Renin is an enzyme produced in the kidney that converts angiotensinogen (produced in the liver) to angiotensin I, which is subsequently converted to angiotensin II (in endothelia throughout the body). Angiotensin II is a potent vasopressor agent responsible for hypertension of renal origin and is a powerful releaser of aldosterone from the adrenal cortex. Both angiotensin II and aldosterone increase blood pressure. Renin levels increase when there is decreased renal perfusion pressure.

The renin–angiotensin–aldosterone system helps maintain a balance of potassium and sodium blood levels. The renin–aldosterone axis regulates sodium and potassium balance and blood volume and pressure. Renal reabsorption of sodium affects plasma volume. Low plasma volume, low blood pressure, low sodium, and increased potassium induce renin release, causing increased aldosterone through stimulation of angiotensin. Potassium loss, acute blood pressure increases, and increased blood volumes suppress renin release.

The PRA test measures the rate at which renin forms angiotensin in plasma and is most useful in the differential diagnosis of hypertension, whether essential, renal, or renovascular. In primary hyperaldosteronism, the findings will demonstrate that aldosterone secretion is exaggerated, and secretion of renin is suppressed. In renal vascular disease, renin is elevated. This test is helpful in identifying renin-producing tumors of the kidney.

Plasma Renin Activity (PRA).

• Adult (upright: sodium repleted):

  • 18–39 years: 0.6–4.3 ng/mL/hr or 0.6–4.3 μg/hr/L

  • 40 years and older: 0.6–3.0 ng/mL/hr or 0.6–3.0 μg/hr/L

• Adult (upright: sodium depleted):

  • 18–39 years: 2.9–24.0 ng/mL/hr or 2.9–24.0 μg/hr/L

  • 40 years and older: 2.9–10.8 ng/mL/hr or 2.9–10.8 μg/hr/L

Renin Direct:

1. Adult supine: 12–79 mU/L or 12–79 mU/L

2. Adult standing: 13–114 mU/L or 13–114 mU/L

3. Procedure

4. Obtain a 5-mL venous blood sample (lavender-topped tube). Fasting is required. Collect specimen with scrupulous attention to detail. Use EDTA as the anticoagulant to aid in preservation of any angiotensin formed before examination. Observe standard precautions. Label the specimen with the patient’s name, date and time of collection, and test(s) ordered.

5. Draw blood in chilled tubes and place samples on ice. Transport samples to laboratory immediately in a biohazard bag. The sample must be centrifuged in a refrigerated centrifuge.

6. Record posture and dietary status of patient at time of blood drawing.

7. A 24-hour urine sodium should be done concurrently to aid in diagnosis.

1. Increased renin levels occur in the following conditions:

   1. Secondary aldosteronism with malignant hypertension

   2. Renovascular hypertension

   3. Reduced plasma volume due to low-sodium diet, diuretic agents, Addison disease, or hemorrhage

   4. CKD

   5. Salt-losing status owing to GI disease (sodium [Na] and potassium [K] wastage)

   6. Renin-producing tumors of kidney

   7. Few patients (15%) with essential hypertension

   8. Bartter syndrome (high in renin hypertension)

   9. Pheochromocytoma

2. Decreased renin levels are found in the following conditions:

   1. Primary aldosteronism

   2. Unilateral renal artery stenosis

   3. Administration of salt-retaining steroids

   4. Congenital adrenal hyperplasia with 17-hydroxylase deficiency

   5. Liddle syndrome (autosomal dominant disorder resulting in abnormal kidney function and hypertension)

Pretest Patient Care

1. Explain test purpose and procedure.

2. A low sodium diet should be maintained for 3 days before the specimen is obtained. A 24-hour urine sodium and potassium should also be done to evaluate salt balance. The blood test should be drawn at the end of the 24-hour urine test.

3. The specimen is drawn with patient in an upright position.

4. Ensure that antihypertensive drugs, cyclic progestogens, estrogens, diuretic agents, and licorice are terminated at least 2 weeks and preferably 4 weeks before a renin–aldosterone workup.

5. If a standing specimen is ordered, the patient must be standing for 2 hours before testing, and blood should be drawn with the patient in the sitting position.

6. Do not allow caffeine ingestion 1 day before the test.

7. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

1. Levels vary in healthy persons and increase under influences that tend to shrink the intravascular fluid volume.

2. Random specimens may be difficult to interpret unless diet and salt intake of patient are regulated.

3. Values are higher when the patient is in an upright position, when the test is performed early in the day, when the patient is on a low-sodium diet, during pregnancy, and with drugs such as diuretic agents and antihypertensive drugs and foods such as licorice. see Appendix E for other drugs that affect outcomes.

4. Recently administered radioisotopes interfere with test results.

5. Indomethacin and salicylates decrease renin levels.