Also known as: blood magnesium
Name: magnesium, Mg
Related tests: Calcium, Potassium, Vitamin D
Why is the test done?
To measure the concentration of magnesium in the blood and sometimes to help find the cause of changes in blood calcium and potassium.
When is the test done?
When there are symptoms linked to changes in blood magnesium, such as weakness, irritability, heart rhythm disturbances, nausea, vomiting and diarrhoea; it may also be done when there are changes in blood calcium and potassium.
What samples are required?
A blood sample from a vein.
What is being tested?
This test measures the concentration of magnesium in the blood. Normally only 1% of the body’s magnesium is in the blood, while 60% is in the bones and 39% inside cells.
Magnesium is a mineral found in all the body’s cells and is involved in many vital processes, such as energy production in cells, muscle contraction, transmission of nerve impulses, and bone mineralisation and development.
The main dietary source of magnesium is green leafy vegetables such as spinach; the body keeps its magnesium content constant by regulating absorption from the gut and elimination by the kidneys.
How is it used?
When is it requested?
What does the test result mean?
Is there anything else I should know?
How is it used?
Changes in magnesium concentration can be found when elimination by the kidneys or absorption from the gut is impaired: magnesium may therefore be requested to assess the extent of these changes, as in kidney dysfunction or intestinal malabsorption.
In addition, since low magnesium levels can be associated with low calcium or low potassium, magnesium testing can help diagnose disorders of calcium and phosphorus metabolism or electrolyte disorders.
Magnesium can also be used to monitor the use of supplements containing magnesium and calcium; in this case calcium and phosphorus may be requested as well as magnesium.
When is it requested?
Magnesium is requested in malabsorption, malnutrition, diarrhoea and alcoholism, or periodically when calcium and magnesium supplements need to be taken.
It may also be requested to monitor chronic low calcium and low potassium, and when there are symptoms linked to changes in blood magnesium, such as weakness, muscle spasms and cramps, confusion and heart rhythm disturbances.
Magnesium may also be requested in kidney disease or when taking medicines that interfere with the elimination of magnesium by the kidneys.
What does the test result mean?
1) low dietary intake (as in malnutrition);
2) reduced absorption from the gut;
3) excessive elimination by the kidneys.
These conditions can occur with:
• old age, malnutrition, alcoholism
• gastrointestinal diseases such as Crohn’s disease or ulcerative colitis
• poorly controlled diabetes
• hypoparathyroidism
• prolonged use of diuretics
• diarrhoea
• after surgery
• burns
Hypermagnesaemia can rarely be caused by excessive dietary intake; more often it is due to impaired elimination by the kidneys or excessive intake of magnesium-containing supplements. The conditions that most often cause a rise in blood magnesium are:
• kidney failure
• hyperparathyroidism
• hypothyroidism
• dehydration
• diabetic acidosis
• Addison’s disease
• prolonged use of medicines containing magnesium (such as some laxatives or antacids).
Is there anything else I should know?
Since magnesium is an electrolyte, it is generally measured together with other electrolytes such as sodium, potassium, chloride, calcium and phosphorus to assess the patient’s fluid and electrolyte balance; a fall in magnesium is often accompanied by a fall in potassium.
Blood magnesium normally falls in the second and third trimesters of pregnancy
1) Why has my doctor requested calcium and phosphorus together with magnesium?
1
) Why has my doctor requested calcium and phosphorus together with magnesium?Plasma calcium is regulated mainly by PTH (parathyroid hormone), vitamin D and phosphorus and is affected by magnesium; it is useful to measure plasma magnesium together with calcium and phosphorus because low calcium can be harder to treat when magnesium is also low in the blood.