The aldosterone-renin ratio (ARR), also known as the aldosterone plasma renin activity (PRA) ratio, is a critical diagnostic tool used in the evaluation of primary aldosteronism, a condition characterized by excessive production of the hormone aldosterone by the adrenal glands.
Aldosterone plays a significant role in regulating blood pressure by controlling sodium and potassium levels in the blood, and its overproduction can lead to hypertension (high blood pressure) and hypokalemia (when the amount of potassium in your blood is too low).
The ARR test measures the levels of aldosterone and renin, an enzyme essential for the production of angiotensin II which stimulates aldosterone release, in the blood.
A high ARR suggests that aldosterone production is not appropriately regulated by renin, which is indicative of primary aldosteronism.
The ARR compares the amount of aldosterone to that of renin, and the resulting number – a ratio – is then compared with a “cutoff” value currently set at 30 (or 750 when measurements are expressed in SI units). Below this value, the result is considered normal. Above this value, primary aldosteronism is suspected.
What is Measured?
Testing laboratories measure the ratio of aldosterone to renin. The test not only shows the level of each hormone, it determines the relative amount of aldosterone to renin present in the blood. In healthy individuals, these levels rise and fall together. In those who have primary aldosteronism, aldosterone is high while renin is low — renin is said to be “suppressed.” The higher the ARR result, the more likely a person has primary aldosteronism.
What Results Mean:
While measuring aldosterone is fairly straightforward, renin can be evaluated through different methods. Until recently, Plasma Renin Activity (PRA) was the most common way to measure “active renin.” It is now frequently replaced with a faster method, Direct Renin Concentration (DRC). Unfortunately, this method may not be optimal to measure low levels of renin, which is exactly what must be ascertained with primary aldosteronism.
ARR Accuracy:
To be accurate, tests must be sensitive (they must detect the condition in people who have it), and specific (they must rule out the condition in people who don’t have it). Highly sensitive tests are usually less specific: they trigger “false-positives,” which means some patients are told they have the condition while they don’t. Highly specific tests tend to have lower sensitivity: they trigger “false-negatives,” which means some patients are told they do not have the condition while they do.
ARRs are considered sensitive tests, and thus lack specificity. Up to 50% of elevated ARRs may be “false-positives.” To remediate this risk, PA patients must undergo confirmatory testing to validate or rule out false positive ARR results.
Determining what is considered an abnormal result is as important as the accuracy of the test. ARR cutoff values are somewhat arbitrary, and based on current understanding of excess aldosterone. The lower the cutoff value, the more people are diagnosed, and vice versa.
Note: The test must be interpreted carefully, considering factors that can affect renin or aldosterone levels, such as medications, posture, and sodium intake. Consequently, the ARR is often used in conjunction with other clinical assessments and diagnostic procedures to confirm the diagnosis.
Aldosterone/Plasma Renin Activity Ratio
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