Plasma renin activity Test

Your healthcare practitioner may order a PRA test to find the cause of your high blood pressure. This test specifically checks for renovascular hypertension, which is high blood pressure that occurs due to narrowing of blood vessels that supply the kidney.

Some of the following tests are also done along with the PRA test to detect the exact cause of elevated blood pressure:

• Aldosterone test: Aldosterone measured simultaneously with PRA helps to diagnose the type of hyperaldosteronism (overproduction of aldosterone). Hyperaldosteronism is one of the many causes of hypertension. There are two types of hyperaldosteronism:
  • Primary hyperaldosteronism: This occurs when problems in the adrenal gland cause overproduction of aldosterone. People with primary hyperaldosteronism will have high aldosterone and low renin activity.
  • Secondary hyperaldosteronism: This occurs when problems outside the adrenal gland trigger excess aldosterone release. People who have secondary hyperaldosteronism due to kidney disease have high levels of both aldosterone and renin. (Read more: Aldosterone blood test)
• Renal vein renin assay: In this test, the PRA in blood samples taken from both renal veins is determined. If the high blood pressure is due to narrowing of a renal vessel or kidney disease, the renal vein renin level of the affected kidney would be much higher than normal. 
• Renin stimulation test: A renin stimulation test is performed to diagnose the type of hyperaldosteronism. This test also shows the effect of low sodium diet and body position on renin levels. In primary hyperaldosteronism, the plasma renin will not increase despite the low sodium or change in body posture. However, in secondary hyperaldosteronism, renin level increases with low sodium diet or upright position.
• Captopril test: The captopril test is also a screening test for renovascular hypertension. In this test, a person is given a blood pressure medication called captopril. Patients with renovascular hypertension will have a greater fall in blood pressure and increase in PRA after captopril administration compared to those with essential hypertension (the most common type of hypertension).

A fasting sample is preferred for this test because the renin value may be high in the morning. Intake of salt intake and liquorice can affect the test results. Other factors that increase renin levels and alter the test results are:

• Pregnancy
• Low salt diet 
• Upright position 
• ACE-inhibiting medicines 
• Diuretics (water pills) 
• Medicines for hypertension
• Oestrogen hormone
• Birth control pills
• Vasodilators (medicines that dilate the blood vessels)

Factors that can decrease renin levels are:

• Recumbent (lying down) position 
• Beta-blockers
• Clonidine
• Nonsteroidal anti-inflammatory agents
• Potassium 
• Reserpine 

Your healthcare practitioner may request you to skip some medicines (mentioned above) two to four weeks before the test. For example spironolactone is stopped four to six weeks before the test. However, do not skip any medicine on your own. 

Also, you may be instructed to restrict salt intake to about 3 grams per day for three days before the test.

Your healthcare practitioner will explain the steps of the test to you. You may be advised to stand or sit upright for two hours before the test. A blood sample will then collected be in the following manner:

• A laboratory technician will tie an elastic band on your upper arm and ask you to tighten your fist. This will help locate a vein for blood withdrawal.
• He/she will then insert a sterile needle into the vein and collect a small amount of blood.

For the renin stimulation test, blood samples are taken both in the recumbent and upright positions. For the recumbent sample, you may be asked to remain in bed in the morning until the sample is taken.

Normal results:

Normal values for PRA, expressed in nanograms per millilitre per hour (ng/mL/hr), are given in the following table:

Abnormal results:

High PRA can be observed in the following conditions:

• Addison disease (a condition in which the adrenal glands cannot produce enough hormones) 
• Bleeding (haemorrhage)
• Heart failure
• Renovascular hypertension
• Liver cirrhosis (scarring of the liver)
• Dehydration 
• Diarrhoea 
• Nephrotic syndrome due to kidney damage
• Kidney tumours that produce renin
• Malignant hypertension (extremely high blood pressure that develops suddenly) 
• Essential hypertension 

The following conditions cause low PRA:

• Primary hyperaldosteronism
• Steroid therapy (it may cause the body to retain salt)
• Congenital adrenal hyperplasia
• Chronic renal impairment
• Fluid overload
• Treatment with antidiuretic hormone (ADH)

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. The above information is provided from a purely educational point of view and is in no way a substitute for medical advice by a qualified doctor.