Magnesium Blood Test

• Serum magnesium (measured): reported in mmol/L. It includes magnesium bound to albumin and other molecules as well as the free (ionised) form.

This is not a red blood cell magnesium test, an ionised magnesium test, a urine magnesium test or a magnesium loading test. Each of those has been used in research to estimate body stores, but the 2018 Nutrients review found none is a validated, routine measure of overall magnesium status.

Calcium, potassium, albumin and kidney function are not included. A clinician may want them alongside an abnormal magnesium result.

Magnesium is a cofactor for a very large number of enzymes. Every cell's main energy molecule, ATP, works in a complex with magnesium. Magnesium also helps regulate the electrical activity of nerves and muscles, including the heart, and the NHS notes that it keeps the parathyroid glands working normally, which matters for calcium balance and bone health.

Most of the body's magnesium is out of reach of a blood test. A 2018 review in Nutrients estimates that about 53% is in bone, 27% in muscle and 19% in other soft tissues, with less than 1% in blood and only around 0.3% in serum. The gut adjusts how much it absorbs, the kidneys adjust how much they excrete, and part of the bone store can be released when the blood level falls. Together these keep serum magnesium in a narrow band, so it can remain normal while the body's overall reserves are being drawn down.

Low magnesium also disrupts other minerals. Lab Tests Online UK explains that it can make low calcium and low potassium harder to correct, which is why the three are often checked together.

Your report will show the laboratory's reference range. Many laboratories use a lower limit of around 0.7 mmol/L, a figure derived from population surveys carried out in the 1970s. Some researchers, including Costello and colleagues writing in Advances in Nutrition in 2016, have proposed raising the lower limit to 0.85 mmol/L. That proposal has not been adopted as a UK standard, so a result between those two figures will usually appear as normal on your report. It is a point for discussion, not a diagnosis.

Things that can affect the result without reflecting true magnesium status:

• Haemolysis: red cells hold more magnesium than serum, so damage to cells during or after sampling can raise the result falsely.
• Low albumin: because part of serum magnesium is protein-bound, low albumin can lower the total figure.
• Pregnancy: levels may be naturally lower.

Recognised causes of a genuinely low result include prolonged diarrhoea, malabsorption, heavy alcohol use, poorly controlled diabetes, and long-term diuretics or proton pump inhibitors. A high result is uncommon from food alone and usually reflects reduced kidney function or magnesium from supplements, antacids or laxatives.

A clearly abnormal result should be reviewed by your GP. Severe imbalance can affect heart rhythm, so palpitations, fainting, muscle spasms or a seizure need urgent medical care through NHS 111 or 999, not an online test. Where a result needs medical attention, we will advise you to see your GP and, with your consent, can write to them.

A 2014 review in the British Journal of Hospital Medicine described low magnesium as relatively common but frequently missed. One reason is medicines. In 2012 the MHRA reported cases of low magnesium in people on long-term proton pump inhibitors (such as omeprazole and lansoprazole) and advised prescribers to consider checking magnesium before treatment and periodically during it, especially for people also taking digoxin or diuretics.

If you are in one of these groups, a result can be a useful thing to share with your GP or pharmacist. Do not stop or change a prescribed medicine because of it.

Muscle cramps, twitching and tiredness are often attributed to magnesium. They have many other causes, and a normal serum result neither confirms nor rules out magnesium as the reason.

May find it useful: people on long-term acid-reducing medicines or diuretics; people with chronic diarrhoea, coeliac disease or inflammatory bowel disease; people who drink heavily; people who have had low calcium or potassium results; people taking magnesium supplements who want to check they are not overshooting, particularly if their kidney function is reduced.

May not need it: healthy people with a varied diet and no risk factors, for whom a normal result adds little.

Should seek care elsewhere: anyone with palpitations, fainting, severe muscle spasms or seizures needs urgent assessment. People with kidney disease should check with their kidney team before taking magnesium supplements. If your GP or hospital already monitors your electrolytes, keep to that plan.

Established: a 2021 review by Barbagallo and colleagues describes how magnesium balance becomes more fragile with age. Intake tends to fall, gut absorption declines, the kidneys lose more, and older adults are more likely to take medicines that lower magnesium.

Associations: the same review links low magnesium status with cardiovascular disease, high blood pressure, type 2 diabetes, bone fragility and other age-related conditions. These links come largely from observational studies, and they do not show that low magnesium causes these conditions or that correcting it would prevent them.

