Plasma renin activity

Plasma renin activity (PRA), also known as the renin (active) assay or random plasma renin, is 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. A PRA measurement may be compared to a plasma aldosterone concentration as an aldosterone-to-renin ratio.

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
Reference ranges for blood tests of plasma renin activity can be given both in mass and in milk clotting units (MCU), with the former being roughly convertable to the latter by multiplying with 11.2. The following table gives the lower limit (2.5th percentile) and upper limit (97.5th percentile) for plasma renin activity by mass and MCU, with different values owing to various factors of variability of reference ranges:

Results and explanations
Please go through the physiology of renin and the renin-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-.
 * Primary Hyperaldosteronsim....-same plus direct inhibition of aldosteron on renin secretion-.