PRA or Plasma renin activity - also known as "Renin (active) assay" or "random plasma renin", is, as implied in the name, a measure of the activity of the plasma enzyme "Renin", which plays a major role in the body's regulation of blood pressure, thirst and urine output. PRA is sometimes measured, specially in case of certain diseases which present with hypertension or hypotension. PRA is also raised in certain tumors.

This test is performed as part of the diagnosis and treatment of high blood pressure, specially, if it shows low response to first-line hypertension. Persons with idiopathic (i.e. "primary" or "of unknown etiology") hypertension may need to have their plasma renin and aldosterone levels checked to evaluate if this blood pressure is sensitive to the amount of salt they take in diet.Thus, the test results helps to guide your doctor in choosing the correct medication. Salt-sensitive patients with high blood pressure associated with low renin levels respond well to diuretic medications or even just a low salt diet, if the blood pressure is only mildly raised.

For details on the physiology of Renin, see this page.

Measurement and Values

Measurement is done from a sample of venous blood using immunological measuring mechanisms like ELISA, RIA, etc. Often these are done by automated machines to minimize human error.

Considerations for variation

These values are quite variable in a normal person. Variation occur with the following:

• Salt intake
• Pregnancy
• Time of the day
• Body position (sitting, lying down)
• State of water intake and dehydration
• Use of anti-hypertensive drugs, estrogen-containing forms of hormonal contraception, anti-anginals drugs, etc (basically, most drugs that are active on the heart, blood vessels and/or the kidneys.
• Certain diseases of the heart, kidneys, etc.

Normal values

Range from 1.9 to 3.7 ng/mlL/hour.

Results and explanations

Please go through the physiology of renin and therenin-angiotensin system to understand why the following occur.

Higher-than-normal levels may indicate:

• Addison's disease................Kidneys trying to counter low aldosterone output.
• Cirrhosis of the liver...........Reduced breakdown of Angiotensin.
• Essential hypertension...............Just more of renin is being secreted by the kidneys.
• Hemorrhage (bleeding)................Kidneys trying to raise falling blood pressure.
• Hypokalemia..........................Kidneys trying to raise falling blood pressure due to reduced cardiac output.
• Malignant hypertension...............Excessive renin is being secreted by the kidneys.
• Renin-producing renal tumors.........Tumors can secrete substances like this. See tumor markers.
• Renovascular hypertension............Renal vascular damage leading to reduced JGA perfusion.

Lower-than-normal levels may indicate:

• ADH therapy..........................Leads to water retention and thus raised blood pressure.
• Salt-retaining steroid therapy...........-same-.
• Salt-sensitive essential hypertension....-same-.

Reference and Acknowledgment

• MedlinePlus Medical Encyclopedia(Public Domain - copy left):
• Brenner & Rector's The Kidney, 7th ed., Saunders, 2004. pp.2118-2119.Full Text with MDConsult subscription.