI-induced changes in the renogram likely reflect a similar fundamental abnormality in the poststenotic kidney: a selective reduction in the postglomerular vascular resistance leading to a sharp fall in the glomerular filtration rate (GFR). This fall in GFR is manifest as a delay in tubular transit of renogram tracers, leading to an abnormal ACEI renogram, and to a reduction in macula densa NaCl delivery, leading to an abnormal ACEI renogram, and to a reduction in macula densa NaCl delivery, leading to an exaggerated rise in PRA. Under protocol conditions, both the ACEI-PRA and the ACEI-renogram tests have sensitivities for the detection of functional renovascular hypertension of greater than 90%.