What it measures
This test measures magnesium in serum, the liquid part of the blood. Magnesium is needed for muscles, nerves and many enzymes. Most of the body's magnesium sits in bone and inside cells, and the blood level is held in a narrow band.
Why people test it
- Checked with muscle cramps, twitching or an irregular heartbeat, often alongside calcium and potassium.
- Measured in people on medicines that can lower it, such as long-term acid-reducing tablets or some diuretics.
- Monitored with long-term diarrhoea, high alcohol intake or kidney disease, which affect magnesium levels.
Typical range
Ranges vary between labs with the method used, age and sex, and your report shows the lab's own range. One NHS trust's lab uses 0.7 to 1.0 mmol/L for adults. Source: Leeds Teaching Hospitals NHS Trust: Blood Sciences Reference Range Database v3.20.
Finger-prick or vein?
A finger-prick sample is usually fine. Red blood cells hold more magnesium than serum, so a sample that breaks down (haemolyses) can read falsely high.
A blood test reads the magnesium in serum, which holds only about 1% of the body's magnesium, so it is a limited measure.
