Assays / Hormones / Renin Activity (plasma)

Clinical Use
1 Diagnosis of, and differentiation between, primary and secondary causes of hyper- or hypoaldosteronism.

2 Management of renal artery stenosis.

3 Diagnosis and location of renin secreting tumours.

4 Monitoring mineralocorticoid replacement therapy.

Applications
1 Hyper- or hypoaldosteronism.

The investigation of disorders of aldosterone production requires both aldosterone and plasma renin activity (PRA) measurements. For detailed protocols, please refer to the Aldosterone section, Patient Preparation

2 Renal artery stenosis.

The measurement of PRA in renal vein samples from both kidneys can assist in the diagnosis of renal artery stenosis. The extent to which renal vein PRA is asymmetrical provides a guide to the likelihood that the hypertension associated with this condition will be corrected successfully by surgery. Measurement of PRA in the peripheral circulation is of no value since levels may be normal or raised.

3 Renin secreting tumour.

Hypertensive patients with very high levels of PRA in the peripheral circulation, and in whom chronic renal disease and renal artery stenosis have been excluded, may have a renin secreting tumour. These tumours, often too small to be visualised by renal arteriography, can be located by the increased PRA in samples from the renal vein on the side of the lesion.

4 Monitoring replacement therapy.

Patients with primary mineralocorticoid deficiency require replacement therapy. Provided the renin control system is intact, measurement of PRA can be used to assess adequacy of treatment.

Patient Preparation
1 Hyper- or hypoaldosteronism.

Please refer to the Aldosterone section, Patient Preparation

2,3 Renal artery stenosis and location of a renin secreting tumour.

Take blood (5 mL) from left and right renal veins by catheterization.

4 Monitoring replacement therapy.

Take blood (5 mL) at any time of day, after the patient has been established on a particular dose regime.

Sample Preparation
Transfer the blood to a heparin tube. Use of serum invalidates the assay. Do not place blood sample in 'fridge' or on ice. Transport to local laboratory rapidly at room temperature. Separate the plasma promptly, transfer to 2 fresh plastic containers and freeze immediately. Send frozen plasma (1 portion) to the SAS laboratory. Ensure the sample remains frozen during transport. Store remaining portion frozen until the result of the renin activity is known.

Reference Ranges
Please contact the appropriate SAS laboratory.

Adults (20 to 40 years)
(Sodium intake 100 - 150 mmol/day, Potassium intake 50 - 100 mmol/day):

08.00h, after overnight recumbency: 1.1 - 2.7 pmol/mL/h,

08.30h, after 30 min mobility: 2.8 - 4.5 pmol/mL/h.

In adults, the baseline PRA and the increment in response to changing from a supine to an upright position decline with advancing age. According to some authorities, mean values for both these indices after 60 years of age are about half those of young adults.

Infants

The reference ranges for PRA are poorly defined in infants, but, in the first few weeks of life, values of up to 50 pmol/mL/h have been reported. There is an initial rapid fall, followed by a slower decrease until normal adult levels are reached at about the age of 6 years.

Interpretation of Results
1 Hyper- and hypoaldosteronism.
The reference ranges above must be considered in association with Aldosterone Reference Ranges

2 Renal artery stenosis.
A high PRA, with a ratio of greater than 1.5:1 in samples from the ischaemic and the contralateral normal kidney, suggests that surgical correction would be successful in reversing the patient's hypertension.

3 Renin secreting tumour.
In the absence of chronic renal disease or renal artery stenosis, a greatly elevated PRA in renal vein samples from one side suggests the presence and location of a renin secreting tumour.

4 Monitoring replacement therapy.
Plasma renin activity should be compatible with age. Changes in PRA may take 6 weeks to develop in response to changes in the dose of mineralocorticoid.

Centres offering this assay
Leeds, London (St. Mary's), London (UCLH).

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