Short answer: the magnesium blood test measures magnesium in the serum, the liquid part of the blood. Less than 1% of the body's magnesium is there; most is in bones and cells. So the blood level can stay in range while total stores are low, and doctors read it with symptoms, medicines and other electrolytes. mmol/L = mg/dL × 0.4114, and mEq/L = mmol/L × 2.
At a glance
| Measures | Magnesium in serum or plasma |
|---|---|
| Units | mg/dL (US), mmol/L (Europe), sometimes mEq/L |
| Conversion | mmol/L = mg/dL × 0.4114; mEq/L = mmol/L × 2 |
| LOINC | 19123-9 |
| Changed by | Some diuretics, proton pump inhibitors taken long term, alcohol, diarrhoea, kidney function |
| Keep | Value, unit, range and date, with potassium and calcium |
What the test measures
Magnesium is needed for muscles, nerves, heart rhythm and hundreds of enzymes. The kidneys keep the blood level stable by excreting more or less. Low magnesium often travels with low potassium and low calcium, which is why the three are often ordered together; see electrolytes and calcium.
How the result is written on the report
| Result on the report | LOINC code | Usual unit | Also seen as |
|---|---|---|---|
| Magnesium, serum or plasma | 19123-9 | mg/dL | mmol/L (×0.4114), mEq/L |
mmol/L = mg/dL × 0.4114: 2.0 mg/dL ≈ 0.82 mmol/L ≈ 1.65 mEq/L. Use the range printed on your report.
Medicines and supplements
Some diuretics and long-term acid-reducing medicines (proton pump inhibitors) can lower magnesium; magnesium supplements and laxatives or antacids that contain magnesium can raise it, especially when the kidneys work less well. Tell your doctor everything you take, including supplements.
Keeping the results in your record
Keep magnesium next to potassium and calcium, with the date. Anpheros Daily keeps each with its unit and charts it over time; the lab results tracker converts mg/dL and mmol/L for you.
What this page does not do
It explains the test. It does not interpret your result or recommend supplements; whether you need magnesium is your doctor's decision.
Sources
Frequently asked questions
Can I have low magnesium with a normal blood test?
Yes. Because most magnesium is inside cells and bone, the blood level can stay in range while stores are low. Your doctor considers symptoms and causes.
How do I convert magnesium from mg/dL to mmol/L?
Multiply by 0.4114. For example, 1.8 mg/dL is about 0.74 mmol/L.
Should I take magnesium supplements if my result is low?
Ask your doctor. The cause matters, and supplements can be unsafe with some kidney problems or medicines.
Try it in Anpheros Daily
Photograph your lab report and Anpheros Daily lines up each value with its reference range, so you can compare reports from different laboratories.
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