Emerging and unproven: the authors suggest that maintaining magnesium balance through life might help limit chronic low-grade inflammation, but they state that this still needs to be shown in long-term studies. They also note that the role of supplements remains unclear. Food sources such as green leafy vegetables, nuts and wholegrains remain the usual starting point.

• No fasting or special preparation is needed.
• Tell us about any magnesium supplements, magnesium-containing antacids or laxatives, diuretics and acid-reducing medicines. Keep taking prescribed medicines as normal.
• If you plan to repeat the test, keep your supplement routine and the time of day similar so the results are comparable.
• Being well hydrated helps the sample to be taken smoothly, which reduces the risk of haemolysis.
• Tell us if you are pregnant.

Choose how your sample is taken when you book. We confirm the collection option, any collection fee and the expected turnaround time before you book.

• Juvenology Clinic, Maidstone (82 King Street, Maidstone ME14 1BH): Available.
• Partner clinic near you, anywhere in the UK: Available on request. We confirm availability for your postcode before you book.
• Postal home finger-prick kit: Ask before ordering. We offer a home kit only where the laboratory confirms this test is suitable for a finger-prick sample.

Sample: Blood sample (serum)

Collection notes: No fasting needed. Haemolysis can falsely raise magnesium, so a venous sample is preferred; a postal finger-prick kit is offered only where the laboratory confirms capillary samples are suitable for this test.

The price of this test covers laboratory analysis and the laboratory's results report. The report lists each result beside the laboratory's reference range.

Beyond the blood test: optional support, priced separately

• Personalised written interpretation report: £50
• General consultation: £50
• Results consultation (results discussion and follow-up advice): £100
• Personalised programme: from £150, after we assess whether a programme suits you

Consultations are with Marina, an NMC-registered nurse. She explains what your results show and how they relate to lifestyle. No prescription service is offered with these tests.

If a result suggests you need medical follow-up, we will recommend that you see your GP and, with your consent, can provide a letter to take with you. A blood test cannot rule out every condition. If you have symptoms, speak to your GP or NHS 111, and call 999 in an emergency.

If my result is normal, are my magnesium stores fine?

Not necessarily. Serum reflects under 1% of body magnesium and is held steady by the body. A normal result makes a serious deficiency less likely but cannot confirm that stores in bone and muscle are full.

Is this the same as a red blood cell magnesium test?

No. This test measures serum magnesium, the form used in routine clinical practice. Red cell magnesium is sometimes offered elsewhere, but it is not a validated routine measure of body stores.

I take omeprazole. Should I be worried?

Low magnesium has been reported with long-term proton pump inhibitor use, and the MHRA advises prescribers to consider monitoring. Share your result with your GP or pharmacist and keep taking your medicine unless they advise otherwise.

Can supplements make my result high?

They can, particularly if kidney function is reduced. The NHS suggests that 400mg a day or less from supplements is unlikely to cause harm.

Will this explain my muscle cramps?

Possibly, if the result is clearly low. Cramps have many causes, so a normal result is a reason to look elsewhere with your GP.

Is interpretation included?

The price covers analysis and the laboratory's report. You can add a personalised written interpretation report (£50) or a results consultation (£100).

1. Workinger JL, Doyle RP, Bortz J. Challenges in the diagnosis of magnesium status. Nutrients, 2018;10(9):1202. https://pubmed.ncbi.nlm.nih.gov/30200431/
2. Costello RB, et al. Perspective: the case for an evidence-based reference interval for serum magnesium: the time has come. Advances in Nutrition, 2016. https://pubmed.ncbi.nlm.nih.gov/28140318/
3. Barbagallo M, Veronese N, Dominguez LJ. Magnesium in aging, health and diseases. Nutrients, 2021;13(2):463. https://doi.org/10.3390/nu13020463
4. Demssie YN, Patel L, Kumar M, Syed AA. Hypomagnesaemia: clinical relevance and management. British Journal of Hospital Medicine, 2014;75(1):39-43. https://doi.org/10.12968/hmed.2014.75.1.39
5. Proton pump inhibitors in long-term use: reports of hypomagnesaemia. Drug Safety Update, Medicines and Healthcare products Regulatory Agency, 2012. https://www.gov.uk/drug-safety-update/proton-pump-inhibitors-in-long-term-use-reports-of-hypomagnesaemia
6. Magnesium. Lab Tests Online UK. https://labtestsonline.org.uk/tests/magnesium
7. Vitamins and minerals: others (magnesium). NHS. https://www.nhs.uk/conditions/vitamins-and-minerals/others/