Aldosterone

This test measures serum aldosterone levels by quantitative analysis and radioimmunoassay. Aldosterone regulates ion transport across cell membranes to promote reabsorption of sodium and chloride in exchange for potassium and hydrogen ions. Consequently, it helps to maintain blood pressure and blood volume and to regulate fluid and electrolyte balance.

This test identifies aldosteronism and, when supported by plasma renin levels, distinguishes between the primary and secondary forms of this disorder.

Purpose

• To aid diagnosis of primary aldosteronism and potential causes of the disorder, including adrenal adenoma and adrenal hyperplasia
• To aid diagnosis of secondary hypoaldosteronism and commonly related conditions, such as salt-losing syndrome, potassium excess, heart failure with ascites, and other conditions that characteristically increase activity of the renin-angiotensin system.

Patient preparation

• Explain to the patient that this test helps determine if symptoms are due to improper hormonal secretion.
• Tell him that the test requires a blood sample and that he may experience some discomfort from the needle Puncture and tourniquet. Tell him who will perform the venipuncture and when.
• Instruct him to maintain a low-carbohydrate, normal-sodium diet (135 mEq or 3 g/day) for at least 2 weeks or, preferably, for 30 days before the test.
• Withhold all drugs that alter fluid, sodium, and potassium balance especially diuretics, antihypertensives, steroids, oral contraceptives, and estrogens - for at least 2 weeks or, preferably, for 30 days before the test.
• Withhold all renin inhibitors (such as propranolol) for 1 week before the test. If they must be continued, note this on the laboratory slip.
• Tell the patient to avoid licorice for at least 2 weeks before the test because it produces an aldosterone-like effect.

Procedure and posttest care

• Perform a venipuncture while the patient is still supine after a night's rest.
• Collect the sample in a 7-ml red-top tube, and send it to the laboratory immediately.
• Draw another sample 4 hours later, while the patient is standing and after he has been up and about, to evaluate the effect of postural change.
• Collect the second sample in a 7-ml red-top tube, and send it to the laboratory immediately.
• If a hematoma develops at the venipuncture site, apply warm soaks.
• Instruct the patient to resume his normal diet and medications discontinued before the test.

Precautions

• Handle the sample gently to prevent hemolysis.
• Record on the laboratory slip whether the patient was supine or standing during the venipuncture.
• If the patient is a premenopausal female, specify the phase of her menstrual cycle because aldosterone levels may fluctuate during the menstrual cycle.
• Send the sample to the laboratory immediately.

Reference values

Laboratory values vary with the patient's age:

• 0 to 30 days: 16.5 to 154 ng/dl
• 1 to 11 months: 6.5 to 86 ng/dl
• 1 to 10 years: 3 to 39.5 ng/dl (supine), 3.5 to 124 ng/dl (upright)
• 11 years and older: 1 to 21 ng/dl

Abnormal findings

Excessive aldosterone secretion may indicate a primary or secondary disease. Primary aldosteronism (Conn's syndrome) may result from adrenocortical adenoma or carcinoma or from bilateral adrenal hyperplasia. Secondary aldosteronism can result from renovascular hypertension, heart failure, cirrhosis of the liver, nephrotic syndrome, idiopathic cyclic edema, and the third trimester of pregnancy.

Low serum aldosterone levels may indicate primary hypoaldosteronism, salt-losing syndrome, eclampsia, or Addison's disease.

Interfering factors

• Failure to observe pretest restrictions
• Hemolysis due to rough handling of the sample
• Some antihypertensives, such as methyldopa, that promote sodium and Water retention (possible decrease)
• Diuretics (possible increase)
• Some corticosteroids, such as fludrocortisone, that mimic mineralocorticoid activity (possible decrease)
• Radioactive scan performed within 1 week before the test.