Renin is an enzyme produced by the kidney, especially when you are eating less salt in your diet or when your blood volume is low. It converts angiotensinogen to angiotensin I. Angiotensin 1 converts to Angiotensin II, which raises blood pressure levels. This is how your kidneys could be involved in causing high blood pressure. The Angiotensin II also stimulates your adrenal gland to produce high levels of aldosterone. 

The renin plasma (blood) test is one that helps the doctor determine what is happening with your high levels of aldosterone.

Forty-six percent of people with high blood pressure don’t know why their blood pressure is high. The renin activity plasma test may help your doctor determine why your blood pressure stays high. For example, if your renin levels are high on this test, it tells your doctor that it’s possible your renal artery is closing down, a condition called renal stenosis. He can then follow up with imaging studies to confirm this.

Normal Ranges for Renin in ng/mL/hr:

0-11 months 2.000-37.000 ng/mL/hr

1 to 3 years 1.700-11.200 ng/mL/hr

4-5 years 1.000-6.500 ng/mL/hr

6-10 years 0.500-5.900 ng/mL/hr

11-15 years 0.500-3.300 ng/mL/hr

>15 years 0.167-5.380 ng/mL/hr

Sodium-depleted, upright peripheral vein specimen:

18-39 years 10.8 ng/mL/hour (mean)

                   2.9-24.0 ng/mL/hour (range)

40 years or older: 5.9 ng/mL/hour (mean)

                            2.9-10.8 ng/mL/hour (range)

Sodium-replete, upright (peripheral vein specimen)

18-39 years: 1.9 ng/mL/hour (mean)

                   < or =0.6-4.3 ng/mL/hour (range)

40 years or older: 1.0 ng/mL/hour (mean)

< or =0.6-3.0 ng/mL/hour (range)

Critical value:  >1.5 renal venous renin ratio  (This is the ratio of serum aldosterone in ng/dL to plasma renin activity in ng/mL/hr. This ratio indicates renal disease. A ratio of > or = 20  and serum aldosterone level of > or – 15 ng/dL means the condition is probably primary aldolsteronism.)

Sources:

1. https://www.labcorp.com/test-menu/34286/renin-activity-plasma#
2. https://www.mayomedicallaboratories.com/test-catalog/Clinical+and+Interpretive/8060
3. https://medlineplus.gov/ency/article/003698.htm