Also Known As
This article was last reviewed on
This article waslast modified on 7 March 2022.
At a Glance
Why Get Tested?

To measure the concentration of magnesium in your blood and to help determine the cause of abnormal calcium and/or potassium levels

When To Get Tested?

If you have symptoms (such as weakness, irritability, cardiac arrhythmia, nausea, and/or diarrhoea) that may be due to too much or too little magnesium or if you have abnormal calcium or potassium levels

Sample Required?

A blood sample taken from a vein in your arm

Test Preparation Needed?


On average it takes 7 working days for the blood test results to come back from the hospital, depending on the exact tests requested. Some specialist test results may take longer, if samples have to be sent to a reference (specialist) laboratory. The X-ray & scan results may take longer. If you are registered to use the online services of your local practice, you may be able to access your results online. Your GP practice will be able to provide specific details.

If the doctor wants to see you about the result(s), you will be offered an appointment. If you are concerned about your test results, you will need to arrange an appointment with your doctor so that all relevant information including age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), can be considered.

Lab Tests Online-UK is an educational website designed to provide patients and carers with information on laboratory tests used in medical care. We are not a laboratory and are unable to comment on an individual's health and treatment.

Reference ranges are dependent on many factors, including patient age, sex, sample population, and test method, and numeric test results can have different meanings in different laboratories.

For these reasons, you will not find reference ranges for the majority of tests described on this web site. The lab report containing your test results should include the relevant reference range for your test(s). Please consult your doctor or the laboratory that performed the test(s) to obtain the reference range if you do not have the lab report.

For more information on reference ranges, please read Reference Ranges and What They Mean.

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, 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, and most magnesium in the body comes from dietary sources. The body maintains magnesium levels in the blood, cells, and bone by regulating how much is absorbed from the intestines and how much it excretes in the urine via the kidneys.

Accordion Title
Common Questions
  • 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 that cause poor absorption in the intestines. 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 and monitoring of gastrointestinal disorders, sometimes characterised by prolonged diarrhoea.

    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, phosphorus, 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 phosphorus testing, to monitor calcium supplementation. Magnesium supplementation is also used in the treatment of pre-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 blood test 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 also be performed. If magnesium and/or calcium supplementation is necessary, then the level of magnesium in the blood 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 a blood test to measure magnesium to help monitor kidney function and to make sure that you are not excreting or retaining excessive amounts of magnesium.

  • What does the test result mean?

    Low levels of magnesium (hypomagnesaemia) in your blood may mean that you are: 1) you are not getting enough magnesium in your diet; 2) your intestines are not absorbing enough magnesium; or 3) your kidneys are excreting too much magnesium. Deficiencies may be due to:

    • Low dietary intake (seen in the elderly or malnourished), and with alcoholism (low intake and increased excretion) 
    • Gastrointestinal disorders (such as Crohn disease) 
    • Uncontrolled diabetes 
    • Hypoparathyroidism (underactive parathyroid gland)
    • Long-term use of certain drugs e.g. diuretics, proton pump inhibitors ( omepraxone, etc)
    • Prolonged diarrhoea 
    • Surgery 
    • Severe burns 
    • Toxaemia of pregnancy

    Increased levels of magnesium are rarely due excess magnesium in the diet and are usually the result of an excretion problem or excessive supplementation. Increased levels are seen in:

    • Kidney failure 
    • Hyperparathyroidism (overactive parathyroid gland)
    • Hypothyroidism 
    • Dehydration 
    • Diabetic acidosis (when first seen) 
    • Addison disease
    • Use of magnesium-containing antacids or laxatives
  • 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 phosphorus to evaluate a patient’s overall 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.

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

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