Magnesium blood test: low and high levels explained

Magnesium is an electrolyte that helps muscles contract and relax, supports nerve signalling and keeps the heartbeat regular. The blood test measures how much is circulating, which is only a small fraction of the body total, so a normal result does not always exclude depletion. Lab Tests Online UK says abnormal levels are most often seen in conditions that impair or increase excretion of magnesium by the kidneys or cause poor absorption in the intestines.

It is usually grouped with other electrolytes, including sodium, potassium, chloride, bicarbonate, calcium and phosphate, to give a picture of overall balance.

The test is typically a follow-up to persistently low calcium or potassium, or is requested when symptoms suggest low magnesium, such as muscle weakness, twitching, cramping, confusion, irregular heartbeat or seizures. According to Lab Tests Online UK, dietary deficiency is rare but the test may be done when evaluating malabsorption, malnutrition, diarrhoea or alcoholism, in people taking drugs that cause kidney magnesium loss, in kidney disorders and uncontrolled diabetes, and to monitor supplementation.

Magnesium is also given intravenously to treat pre-eclampsia in pregnancy, and blood levels are checked to ensure treatment is adequate.

No fasting or preparation is needed and blood is taken from a vein in the arm. Tell the clinician about supplements, antacids and laxatives containing magnesium, because they can raise levels, and about diuretics and acid-reducing tablets, which can lower them.

Low levels mean you are not getting enough in the diet, your intestines are not absorbing enough, or your kidneys are excreting too much. Lab Tests Online UK lists low dietary intake (in older or malnourished people and with alcoholism, which also increases excretion), gastrointestinal disorders such as Crohn disease, uncontrolled diabetes, hypoparathyroidism, long-term use of diuretics and proton pump inhibitors, prolonged diarrhoea, surgery, severe burns and toxaemia of pregnancy.

The knock-on effects matter. Low magnesium can lead over time to persistently low calcium and potassium, and can make low calcium levels more resistant to treatment. That is why calcium, potassium, phosphate and parathyroid hormone are often reviewed together.

Increased levels are rarely due to excess dietary magnesium; they are usually the result of an excretion problem or excessive supplementation. Lab Tests Online UK lists kidney failure, hyperparathyroidism, hypothyroidism, dehydration, diabetic acidosis when first seen, Addison disease and use of magnesium-containing antacids or laxatives. Very high levels can slow reflexes and breathing and affect the heart, which is a reason to review products such as laxatives in people with poor kidney function.

The cause is identified and treated. Mild deficiency is often corrected with diet or oral supplements and more severe deficiency with intravenous magnesium, with blood tests to monitor the response. Any medicine thought to be the cause is reviewed, but never stop a prescribed medicine without advice. Seek urgent care for palpitations with fainting, seizures or severe weakness; call 999 or attend A&E.