Adrenal Vein Sampling (AVS) is a specialized, minimally invasive diagnostic procedure used to determine the source of excessive hormone production, most often aldosterone, in patients with high blood pressure. An interventional radiologist threads a catheter through a vein, typically in the groin, navigating it to the tiny veins that drain the adrenal glands. Blood samples are collected directly from each adrenal vein, as well as a peripheral vein, for laboratory analysis. The specific hormone concentrations provide a functional assessment of each gland’s activity. Interpreting AVS results relies on calculating specific numerical ratios, which dictate the most effective treatment path, guiding the choice between surgery and lifelong medical management.
Why Adrenal Vein Sampling is Necessary
The clinical context for AVS is Primary Aldosteronism (PA), a condition where the adrenal glands produce too much aldosterone, leading to hypertension. PA is the most common endocrine cause of high blood pressure and is associated with an increased risk for cardiovascular damage. Excess aldosterone can originate from only one adrenal gland (unilateral disease) or from both glands (bilateral disease).
Distinguishing between these sources is essential for treatment planning, but standard imaging techniques like CT or MRI are often insufficient. Imaging may show a mass or nodule, but it cannot confirm if that structure is the actual source of the excess aldosterone; an overactive gland might even appear normal. AVS provides a functional map of hormone secretion, allowing clinicians to definitively compare the output of the right and left adrenal glands. AVS is the gold standard for subtype differentiation in PA, informing the decision between surgery or medication.
Confirming the Sample Quality (The Selectivity Index)
Before any diagnostic conclusion, the technical success of the blood collection must be confirmed by calculating the Selectivity Index (SI). The adrenal veins are difficult to cannulate, risking sample dilution with blood from non-adrenal veins. Dilution would invalidate hormone measurements and lead to an incorrect diagnosis.
The SI uses cortisol as a marker for a successful sample because cortisol is produced exclusively and in high concentrations by the adrenal glands. The cortisol concentration in the adrenal vein must be significantly higher than the concentration in the peripheral blood, which serves as the reference. The SI is calculated as the ratio of the cortisol concentration in the adrenal vein divided by the cortisol concentration in the peripheral vein.
A threshold of \(2.0\) or greater indicates a selective sample when the procedure is performed without stimulation. If the procedure involves the continuous infusion of cosyntropin (synthetic ACTH), which stimulates cortisol production, the threshold is typically set higher, often at \(3.0\) or \(5.0\). If the SI falls below the accepted cutoff on either side, the sample is non-selective, and the AVS result cannot be reliably interpreted.
Determining the Source of Hormone Overproduction (The Lateralization Index)
Once the Selectivity Index confirms successful, undiluted samples from both adrenal veins, the Lateralization Index (LI) is calculated to pinpoint the source of aldosterone overproduction. To accurately compare the output, the measured aldosterone concentration must be corrected for any residual dilution. This correction uses the Aldosterone-to-Cortisol (A/C) ratio, where the concentration of aldosterone is divided by the concentration of cortisol in the same sample.
The LI is calculated as the ratio of the corrected A/C ratio from the dominant adrenal vein (the higher A/C ratio) to the corrected A/C ratio from the non-dominant adrenal vein (the lower A/C ratio). This comparison reveals if one gland is producing significantly more aldosterone than the other. A high LI indicates a unilateral source, while an LI close to \(1.0\) suggests a bilateral source.
Specific numerical thresholds define lateralization and depend on whether cosyntropin was used. With cosyntropin stimulation, an LI of \(4.0\) or greater is a common criterion for unilateral aldosterone excess. If the LI is less than \(3.0\), the result is interpreted as bilateral secretion. The LI is the most important number derived from AVS, translating functional hormone data into a statement about disease distribution.
Diagnosis and Next Steps
The final interpretation of the Lateralization Index directly dictates clinical management and is the ultimate goal of AVS.
A finding of unilateral disease, defined by an LI that meets or exceeds the established threshold (e.g., \(4.0\) with stimulation), indicates a single, overactive adrenal gland. This diagnosis points toward Unilateral Primary Aldosteronism, which is a surgically curable form of hypertension.
The recommended next step for unilateral disease is a unilateral adrenalectomy, the surgical removal of the overactive gland. This intervention has a high probability of resolving the PA, often leading to significant improvement or normalization of blood pressure and potassium levels. The LI provides the necessary confidence to proceed with surgery, ensuring the correct gland is targeted.
Conversely, an AVS result that does not show lateralization (a non-lateralizing LI, such as less than \(3.0\)) indicates Bilateral Primary Aldosteronism. Since both glands are overproducing the hormone, surgery would be ineffective and unnecessary. Removing one gland would not cure the condition and could lead to adrenal insufficiency.
The treatment for bilateral disease is medical management, typically involving mineralocorticoid receptor antagonists like spironolactone or eplerenone. These medications block the effects of aldosterone. Accurate interpretation of AVS prevents patients with bilateral disease from undergoing an invasive procedure that would not provide a cure.