Plasma renin activity also known as the renin (active) assay or random plasma renin, is a measure of the activity of the plasma

Blood plasma

Blood plasma is the straw-colored liquid component of blood in which the blood cells in whole blood are normally suspended. It makes up about 55% of the total blood volume. It is the intravascular fluid part of extracellular fluid...

 enzyme

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

 renin

Renin

Renin , also known as an angiotensinogenase, is an enzyme that participates in the body's renin-angiotensin system -- also known as the Renin-Angiotensin-Aldosterone Axis -- that mediates extracellular volume , and arterial vasoconstriction...

, 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

Hypotension

In physiology and medicine, hypotension is abnormally low blood pressure, especially in the arteries of the systemic circulation. It is best understood as a physiologic state, rather than a disease. It is often associated with shock, though not necessarily indicative of it. Hypotension is the...

. PRA is also raised in certain tumors. A PRA measurement may be compared to a plasma aldosterone

Aldosterone

Aldosterone is a hormone that increases the reabsorption of sodium ions and water and the release of potassium in the collecting ducts and distal convoluted tubule of the kidneys' functional unit, the nephron. This increases blood volume and, therefore, increases blood pressure. Drugs that...

 concentration as an aldosterone-to-renin ratio

Aldosterone-to-renin ratio

Aldosterone-to-renin ratio is the mass concentration of aldosterone divided by the plasma renin activity in blood plasma. The aldosterone/renin ratio is recommended as screening tool for primary hyperaldosteronism.-Interpretation:...

.

Measurement and Values

Measurement is done from a sample of venous blood using immunological measuring mechanisms like ELISA

ELISA

Enzyme-linked immunosorbent assay , is a popular format of a "wet-lab" type analytic biochemistry assay that uses one sub-type of heterogeneous, solid-phase enzyme immunoassay to detect the presence of a substance in a liquid sample."Wet lab" analytic biochemistry assays involves detection of an...

, RIA

Radioimmunoassay

Radioimmunoassay is a very sensitive in vitro assay technique used to measure concentrations of antigens by use of antibodies...

, 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
  Pregnancy
  Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...
  
  
• 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
  Hormonal contraception
  Hormonal contraception refers to birth control methods that act on the endocrine system. Almost all methods are composed of steroid hormones, although in India one selective estrogen receptor modulator is marketed as a contraceptive. The original hormonal method—the combined oral contraceptive...
  
  , anti-anginals drugs, etc. (basically, most drugs that are active on the heart
  Heart
  The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
  
  , 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:

Unit	Lower limit	Upper limit
ng/(mL*hour)	0.29, 1.9	3.7
mcU/mL	3.3, 21	41

Results and explanations

Please go through the physiology

Physiology

Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...

 of renin

Renin

Renin , also known as an angiotensinogenase, is an enzyme that participates in the body's renin-angiotensin system -- also known as the Renin-Angiotensin-Aldosterone Axis -- that mediates extracellular volume , and arterial vasoconstriction...

 and the renin-angiotensin system

Renin-angiotensin system

The renin-angiotensin system or the renin-angiotensin-aldosterone system is a hormone system that regulates blood pressure and water balance....

 to understand why the following occur.

Higher-than-normal levels may indicate:

Disease	Brief Description
Addison's disease Addison's disease Addison’s disease is a rare, chronic endocrine disorder in which the adrenal glands do not produce sufficient steroid hormones...	Kidneys trying to counter low aldosterone output.
Cirrhosis Cirrhosis Cirrhosis is a consequence of chronic liver disease characterized by replacement of liver tissue by fibrosis, scar tissue and regenerative nodules , leading to loss of liver function...  of the liver	Reduced breakdown of Angiotensin.
Essential hypertension Essential hypertension Essential hypertension is the form of hypertension that by definition, has no identifiable cause. It is the most common type of hypertension, affecting 95% of hypertensive patients, it tends to be familial and is likely to be the consequence of an interaction between environmental and genetic...	Just more of renin is being secreted by the kidneys.
Hemorrhage (bleeding)	Kidneys trying to raise falling blood pressure.
Hypokalemia Hypokalemia Hypokalemia or hypokalaemia , also hypopotassemia or hypopotassaemia , refers to the condition in which the concentration of potassium in the blood is low...	Kidneys trying to raise falling blood pressure due to reduced cardiac output.
Malignant hypertension Malignant hypertension Malignant hypertension or hypertensive emergency is severe hypertension with acute impairment of an organ system and the possibility of irreversible organ-damage...	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 Juxtaglomerular apparatus The juxtaglomerular apparatus is a microscopic structure in the kidney, which regulates the function of each nephron. The juxtaglomerular apparatus is named for its proximity to the glomerulus: it is found between the vascular pole of the renal corpuscle and the returning distal convoluted tubule...  perfusion.

Lower-than-normal levels may indicate:

Disease	Brief Description
ADH Vasopressin Arginine vasopressin , also known as vasopressin, argipressin or antidiuretic hormone , is a neurohypophysial hormone found in most mammals, including humans. Vasopressin is a peptide hormone that controls the reabsorption of molecules in the tubules of the kidneys by affecting the tissue's...  therapy	Leads to water retention and thus raised blood pressure.
Salt-retaining steroid Steroid A steroid is a type of organic compound that contains a characteristic arrangement of four cycloalkane rings that are joined to each other. Examples of steroids include the dietary fat cholesterol, the sex hormones estradiol and testosterone, and the anti-inflammatory drug dexamethasone.The core... therapy	see above
Salt-sensitive essential hypertension	see above
Primary Hyperaldosteronsim	see above and direct inhibition of aldosteron on renin secretion

Further reading

• MedlinePlus Medical Encyclopedia(Public Domain - copy left):http://www.nlm.nih.gov/medlineplus/ency/article/003698.htm.

• Brenner & Rector's The Kidney, 7th ed., Saunders, 2004. pp. 2118–2119.Full Text with MDConsult subscription.