Serum Magnesium Blood Test

Page content

A serum magnesium blood test is done to measure a patient’s blood-magnesium level. Approximately half of the magnesium in the body is within bone. The other half can be found within organs and body tissue cells.

Why is This Test Performed?

When a doctor feels that his or her patient may have abnormal blood-magnesium levels, he or she will often order this blood test. Magnesium is a mineral the body needs for almost every biochemical process in the body. It helps keep the bones strong, helps to regulate blood pressure, helps in maintaining normal nerve and muscle function, and controls the heartbeat. Magnesium also plays an important role in supporting the immune system and controlling blood sugar levels.

Preparation and Procedure

For most patients, preparation is not necessary for a serum magnesium blood test. A needle will be inserted into a vein to obtain a blood sample. This may cause some patients discomfort ranging from a slight prick to moderate pain. This test is very fast so if a patient does feel moderate pain, it should not last long. On average, this test will take no longer than five minutes from start to finish. If the patient’s health care provider has trouble finding a good vein, they may have to “stick” the patient more than once in order to ensure they get an adequate blood sample.

A lancet is often used to obtain a blood sample from infants or young children. Just like using a needle, some patients may experience more discomfort than others. Using a lancet is also very quick so any discomfort should not last long.


If the patient’s level of magnesium in their blood is normal it will range from about 1.7 to 2.2 mg/dL.

If a patient has high levels of magnesium in his or her blood, it may be caused by:

  • Chronic renal failure
  • Diabetic acidosis
  • Addison’s disease
  • Dehydration
  • Decreased urine output

If a patient has low levels of magnesium in his or her blood, it may be caused by:

  • Chronic diarrhea
  • Hemodialysis
  • Hyperaldosteronism
  • Pancreatitis
  • Toxemia of pregnancy
  • Alcoholism
  • Delirium tremens
  • Hepatic cirrhosis
  • Hypoparathyroidism
  • Too much insulin
  • Ulcerative colitis


This test is a routine blood test, therefore, there is very little risk involved. Some patient’s veins may be harder to draw blood from than others and this may result in them needing to be stuck with a needle more than once. Other possible risks are not common, but may include:

  • Fainting or feeling lightheaded
  • Slight risk of infection
  • Bleeding excessively
  • A hematoma may occur if blood accumulates beneath the skin


University of Maryland Medical Center. (2010). Serum Magnesium Test Overview. Retrieved on November 25, 2010 from the University of Maryland Medical Center:

MedlinePlus. (2009). Serum Magnesium - Test. Retrieved on November 25, 2010 from MedlinePlus: