The saline suppression test (SST) is a specialized diagnostic procedure used in endocrinology to investigate the body’s regulation of the hormone aldosterone. This test is specifically employed to confirm or rule out the presence of Primary Aldosteronism, a condition characterized by excessive and unregulated aldosterone production. The underlying goal of the SST is to challenge the body’s hormonal feedback loop by temporarily increasing the fluid and sodium volume in the bloodstream. This increase should normally cause a significant drop in aldosterone output. By measuring the hormone levels before and after this challenge, clinicians can determine if aldosterone secretion is appropriately responsive or abnormally independent.
Diagnostic Context: Why the Test is Necessary
Aldosterone, a hormone produced by the adrenal glands, plays a central role in regulating blood pressure by controlling the balance of sodium and potassium in the body. It causes the kidneys to retain salt and water, which increases blood volume and, consequently, blood pressure. The need for a saline suppression test arises when initial screening suggests an imbalance in the renin-angiotensin-aldosterone system.
Primary Aldosteronism, also known as Conn’s syndrome, is a disorder where the adrenal glands produce too much aldosterone autonomously. This means its production is independent of the body’s normal control mechanisms. This excess aldosterone leads to fluid retention and sodium overload, resulting in hypertension that is often difficult to treat with standard medications, known as resistant hypertension. Initial screening involves measuring the plasma aldosterone concentration (PAC) and plasma renin activity (PRA) to calculate the Aldosterone-Renin Ratio (ARR).
A high ARR, typically indicating a high aldosterone level paired with a very low or suppressed renin level, suggests that the condition is present. Renin, released by the kidneys, normally stimulates aldosterone production, but in Primary Aldosteronism, the adrenal glands are overactive and no longer need this signal. The saline suppression test serves as a confirmatory step to definitively prove that the aldosterone production is inappropriately independent of the body’s volume status.
Essential Preparation Requirements
The accuracy of the saline suppression test relies heavily on meticulous patient preparation, particularly concerning diet and medication management. Patients are instructed to ensure an adequate salt intake for several days leading up to the procedure, sometimes through supplemental salt tablets or a high-sodium diet, to ensure a state of volume expansion. A high sodium load is necessary for the test to function correctly, as it provides the signal that should suppress aldosterone production in a healthy person.
Adjusting blood pressure medications is also necessary, as many common antihypertensive drugs can interfere with the measurement of aldosterone and renin, leading to inaccurate results. The following medications are typically discontinued for a period ranging from two to six weeks before the test:
- Diuretics
- ACE inhibitors
- Angiotensin Receptor Blockers (ARBs)
- Spironolactone
When these medications must be stopped, the healthcare team will substitute them with non-interfering alternatives, such as certain alpha-adrenoceptor blockers or calcium channel blockers, to safely manage blood pressure.
Furthermore, the patient’s potassium levels must be within the normal range before the test can proceed. Aldosterone excess can cause hypokalemia (low potassium). If potassium levels are low, the patient will be prescribed supplements to correct the deficiency, as hypokalemia itself can artificially lower aldosterone levels and potentially mask the condition.
The Step-by-Step Procedure
The saline suppression test generally takes about four to five hours and is performed in a controlled clinical setting. Upon arrival, a small intravenous (IV) catheter is placed into a vein, typically in the arm, which will be used for both the fluid infusion and blood sample collection. The patient is required to rest in a supine or seated position for a period before the infusion begins to stabilize baseline hormone levels.
A baseline blood sample is drawn through the IV line to establish the initial plasma aldosterone and renin concentrations. Following this, an infusion of two liters of normal saline solution is administered over a four-hour period. The saline, a sterile salt-water solution, rapidly expands the volume of fluid in the bloodstream.
Throughout the four-hour infusion, the patient must remain in the designated position, and vital signs, including blood pressure and heart rate, are closely monitored. The large volume of fluid is intended to signal to the adrenal glands to halt aldosterone production. Once the infusion is complete, a final blood sample is collected from the IV line to measure the post-infusion aldosterone and renin levels, and the catheter is then removed.
Interpreting the Results
The final step in the diagnostic process is the careful interpretation of the post-infusion plasma aldosterone concentration (PAC). The principle of the SST is that the massive salt and fluid load should suppress aldosterone production in a person with a healthy hormonal system. The finding is whether the aldosterone level has successfully suppressed or if it remains inappropriately high.
A final PAC below a specific threshold, often cited as less than 5 ng/dL, indicates a normal response and makes the diagnosis of Primary Aldosteronism unlikely. This result confirms that the adrenal glands are appropriately responsive to the volume expansion signal. Conversely, a PAC that remains elevated after the infusion, typically greater than 10 ng/dL, is considered a positive result for Primary Aldosteronism.
This lack of suppression demonstrates that the aldosterone production is autonomous and uncontrolled by the body’s volume status. For results that fall within an indeterminate range, further investigation may be warranted. Following a confirmed positive result, the next steps typically involve additional testing, such as imaging or adrenal vein sampling, to determine whether the excess hormone is coming from one adrenal gland or both, which guides the choice between surgical removal of a gland or long-term medical management.