Definition

A serum magnesium test measures the level of magnesium in the blood.

Alternative Names

Magnesium - blood

How the Test is Performed

A blood sample is needed.

How to Prepare for the Test

No special preparation is needed.

How the Test will Feel

When the needle is inserted to draw blood, some people feel slight pain. Others feel a prick or stinging. Afterward, there may be some throbbing or a slight bruise. This soon goes away.

Why the Test is Performed

This test is done when your health care provider suspects you have an abnormal level of magnesium in your blood.

Only a small amount of the body’s total amount of magnesium is found in the blood. About one half of the body's magnesium is found in bone. The other half is found inside cells of body tissues and organs.

Magnesium is needed for many chemical processes in the body. It helps maintain normal muscle and nerve function and helps keep bones strong. Magnesium is also needed for the heart to function normally and to help regulate blood pressure. Magnesium also helps the body control blood sugar level and helps support the body's defense (immune) system.

Normal Results

The normal range for blood magnesium level is 1.7 to 2.2 mg/dL (0.85 to 1.10 mmol/L).

Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your provider about the meaning of your specific test results.

What Abnormal Results Mean

A high blood magnesium level may be due to:

• Adrenal insufficiency (the glands are not producing enough hormones)
• Diabetic ketoacidosis, a life-threatening problem in people with diabetes
• Lithium use
• Loss of kidney function (acute or chronic renal failure)
• Loss of body fluids (dehydration)
• Milk alkali syndrome (a condition in which there is a high level of calcium in the body)

A low blood magnesium level may be due to:

• Alcohol use disorder
• Diabetes that is uncontrolled
• Diarrhea that is chronic
• Hyperaldosteronism (adrenal gland produces too much of the hormone aldosterone)
• Hypercalcemia (high blood calcium level)
• Kidney disease
• Medicines, such as proton pump inhibitors for acid reflux, diuretics (water pills), aminoglycoside antibiotics, amphotericin, cisplatin, calcineurin inhibitors
• Inflammation of the pancreas (pancreatitis)
• High blood pressure and protein in the urine in a pregnant woman (preeclampsia)
• Inflammation of the lining of the large intestine and rectum (ulcerative colitis)

Risks

There is little risk in having your blood taken. Veins and arteries vary in size from one person to another and from one side of the body to the other. Taking blood from some people may be more difficult than from others.

Other risks may include:

• Excessive bleeding
• Fainting or feeling lightheaded
• Multiple punctures to locate veins
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

References

Klemm KM, Klein MJ, Zhang Y. Biochemical markers of bone metabolism. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 16.

Mason JB, Booth SL. Vitamins, trace minerals, and other micronutrients. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 26th ed. Philadelphia, PA: Elsevier; 2020:chap 205.