The Plasma Aldosterone Concentration (PAC) test measures the amount of the hormone aldosterone circulating in the bloodstream. This test is used in the diagnostic workup for patients with hypertension when the cause is not immediately clear. By assessing this hormone, doctors investigate whether an imbalance in the body’s salt and water regulation system is driving the elevated blood pressure. The PAC test helps identify a potentially treatable form of hypertension.
Defining the Plasma Aldosterone Concentration Test
The Plasma Aldosterone Concentration test quantifies the amount of aldosterone present in a patient’s plasma. Aldosterone is a steroid hormone produced by the adrenal glands, which sit atop the kidneys. Its primary function is to manage the body’s balance of salt (sodium) and water, which directly influences blood volume and blood pressure. When aldosterone levels rise, the kidneys retain more sodium and water while excreting potassium, leading to an increase in blood volume and pressure.
The PAC test is almost always performed with a Plasma Renin Activity (PRA) test, which measures the activity of the enzyme renin. Renin is released by the kidneys and signals the system that controls aldosterone production. When blood pressure drops, renin tells the adrenal glands to produce aldosterone to raise pressure back up. Measuring both aldosterone and renin provides a more complete picture of the body’s blood pressure control mechanisms.
Why Doctors Order a PAC Test
Doctors primarily order the PAC test to screen for Primary Aldosteronism (PA), which is characterized by the overproduction of aldosterone by the adrenal glands. This excessive and inappropriate aldosterone release occurs independently of normal regulatory signals like renin activity. PA is estimated to affect 5% to 10% of all hypertensive patients.
The test is recommended for patients with resistant hypertension, defined as high blood pressure uncontrolled despite three different classes of optimal antihypertensive medications, including a diuretic. PA represents a form of hypertension that can often be cured surgically or effectively managed with specific hormone-blocking medications. Screening is also advised for patients who meet the following criteria:
- Hypertension and low potassium levels (hypokalemia), as aldosterone excess causes potassium wasting.
- Diagnosis of hypertension at a young age.
- Severe hypertension (blood pressure greater than 160/100 mmHg).
- A family history of early-onset hypertension or stroke.
Preparation and the Testing Procedure
Preparation for the PAC test requires strict adherence to instructions because many factors can interfere with the results. Physicians must first address the patient’s potassium levels, as hypokalemia can suppress aldosterone production and lead to a false negative result. Low potassium levels must be corrected before the test is performed.
Medication Interference
A patient’s medication regimen is the most challenging variable, as numerous blood pressure drugs can artificially suppress or elevate aldosterone and renin levels. Medications that directly block aldosterone, such as spironolactone or eplerenone, must be stopped for at least four weeks. Other common antihypertensives, including diuretics, ACE inhibitors, and beta-blockers, may need to be temporarily withdrawn or substituted for two weeks under medical supervision. The blood sample is usually drawn in the morning after the patient has been out of bed for at least two hours, as posture can affect results.
Interpreting the Test Results
The results are interpreted by calculating the Aldosterone-to-Renin Ratio (ARR), which is the PAC divided by the PRA. This ratio is the primary screening tool because it assesses whether the aldosterone level is inappropriately high relative to the body’s renin level.
In a healthy individual, high aldosterone should be accompanied by low renin, maintaining a dynamic balance. A high ARR strongly suggests Primary Aldosteronism, as it indicates an elevated PAC alongside a suppressed PRA. The adrenal glands are producing too much aldosterone, which then suppresses the release of renin from the kidneys. A ratio greater than 20 to 30 (depending on the measurement units used) combined with a sufficiently high PAC, often above 15 ng/dL, is considered a positive screening result for the condition.
A positive ARR screening test is not a final diagnosis, but rather a strong indicator that requires further investigation. The next step is typically a confirmatory test, such as a saline suppression test, to definitively prove that aldosterone production is autonomous and cannot be suppressed by typical physiological stimuli. This comprehensive process ensures an accurate diagnosis, which is crucial for determining the correct, targeted treatment plan for the patient’s hypertension.