Definition of ADH:

ADH is a test that measures the amount of antidiuretic hormone (ADH) in blood. ADH is found in the body, and it may be given as a medication.

How the test is performed:

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

How to prepare for the test:

Talk to your health care provider about your medications before the test. Many medications can affect ADH measurements, including:

• Alcohol
• Clonidine
• Diuretics
• Haloperidol
• Insulin
• Lithium
• Morphine
• Nicotine
• Steroids

How the test will feel:

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed:

This test is performed if your health care provider suspects you have a disorder that affects your ADH level.

ADH is a hormone stored in the posterior pituitary gland in the brain. It regulates water in the body. ADH acts on the kidneys to increase total body water. This increases blood volume and blood pressure.

The release of ADH is controlled by cells called osmoreceptors and baroreceptors. Osmoreceptors are specialized areas in a part of the brain called the hypothalamus.

These cells sense the concentration of particles in the blood. When the concentration is high, the pituitary releases more ADH. This causes more water to be retained to dilute the body fluids. When the concentration is low, the pituitary releases less ADH.

Special areas in the heart sense blood volume and blood pressure. The heart signals the pituitary to release more ADH when blood volume or blood pressure are low and less ADH when they are high.

Certain diseases affect the normal release of ADH. The blood level of ADH must be tested to determine the cause of the disease. ADH may be measured as part of a "water restriction test" to find the cause of a disease.

Normal Values:

Normal values -- 0 - 4.7 pg/mL

Note: pg/mL = picograms per milliliter

Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

What abnormal results mean:

Higher than normal levels may indicate:

• Acute porphyria (very rare)
• Central nervous system infection
• Central nervous system tumor
• Lung infections
• Lung or mediastinal tumors
• Post-surgery fluid imbalance
• Syndrome of inappropriate ADH (SIADH)

Lower than normal levels may indicate:

• Damage to the pituitary gland
• Diabetes insipidus -- central or nephrogenic
• Primary polydipsia

What the risks are:

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)