Magnesium is a mineral involved in muscle and nerve function, energy use, and bone, and most of it sits inside cells and in bone. A blood test measures the small share circulating in serum or plasma. It is ordered as part of an electrolyte assessment. The range printed on your report decides how the value is read.
Because the blood holds so little of the body's magnesium, a result within the range is not proof that the body's stores are full. That gap is one of the most important things to understand about this test.
What the test measures
Magnesium is one of the body's main positively charged ions, and it is a cofactor for a large number of enzymes. Most of it is stored in bone and inside cells, with only a small fraction in the fluid outside cells. A routine blood test measures total magnesium in serum or plasma: the part bound to albumin, the part complexed with small molecules, and the free part, all together.
The result is a concentration, not a measure of the body's total magnesium. Because the body can draw on bone and cells to keep the blood concentration steady, the blood value can stay inside the report's range while the body's stores are low. Some laboratories offer a red cell magnesium measurement, which is a different test on a different sample and not interchangeable with the serum result.
Why it may be ordered
- As part of an electrolyte assessment, with sodium, potassium, and calcium.
- When a low potassium or low calcium result is being assessed, because magnesium is part of the same picture.
- When kidney function is being reviewed, because the kidneys decide how much magnesium is passed in urine.
- When a medicine that changes magnesium handling is being taken, because the result is read with the medicine list.
- When a clinician is assessing muscle cramps, tremor, weakness, or an irregular heartbeat, alongside the rest of the assessment.
- When gut absorption is a concern, as with some long-term gut conditions or after gut surgery.
The unit on your report
| What you may see | Unit | What it reflects |
|---|---|---|
| Magnesium, Mg | mmol/L | The total magnesium concentration in serum or plasma, in millimoles per litre. |
| Magnesium | mg/dL | The same measurement in milligrams per decilitre. The two scales are not the same number. |
| Magnesium | mEq/L | Another scale used for magnesium. It does not give the same number as mmol/L, because magnesium carries two charges. |
| Red cell magnesium | as the laboratory reports | A different test on a different sample, not interchangeable with the serum value. |
Most reports print one unit and one range. Read the range beside the unit you are looking at, and do not compare a serum magnesium value with a red cell magnesium value.
Why this page does not print a range
Magnesium ranges differ by laboratory and method, and total magnesium depends on albumin, because a share of it is protein bound. Age, sex, and pregnancy can shift the expected interval, and serum and plasma do not always give exactly the same result. A range copied from a website cannot account for any of that.
The range printed on the report that produced your value is the one that applies. Doctano keeps that printed range with the result and does not replace it with a general range. When a report notes haemolysis, read that note with the value.
What a high value can mean
A result above the report's range means the magnesium concentration is higher than the laboratory's interval. The term hypermagnesaemia describes the finding; it names the value, not a diagnosis.
Possible contributors include reduced kidney excretion and magnesium-containing laxatives or antacids taken in large amounts. Haemolysis can also raise the measured value, because red cells contain magnesium that is released into the sample. The result does not confirm or rule out any of these, and a clinician reads it with kidney function and the medicine and product list.
What a low value can mean
A result below the report's range means the magnesium concentration is lower than the laboratory's interval. The term hypomagnesaemia describes the finding. Because the blood concentration is buffered from bone and cells, a low value can appear after the body's stores have already been low for some time.
Possible contributors include reduced intake or absorption through the gut, as with malabsorption or after gut surgery; increased loss through the kidneys, as with some diuretics; increased loss through the gut, as with diarrhoea; some medicines, including long-term proton pump inhibitors; alcohol use; and a shift of magnesium into cells. Low magnesium often accompanies low potassium and low calcium, and magnesium status can affect how those values behave.
What can move the value
- Albumin. A share of magnesium is bound to albumin, so a change in albumin can move the total value.
- Haemolysis. Red cells release magnesium, so a damaged sample can read higher than the body.
- Kidney function. The kidneys decide how much magnesium is passed in urine.
- Gut absorption. Long-term gut conditions and gut surgery can change how much magnesium is taken up.
- Medicines. Some diuretics and long-term proton pump inhibitors can change magnesium balance, and a clinician can say whether a medicine is relevant.
- Pregnancy. Pregnancy changes albumin and mineral handling, so the expected interval can differ.
What to compare it with
Read magnesium with potassium and calcium, because the three move together in several settings. Add albumin, because a share of magnesium is protein bound; creatinine for the kidney view; and glucose, because high glucose increases urine output and can affect mineral losses. Where a report prints phosphate and parathyroid hormone, those add context too.
How to see the trend
Doctano reads the magnesium value, its unit, and the reference range printed on the report. It keeps the range with the result, normalizes the marker name across laboratories, and places the value on the profile's timeline. Each point still opens the report it came from, so a note about haemolysis stays with the value it describes.
Verify the value and the unit against the original report before treating a change as real, especially when the two results came from different laboratories or used different units.
What this page is not
This page explains the test in general. It does not interpret your result, diagnose a condition, or recommend treatment. It is not medical advice. Verify the value and the unit against the original report, and discuss the result with a qualified professional.