Magnesium blood test: what your result means

How is magnesium tested?

Usually with a standard blood sample from a vein. The lab measures serum magnesium, the magnesium in the liquid part of the blood. Doctors often check calcium, potassium and kidney function at the same time, because the four are closely linked.

What is a normal result?

Normal ranges vary slightly between laboratories, so always compare your result with the range printed on the report. The range most widely quoted internationally is 0.75 to 0.95 mmol/L, and UK labs commonly use a range close to 0.7 to 1.0 mmol/L. Below the range is called hypomagnesaemia; above it is hypermagnesaemia, which is uncommon and usually linked to kidney disease or high-dose magnesium medicines. Side effects lists the signs of too much.

Why can a normal result miss low magnesium?

Less than 1% of the body’s magnesium is in the blood. The rest is in bone and inside cells. When intake falls, the body pulls magnesium out of bone and the kidneys hold on to more, so the blood level can stay normal while stores drain.

In 2016 a group of researchers argued that the usual reference range was set from population averages in the 1970s, not from health outcomes, and that people with results near the bottom of the range may already be short. They proposed a higher lower limit of about 0.85 mmol/L. This is not yet standard practice in the UK, but it explains why some doctors treat low-normal results in people with symptoms or risk factors.

Are there better tests?

Researchers use several, none perfect:

• Red blood cell magnesium, sold by private labs, but with no agreed normal range.
• 24-hour urine magnesium, which shows how much the kidneys are releasing and helps work out the cause of a low level.
• Magnesium loading tests, which measure how much of an infused dose is retained. These are research or specialist tools.

Are home finger-prick tests reliable?

They can be, if the lab is accredited, but there are catches. Squeezing a finger can burst red blood cells, which hold more magnesium than serum and can push the result up. Some kits measure red blood cell magnesium, which has no agreed normal range. If a home test comes back low or borderline, it’s worth a repeat through your GP, who can test from a vein and look at the result alongside calcium, potassium and kidney function.

What happens if your result is low?

Your GP will look for the cause, such as a medicine, diarrhoea or alcohol, and may suggest a supplement, usually as a well-absorbed form like citrate. Very low levels, or low levels with heart rhythm changes, are treated in hospital with magnesium through a drip. Most people need a repeat test after a few weeks.

When is testing worth it?

Testing makes most sense if you take a medicine that lowers magnesium, especially a proton pump inhibitor for more than a year, have diabetes, drink heavily or have a bowel condition, or have symptoms such as persistent cramps, twitching or palpitations. See signs of low magnesium and how much magnesium you need.