The Renin Aldosterone Ratio (RAR) is a laboratory screening tool used to identify a secondary, potentially correctable cause of high blood pressure. The ratio is calculated from blood tests that measure the levels of the hormones aldosterone and renin in the plasma. The primary purpose of the RAR is to screen for a condition called primary hyperaldosteronism, also known as Conn’s syndrome. This condition involves the adrenal glands producing too much aldosterone, which frequently causes resistant hypertension. Detecting this condition allows for targeted treatment that can significantly improve blood pressure control and reduce associated health risks.
The Regulatory Roles of Renin and Aldosterone
Renin and aldosterone are components of the Renin-Angiotensin-Aldosterone System (RAAS), a hormonal pathway that regulates blood pressure and fluid balance. Renin is an enzyme released by specialized cells in the kidneys, primarily in response to a drop in blood pressure, decreased blood volume, or low sodium levels.
Once released into the bloodstream, renin initiates a cascade of reactions. Renin converts angiotensinogen, a protein produced by the liver, into angiotensin I. Angiotensin I is then converted into the active hormone angiotensin II by an enzyme found mainly in the lungs.
Angiotensin II acts on blood vessels, causing them to constrict, which increases blood pressure. It also stimulates the adrenal glands to release aldosterone. Aldosterone is a steroid hormone that acts on the kidneys to promote the reabsorption of sodium back into the blood.
The body retains water alongside the sodium, which increases blood volume and helps raise blood pressure back to a normal range. Aldosterone also causes the kidneys to excrete potassium, maintaining electrolyte balance. In a healthy person, high levels of renin should lead to high levels of aldosterone, and the system shuts down once blood pressure is restored.
Preparing for the Renin Aldosterone Ratio Test
The Renin Aldosterone Ratio test requires specific pre-analytic conditions for accurate results. Many common medications interfere with the production or action of renin and aldosterone, potentially leading to false results. Healthcare providers often require a “washout” period where interfering drugs are temporarily stopped or replaced.
Medications that directly block aldosterone, such as mineralocorticoid receptor antagonists (spironolactone or eplerenone), must be stopped for at least four to six weeks before the test. Other common blood pressure drugs, including ACE inhibitors, angiotensin receptor blockers (ARBs), and diuretics, require a washout period of approximately two weeks. Beta-blockers and certain pain relievers like nonsteroidal anti-inflammatory drugs (NSAIDs) may also need to be discontinued one to two weeks prior.
The patient’s diet and potassium levels must also be addressed. Patients are instructed to maintain a regular, unrestricted sodium intake for several days leading up to the test. If a patient has low potassium levels (hypokalemia), this must be corrected before the blood draw, as low potassium can suppress aldosterone release and interfere with reliability.
The patient’s posture and the time of the blood draw affect the outcome because RAAS hormones fluctuate throughout the day and in response to gravity. The blood sample is usually collected in the morning, often after the patient has been out of bed for at least two hours. The patient is typically seated for five to fifteen minutes before the blood is drawn to standardize the effect of gravity on the hormones.
Interpreting the Renin Aldosterone Ratio
The Renin Aldosterone Ratio (RAR) is calculated by dividing the plasma aldosterone concentration by the plasma renin activity (A/R). This calculation looks for an inappropriate relationship between the two hormones, which is the hallmark of primary hyperaldosteronism. Interpretation relies on specific cutoff values, which may vary depending on the laboratory’s measurement units.
A high RAR is the main indicator suggesting primary hyperaldosteronism. This occurs because the adrenal gland produces aldosterone autonomously, independent of the body’s normal regulatory signals. The normal feedback loop attempts to compensate by drastically reducing the release of renin. This combination of high aldosterone and suppressed renin activity mathematically yields a significantly elevated ratio.
A common cutoff value suggesting a positive screen is a ratio greater than 30, provided the aldosterone level is also sufficiently high (usually above 10 ng/dL). A high ratio indicates that the patient’s hypertension is likely driven by this hormonal imbalance. The excess aldosterone causes sodium retention and potassium loss, which can lead to resistant high blood pressure and potentially low potassium levels.
An elevated RAR is only a screening test, not a definitive diagnosis. A normal or low ratio suggests the high blood pressure is likely due to other causes, such as essential hypertension. However, a high ratio necessitates further confirmatory testing to formally diagnose primary hyperaldosteronism, such as the saline suppression test or oral salt loading test.