Magnesium

Magnesium is a mineral your body uses in hundreds of enzyme reactions, including muscle contraction, nerve signaling, blood pressure regulation, and energy production. A serum test measures the small fraction circulating in your blood.

Ranges vary between labs, so compare to the range on your own lab report.

What serum magnesium measures

Magnesium is a cofactor in more than 300 enzymatic reactions, and your body guards its blood level tightly. Roughly 99 percent of your magnesium sits inside bone, muscle, and soft tissue, with only about 1 percent circulating in serum.

That distribution is the key to reading this test. Because the body pulls magnesium out of storage to hold serum levels steady, a normal serum result does not rule out depleted tissue stores. What serum magnesium does reliably catch is a level that has fallen far enough that compensation is failing, or one that has risen too high.

What high or low levels mean

Low magnesium can cause muscle cramps, tremors, irregular heart rhythm, and in more significant cases confusion or seizures. Common contributors include chronic alcohol use, poorly controlled diabetes, gastrointestinal conditions that impair absorption, and medications such as proton pump inhibitors and certain diuretics. Low magnesium also frequently accompanies low potassium and low calcium, and correcting it is often necessary before those normalize.

High magnesium is uncommon in people with healthy kidneys, since excess is excreted efficiently. It appears most often with reduced kidney function, or with heavy use of magnesium-containing supplements, antacids, or laxatives.

Magnesium FAQ

Can my magnesium be normal and I still be deficient?

Yes. The body maintains serum magnesium at the expense of tissue stores, so a normal blood level can coexist with depleted reserves. Symptoms in the presence of a normal result are worth discussing with a clinician rather than dismissing.

What is the difference between serum and RBC magnesium?

Serum measures what circulates in blood. RBC magnesium measures what is inside red blood cells and is sometimes used as a proxy for tissue stores. Serum is the standard test and the one most reference ranges and clinical decisions are built around.

Do I need to fast?

No. Magnesium does not require fasting, though it is commonly drawn alongside other tests that do.

Read alongside Calcium and Potassium, which are closely linked to magnesium balance.

Sources

• MedlinePlus Medical Encyclopedia, Magnesium blood test.

• Cleveland Clinic, Magnesium Blood Test.