Plasma Renin Activity

Function

Plasma Renin Activity (PRA) measures the capacity of the enzyme renin to generate angiotensin I from angiotensinogen. Renin is secreted by the juxtaglomerular cells of the kidneys in response to low blood pressure, low sodium levels, or sympathetic nervous system activation. This test is a critical component of the Renin-Angiotensin-Aldosterone System (RAAS), which regulates long-term blood pressure and fluid balance.

Why it is Ordered

The PRA test is primarily ordered to evaluate patients with hypertension (high blood pressure), especially those who do not respond to standard medications or those suspected of having primary aldosteronism (Conn's syndrome). It is often performed alongside a plasma aldosterone concentration (PAC) test to calculate the Aldosterone-to-Renin Ratio (ARR).

Associated Conditions

• Primary Aldosteronism: Characterized by low PRA and high aldosterone.
• Renovascular Hypertension: High PRA caused by narrowed renal arteries.
• Addison's Disease: Often presents with high PRA due to low blood volume and sodium.
• Essential Hypertension: Most patients have normal or low renin levels.

Monitoring PRA helps clinicians distinguish between different types of hypertension and guides the selection of antihypertensive therapy, such as ACE inhibitors or mineralocorticoid receptor antagonists.