Magnesium Test

About the Test

Purpose of the test

A test measures magnesium level in the blood (or sometimes urine). Abnormal magnesium levels are most frequently seen in conditions or diseases that cause impaired or excessive excretion of the mineral by the kidneys, or impaired absorption in the intestines.

Magnesium levels may be checked as part of an evaluation of the severity of kidney problems and/or uncontrolled diabetes and may help diagnose gastrointestinal disorders.

Since a low magnesium blood level can, over time, cause persistently low calcium and potassium levels, it may be checked to help diagnose problems with calcium, potassium, phosphorus, and/or parathyroid hormone – another component of calcium regulation.

Magnesium levels may be measured frequently to monitor the response to oral or intravenous (IV) magnesium supplements. The magnesium test may be ordered, along with calcium and phosphorus testing, to monitor calcium supplementation.

What does the test measure?

Magnesium is stored in the bones, cells, and tissues. Normally, only about 1% of total body magnesium is present in the liquid portion of blood, making it difficult to accurately measure total magnesium content from blood tests alone. However, this test is still useful for evaluating your magnesium status.

A variety of foods contain small amounts of magnesium, especially green vegetables such as spinach, whole grains, and nuts. Foods that have dietary fiber are usually also sources of magnesium. The body maintains its magnesium level by regulating how much it absorbs, excretes, or conserves in the kidneys.

Magnesium deficiencies (hypomagnesemia) may be seen with malnutrition, conditions that cause malabsorption, and excess mineral loss by the kidneys. An excess of magnesium (hypermagnesemia) may happen with the ingestion of antacids that contain magnesium and with a decreased ability of the kidneys to excrete the mineral. But clinically significant hypermagnesemia is rare due to the kidney’s ability to increase excretion when needed.

Someone with mild to moderate magnesium deficiency may have no or few nonspecific symptoms. Persistent or severe deficiencies can cause nausea, loss of appetite, fatigue, confusion, muscle cramps, seizures, changes in heart rate, and numbness or tingling. They can also affect calcium metabolism and exacerbate calcium deficiencies.

Symptoms of excess magnesium can be similar to those of deficiency and include nausea, muscle weakness, loss of appetite, and an irregular heart rate.

When should I get this test?

Magnesium testing may be a follow-up to chronically low blood calcium and potassium levels. It also may be ordered when you have symptoms possibly due to a magnesium deficiency, such as muscle weakness, twitching, cramping, confusion, cardiac arrhythmias, and seizures. Magnesium depletion is known to be linked to severe alcohol intake or irregular or bad absorption of magnesium in the intestine.

A health care practitioner may order a magnesium test to check for a deficiency as part of an evaluation of malabsorption, malnutrition, diarrhea, or alcoholism. Testing may also be performed when someone is taking medications that can cause the kidneys to excrete magnesium.

If magnesium and/or calcium supplementation is necessary, the level of magnesium in the blood may be checked at intervals to monitor the effectiveness of treatment.

When you have a kidney disorder or uncontrolled diabetes, a magnesium test may be ordered periodically. Tests such as a BUN and creatinine may also be used to help monitor kidney function and ensure you are not excreting or retaining excessive amounts of magnesium.

Since magnesium is an electrolyte, a magnesium test may be ordered along with other electrolytes such as sodium, potassium, chloride, bicarbonate (or total CO2), calcium, and phosphorus to evaluate your electrolyte balance. If magnesium is low, it is not unusual for potassium also to be low, because prolonged hypomagnesemia (low magnesium in blood) can cause refractory hypokalemia (low potassium in blood).

Magnesium blood levels decrease in the second and third trimesters of pregnancy due to dilution of blood.

Normal magnesium levels do not necessarily reflect the total body stores of the mineral. The body attempts to keep blood magnesium levels relatively stable and will release it from bone and tissues to accomplish this. So, the blood level may be normal with early magnesium deficiencies.

Drugs that can increase magnesium levels include lithium, aspirin, thyroid medication, some antibiotics, laxatives, and magnesium salts. Digoxin, cyclosporine, diuretics, insulin, some antibiotics, phenytoin, antacids, and other drugs can decrease magnesium levels.