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Plasma Renin Activity (PRA) Clinical Use • Assess renin-aldosterone axis • Differential diagnosis of hypertension Clinical Cut-off Values†† Sodium/volume hypertension likely Primary aldosteronism possible§ Renin-mediated hypertension likely Renovascular hypertension possible Renovascular hypertension more likely ng/mL/h 0.3-3.0 0.4-8.8 0.65-5.0 Upright – – <15.0 <17.0 <16.0 <16.0 <0.65 <0.65 >0.65 >1.5 >10.0 Clinical Background Renin is a proteolytic enzyme produced by the kidney. Secretion is modulated by changes in renal blood flow. Erect posture, sodium depletion, hemorrhage, and low cardiac output all increase renin secretion by reducing flow. The measurement of plasma renin activity (PRA) is useful in evaluating hypertension. Primary hyperaldosteronism is associated with sodium retention, increased blood volume, increased renal blood flow, and low PRA. Thus, a normal or high PRA rules out primary aldosteronism. Conversely, a normal or low PRA helps rule out renal hypertension. Additionally, an elevated PRA may indicate renovascular hypertension due to renal artery stenosis. The interpretation of PRA values is facilitated by measurements of aldosterone and serum potassium levels. Renin Aldosterone Potassium 1° aldosteronism Low Hyporeninemic Low hypoaldosteronism *In apparently healthy, non-hypertensive, non-medicated, non-pregnant individuals on unrestricted diets. †Pediatric data from J Pediatr. 1976;89:256; Pediat Res. 1979;13:817; and Eur J Pediatr. 1994;153:284. †† In ambulatory, briefly seated hypertensive patients. §To be used in conjunction with aldosterone concentration; this is a rare condition. High Low Low High Method • Angiotensin I generation, radioimmunoassay (RIA) • Analytical sensitivity: 0.37 ng/mL/h Specimen Requirements 3 mL frozen EDTA plasma Interpretive Information • Renal hypertension • Addison’s disease • Secondary hypoaldosteronism • Medications (eg, diuretics, ACE inhibitors) 0.4 mL minimum Avoid refrigerated temperatures. When submitting catheterization studies, retain a portion of each sample at the referring laboratory. • Hyporeninemic hypoaldosteronism • Primary aldosteronism • Essential hypertension (1/3rd of cases; PRA <0.65 ng/mL/h) Patient preparation: moderate sodium diet; ambulatory for 30 minutes; no medications, preferably for 3 weeks prior to sample collection. 137 Alphabetical Test Section Reference Range Adults* ng/mL/h Supine Upright Upright/sitting Supine Children† 3-12 mo <15.0 1-3 y <10.0 4-6 y <7.5 7-9 y <5.9 10-12 y <5.3 13-15 y <4.4 10537X