Aldosterone-renin ratio explained

Aldosterone is a hormone made by the adrenal glands that tells the kidneys to keep sodium and water and to remove potassium, which raises blood volume and blood pressure. Renin is an enzyme made by the kidneys that starts the chain that ends in aldosterone production, with angiotensin I and II as intermediate steps, according to Lab Tests Online UK.

Normally, when renin increases, aldosterone increases, and when renin is low, aldosterone falls. The aldosterone-renin ratio (ARR) identifies when that normal relationship has broken down. In primary hyperaldosteronism the adrenals make aldosterone independently, which pushes renin down, so a high aldosterone alongside a low renin produces a high ratio.

Lab Tests Online UK explains that the most common reason for aldosterone and renin tests is high blood pressure with particular features. These tests are also used if symptoms suggest low aldosterone.

• Sustained high blood pressure.

• Difficult-to-control blood pressure needing multiple medicines.

• High blood pressure with low potassium levels.

• High blood pressure starting under the age of 30.

• A family history of high blood pressure or early stroke (before 40).

• Symptoms suggesting low aldosterone, such as low blood pressure, dizziness on standing, muscle weakness, fatigue, salt cravings or excessive thirst.

Lab Tests Online UK stresses that these tests need careful preparation. Medicines that can affect results include beta-blockers and ACE inhibitors (ideally stopped 2 weeks before testing), spironolactone and other diuretics (ideally stopped 6 weeks before), NSAIDs such as ibuprofen, and oral contraceptives. It stresses that you should never stop any medicine without your doctor's advice, and your doctor may prescribe alternative blood pressure medicines during the preparation period.

The NHS Highland guideline takes a pragmatic view: although it is optimal to test off interfering drugs, this is often impractical and may carry risk, so the first screening test can usually be done on treatment provided the medicines are noted and the requesting clinician knows their likely effects. If the first result is abnormal, it may be repeated off the interfering drugs.

Other advice includes keeping a normal salt intake, avoiding real liquorice for at least 2 weeks, avoiding strenuous exercise and trying to be relaxed. Low potassium should be corrected because it affects aldosterone. The blood sample is usually taken after a short rest seated, because standing or walking beforehand may affect results. The NHS Highland protocol describes a mid-morning sample after being up and about for 2 hours and then sitting upright for 15 minutes, and sending the sample straight to the laboratory.

Doctors look at aldosterone and renin together, often along with cortisol, because different patterns point to different conditions.

There is no single ratio that applies everywhere. As an example of one UK service's approach, NHS Highland's guideline treats an aldosterone to renin ratio under 35 as normal (using plasma renin concentration), and a ratio of 35 or above as unable to exclude primary hyperaldosteronism, especially if plasma aldosterone is above 410 pmol/L. The aldosterone reference range quoted there is 130 to 800 pmol/L.

These figures belong to that health board's assay and units. Other laboratories report renin as activity rather than concentration and use different thresholds, so do not apply a number from one report to another. Ask the clinician or the laboratory report for the reference range that applies to your result.

Several things can distort the ratio. Lab Tests Online UK says beta-blockers and NSAIDs can falsely increase it, while diuretics, ACE inhibitors and calcium channel blockers can falsely decrease it. Pregnancy, kidney impairment, older age, low potassium and severe illness also matter.

A high ratio suggests primary hyperaldosteronism but does not prove it. Lab Tests Online UK describes the usual next steps as confirmatory tests such as a salt loading (saline infusion) test, then CT or MRI of the adrenal glands and sometimes adrenal vein sampling to find which side is producing excess aldosterone.

Treatment depends on the cause. If one adrenal gland is responsible, surgical removal (adrenalectomy) can cure the high blood pressure, and if both glands are involved, medicines such as spironolactone or eplerenone are used to block aldosterone. The NHS Highland guideline notes that a saline infusion test is contraindicated in some people, including those with heart failure, kidney impairment, severe uncontrolled hypertension and low potassium, which is one reason this work-up is specialist-led.