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.[1] A PRA measurement may be compared to a plasma aldosterone concentration as an aldosterone-to-renin ratio.


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.


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.


Reference ranges for blood tests of plasma renin activity can be given both in mass and in international units (μIU/mL or equivalently mIU/L, improperly shown as μU/mL or U/L, confusing mcU/mL used where Greek μ not available), with the former being roughly convertible to the latter by multiplying with 11.2.[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:


Unit                Lower limit                Upper limit
ng/(mL*hour)     0.29, 1.9                        3.7[3][4]
μIU/mL               3.3, 21                             41[2][5]


Higher-than-normal levels may indicate:
Disease                      Brief Description
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:

Disease                   Brief Description
ADH therapy                    Leads to water retention and thus raised blood pressure.
Salt-retaining steroid therapy     see above
Salt-sensitive essential hypertension     see above
Primary Hyperaldosteronsim     see above and direct inhibition of aldosteron on renin Secretion