Aldosterone-to-renin ratio (ARR)

The aldosterone-to-renin ratio (ARR) is the standard first-line screen for primary hyperaldosteronism (Conn's syndrome), in which the adrenal glands make too much aldosterone independently of the body's normal controls. Aldosterone is a hormone that tells the kidneys to hold on to salt and water and get rid of potassium, raising blood pressure. Renin is the kidney signal that normally switches aldosterone production on and off. In health, when aldosterone is high, renin should be low, and vice versa. In primary hyperaldosteronism the adrenals make aldosterone autonomously, so aldosterone is inappropriately high while renin is strongly suppressed — and it is this combination, captured as a high ratio, that flags the disorder. The ARR is calculated from a blood sample, ideally taken under standardised conditions (mid-morning, seated, with attention to salt intake, potassium level and blood-pressure medicines, several of which alter the result). It is a screening tool, not a final diagnosis: a raised ratio needs a confirmatory test and then imaging to distinguish a single adenoma (often curable with surgery) from both glands being overactive (treated with medication). Its importance is that primary hyperaldosteronism is far commoner than once thought, especially in resistant hypertension, and is very treatable once found.

• Primary hyperaldosteronism (Conn's syndrome) — a high ratio (high aldosterone with suppressed renin) is the key screening signal; needs confirmatory testing.
• Resistant or early-onset high blood pressure with low potassium — a classic setting for a positive screen.
• A borderline ratio — can be affected by medications, salt intake, posture and a low potassium, and is often repeated under controlled conditions.

• A normal ratio — makes primary hyperaldosteronism unlikely; high blood pressure is then attributed to other causes.
• A low aldosterone with high renin — points away from this diagnosis, and can reflect other conditions.