Diagnostic Testing and Genetic Confirmation of FH-III

Navigating the diagnostic process for a rare adrenal condition requires patience and precise coordination with your medical team. Confirming a diagnosis of Familial Hyperaldosteronism Type III (FH-III) requires a careful, multi-step approach. Because this condition is driven by a “leaky” genetic valve in the adrenal glands, the diagnostic process focuses on proving that your body is producing too much aldosterone and then finding the genetic cause ^[1][2].

Diagnostic Flowchart

Suspected primary aldosteronism → ARR with supervised preparation → Selected confirmatory testing → Germline panel and genetic counseling → Imaging and individualized surgical planning.

Step 1: The Screening Blood Test (ARR)

The first step for anyone with early-onset or resistant high blood pressure is the Aldosterone-Renin Ratio (ARR) test ^[3][4]. This test compares the level of aldosterone to the level of renin (an enzyme that usually tells the body to make aldosterone). In FH-III, aldosterone will be high while renin is “suppressed” (very low) because the body is trying to shut down hormone production that it doesn’t actually need ^[5].

Preparing for the Test

⚠️ WARNING: Never adjust or withdraw antihypertensive medications, salt, or potassium independently. Withdrawal of antihypertensives can cause a dangerous blood-pressure elevation or severe potassium drops.

Accuracy is critical, as several factors can cause “false” results. Your clinician will supervise a tailored plan:

• Correct Potassium First: Low potassium can artificially lower your aldosterone levels, making a positive case look negative. Your doctor will likely provide potassium supplements to ensure your levels are in the normal range before drawing blood ^[6][7].
• Manage Medications: Many common blood pressure drugs interfere with the results. Beta-blockers can cause false-positive results, while ACE inhibitors, ARBs, and diuretics can cause false-negatives ^[8][9][10].
• Mineralocorticoid Antagonists (MRAs): Drugs like spironolactone or eplerenone are often stopped for at least 4 weeks before testing under strict clinical supervision. However, an MRA does not always have to be stopped if discontinuation is clinically unsafe, particularly if renin remains suppressed during testing ^[11][12].
• Salt Intake: Do not restrict salt before the test; a low-salt diet can confuse the results by raising renin levels ^[13].

Step 2: Bypassing the “Confirmatory” Tests

Normally, if an ARR test is positive, doctors perform a “confirmatory suppression test” (like a salt-loading or fludrocortisone test) to see if the body can turn off aldosterone production. These tests can be long, uncomfortable, and sometimes risky for people with very high blood pressure.

However, the 2025 Endocrine Society guidelines allow for the possible omission of confirmatory testing for many patients if they meet specific assay-dependent criteria ^[3][6]:

1. Spontaneous Hypokalemia: Low potassium that occurs without the use of diuretics.
2. Suppressed Renin: Renin levels that are at or below the lab’s detection limit.
3. High Aldosterone: A plasma aldosterone concentration (PAC) that is significantly elevated (often >20 ng/dL, though cutoffs are assay- and unit-specific) ^[14].

These criteria establish a high-probability biochemical diagnosis of primary aldosteronism (not a genetic diagnosis of FH-III).

Step 3: Genetic Testing and Consultation

Genetic testing is highly recommended for patients with early-onset disease or a suggestive family history ^[15][16]. This involves a simple blood or saliva sample to look for germline mutations in the KCNJ5 gene ^[17].

A pathogenic or likely pathogenic germline KCNJ5 variant supports a diagnosis of FH-III. However, a negative result does not exclude all familial primary aldosteronism, and variants of uncertain significance should not direct treatment on their own. Genetics consultation is advised.

Imaging and Next Steps

While a CT or MRI scan is often performed to look for adrenal growths, imaging alone cannot diagnose FH-III ^[18]. In many cases of FH-III, the glands may simply look enlarged (hyperplasia) or even appear normal on a scan ^[19].

Adrenal Venous Sampling (AVS)—a procedure where a catheter is used to take blood directly from the adrenal veins—may be unnecessary when a confirmed familial bilateral disorder makes bilateral adrenalectomy the clear plan ^[20][18]. However, AVS remains an individualized specialist decision. It may still be considered when unilateral disease is suspected, the genetic result is uncertain, or the patient and team are considering a less extensive operation.