How Do I Prepare for an Aldosterone-to-Renin Ratio Test?

Yes, the time of day you get your aldosterone and renin blood test done is critical to the accuracy of your results. Blood for an aldosterone-to-renin ratio (ARR)—the primary screening test for primary aldosteronism—should be drawn under specific, standardized conditions. This usually means testing in the morning, after you have been out of bed and awake for at least two hours ^[1][2].

Aldosterone is a hormone produced by your adrenal glands that helps control salt, water, and potassium in your body. Renin is a protein made by your kidneys that helps regulate blood pressure. The ARR test compares the levels of these two substances to see if your body is making too much aldosterone independently of renin. Because it is a screening test and not a final diagnosis, the conditions under which your blood is drawn are very important for interpreting the results accurately ^[3].

The Morning of Your Test

Hormones like aldosterone and renin naturally fluctuate throughout the day and change significantly based on your body position. To ensure your doctor gets an accurate baseline, clinical guidelines recommend specific preparation ^[4][2]:

• Time of Day: Schedule your blood draw for the morning, ideally between 8:00 AM and 10:00 AM, though you should always follow your clinic’s specific instructions ^[2]. If you work night shifts or have an unusual sleep schedule, ask your doctor how to time your test appropriately.
• Being Awake and Upright: You should be awake and out of bed for at least two hours before the blood is drawn ^[2]. This ensures your body has adjusted from lying down overnight.
• Resting Before the Draw: Immediately before the needle is inserted, you must sit quietly in a chair for 5 to 15 minutes ^[2]. This brief, seated resting period stabilizes the hormone levels in your blood ^[4]. It is best to confirm these collection conditions with the ordering clinic in advance and ask to have the seated rest period written directly on your lab order.
• Fasting and Hydration: Fasting is not generally required for the ARR test itself, but you should follow your laboratory’s hydration and food instructions, especially if other blood tests are being drawn at the same time.

Your Diet Before Testing

A common mistake patients make before an ARR test is independently restricting their salt intake.

• Maintain Your Usual Salt Diet: You should maintain your usual, everyday sodium intake and avoid deliberately restricting or increasing your salt in the days leading up to your blood draw ^[2]. If you have a prescribed sodium restriction for conditions like heart failure or kidney disease, ask your clinician how to proceed safely.
• The Risk of Low Sodium: If you eat a low-sodium diet, your body naturally produces more renin to help hold onto salt. This elevated renin can make your ARR look normal even if you actually have primary aldosteronism, leading to a false-negative result ^[5]. In one study of untreated hypertensive patients, 52% of those with confirmed primary aldosteronism had a negative screening test when placed on a low-sodium diet ^[5].
• Potassium Levels: Your blood potassium levels should be in the normal range during the test ^[2], as low potassium can suppress aldosterone production. Your doctor may check your potassium levels beforehand and prescribe a correction if they are too low ^[2]. Never start a potassium supplement on your own, as this can be dangerous, especially if you have kidney disease or take certain blood pressure medications.

A Note on Blood Pressure Medications

Many common medications and supplements can significantly alter your renin and aldosterone levels ^[6][7]. This includes prescription drugs, over-the-counter anti-inflammatories (NSAIDs), oral contraceptives, and even licorice-containing products.

Your doctor may need to adjust your medications for a period before your test, often temporarily replacing them with alternatives that don’t interfere with the results ^[6]. The required “washout” time varies by drug. However, medication safety always takes priority over a perfectly standardized test. Never stop or change your blood pressure medications without explicit instructions and close supervision from your doctor.

What Happens After the Test?

Because the ARR is a screening tool, your doctor will interpret the ratio by looking at both your aldosterone and renin levels, your potassium status, your current medications, and the specific assay used by the laboratory ^[3][8]. An abnormal result does not immediately mean you have a disease; it often means you will need repeat testing or a confirmatory test to verify the diagnosis ^[9].