What is being tested?

This test measures the amount of magnesium in your blood. Normally, only a very small amount (about 1%) of the total magnesium found in the body is present in the blood.

Magnesium is a mineral that is found in every cell of your body. It is vital to energy production, muscle contraction, nerve function, cardiac rhythm and maintenance of strong bones. About half of the body’s magnesium is combined with calcium and phosphorus to form bone.

A wide variety of foods contain small amounts of magnesium, especially green vegetables such as spinach, also whole grains and nuts. Most magnesium in the body comes from dietary sources. The body maintains magnesium levels in its blood, cells, and bone by regulating how much it absorbs from the intestines and by how much it excretes or conserves in the kidneys.

How is it used?

Abnormal levels of magnesium are most frequently seen in conditions or diseases that cause impaired or excessive excretion of magnesium by the kidneys or cause increased loss from the intestines, such as through vomiting or diarrhoea. Magnesium levels may be checked as part of an evaluation of the severity of kidney problems or of uncontrolled diabetes and may help in the diagnosis of gastrointestinal disorders.

Since a low magnesium level can, over time, cause persistently low calcium and potassium levels, it may be checked to help diagnose problems with calcium, potassium, phosphate and/or parathyroid hormone (involved with calcium regulation).

Magnesium levels may be measured frequently to monitor the response to oral or intravenous (IV) magnesium supplements and may be used, along with calcium and phosphate testing, to monitor calcium supplementation. Magnesium supplementation is also used in the treatment of pre-eclampsia and eclampsia in pregnancy, where supplementation is given intravenously and blood levels are then checked to ensure adequacy of treatment.

When is it requested?

Magnesium testing may be requested as a follow-up to persistently low levels of calcium and potassium. It may also be requested if you have symptoms of an abnormally low magnesium level such as muscle weakness, twitching, cramping, confusion, cardiac arrhythmias, and seizures.

Although dietary deficiencies of magnesium are rare, your doctor may request a magnesium level to check for a deficiency as part of an evaluation of malabsorption, malnutrition, diarrhoea, or alcoholism. If you are taking certain drugs that can cause the kidneys to excrete magnesium, testing may be performed as well. If magnesium and/or calcium supplementation is necessary, then the magnesium blood level is likely to be checked at intervals to monitor the effectiveness of treatment.

If you have a kidney disorder or uncontrolled diabetes, your doctor may request magnesium levels to help monitor kidney function and to make sure that you are not excreting or retaining excessive amounts of magnesium.

What does the result mean?

Low levels of magnesium (hypomagnesaemia) in your blood may mean that you are:

• not getting enough magnesium in your diet;
• you are losing magnesium from your gut
• your intestines are not absorbing enough magnesium; or
• your kidneys are excreting too much magnesium.

Deficiencies may be due to:

• Low dietary intake (seen in the elderly, malnourished, and with alcoholism)
• Gastrointestinal disorders (such as Crohn’s disease)
• Uncontrolled diabetes
• Long-term diuretic use that causes increased excretion from the kidneys
• Prolonged diarrhoea
• Any critical illness in Intensive Care Unit patients

Increased levels of magnesium are rarely due to dietary sources but are usually the result of an excretion problem or excessive supplementation. Increased levels are seen in:

• Kidney failure
• Hyperparathyroidism (overactive parathyroid gland)
• Hypothyroidism
• Use of magnesium-containing antacids or laxatives, especially when kidney function is impaired

Reference Intervals
Adult
0.70- 1.10 mmol/L

Paediatrics
0 days to 1 week        0.60–1.00 mmol/L
1 week - 18 years       0.65–1.10 mmol/L

The reference intervals shown above are known as a harmonised reference interval. This means that eventually all laboratories in Australia will eventually use this same interval so wherever your sample is tested, the reference interval should be the one shown above. Laboratories are in the process of adopting these harmonised intervals so it is possible that the intervals shown on the report of your results for this test may be slightly different until this change is fully adopted.

Is there anything else I should know?

Since magnesium is an electrolyte, it may be requested with other electrolytes such as sodium, potassium, chloride, bicarbonate, calcium, and phosphate to evaluate a patient’s electrolyte balance. If magnesium is low, it is not unusual for calcium and potassium also to be low.

Magnesium blood levels may be low normally in the second and third months of pregnancy.

Common questions

• Why is my doctor checking my calcium and phosphate along with my magnesium level?

Low magnesium levels can affect calcium level regulation. Parathyroid hormone and vitamin D ordinarily work together along with phosphorus (phosphate) to regulate calcium levels. Low magnesium levels can make low calcium levels more resistant to change.

More information

RCPA Manual: Magnesium

RCPA Manual: Magnesium Faeces

RCPA Manual: Magnesium Urine