What this test measures
Magnesium is the second most abundant intracellular cation (after potassium). It is a cofactor for over 300 enzymes, essential for ATP function, nerve conduction, muscle contraction, and the secretion and action of parathyroid hormone (PTH). Only about 1% of total body magnesium is in serum — so a normal serum value does not rule out total-body depletion.
Magnesium is absorbed in the small intestine and excreted by the kidneys. Common causes of depletion are gut loss (diarrhoea, malabsorption, proton pump inhibitor use long-term), kidney loss (diuretics, alcohol, drugs), and shifts into cells (refeeding syndrome).
Why it matters
Magnesium deficiency is under-recognised in India — it is common in alcohol users, in chronic diarrhoea, in patients on long-term PPIs (proton pump inhibitors) and diuretics, in malnutrition, and in chronic kidney disease. It often presents as unexplained refractory hypokalaemia or hypocalcaemia — neither corrects until magnesium is replaced first.
Magnesium deficiency can cause arrhythmias (especially Torsades de pointes, ventricular ectopy), neuromuscular irritability (tremor, tetany, seizures), and worsens hypertension, diabetes control, and asthma. High magnesium is mostly seen with chronic kidney disease and excessive magnesium-containing antacids / laxatives.
How to prepare
No fasting required. Avoid magnesium-containing antacids and laxatives for 1–2 days before. Mention diuretic use and chronic PPI / H2-blocker use.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Magnesium (mg/dL)[1][2] | 1.7 – 2.2 (or 0.7 – 1.0 mmol/L) | Hypomagnesaemia (<1.7). Causes — alcohol use, chronic diarrhoea, malabsorption, long-term PPI use, loop / thiazide diuretics, aminoglycoside antibiotics, refeeding syndrome, type 2 diabetes (renal loss), kidney tubular disease. Common clue — refractory hypokalaemia / hypocalcaemia that will not correct until magnesium is given first. | Hypermagnesaemia (>2.2) — rare. Causes — chronic kidney disease (commonest), excessive magnesium antacids / laxatives, magnesium sulphate therapy (eclampsia), Addison's disease, lithium toxicity. Values >5 cause loss of deep tendon reflexes, hypotension, respiratory depression, cardiac arrest. |
When to suspect magnesium deficiency
| Clinical clue | Reason |
|---|---|
| Hypokalaemia that will not correct | Magnesium deficiency causes urinary potassium wasting — replacing potassium alone fails |
| Hypocalcaemia that will not correct | Magnesium is needed for PTH secretion and action |
| Alcohol use | Poor intake, GI loss, renal wasting all contribute |
| Long-term PPI use | Reduces magnesium absorption — FDA warning |
| Diuretic use | Loop and thiazide diuretics increase urinary magnesium loss |
| Type 2 diabetes | Glucosuria pulls magnesium with it; deficiency worsens insulin resistance |
| Unexplained tremor, tetany, ventricular ectopy | Magnesium deficiency is a treatable cause |
Frequently asked questions
Do I need to fast?
No fasting required. Avoid magnesium-containing antacids and laxatives for a day or two before to avoid false elevation.
My potassium is low and will not correct. Should magnesium be checked?
Yes. Low magnesium causes urinary potassium wasting — until magnesium is replaced, potassium often will not correct. The same is true for refractory hypocalcaemia.
Can long-term PPIs cause low magnesium?
Yes. The US FDA issued a warning in 2011 — prolonged PPI use (>1 year) can cause significant hypomagnesaemia in some people. Anyone on long-term PPIs with unexplained tremor, tetany, arrhythmia or low potassium should have magnesium checked.
Can a serum magnesium be normal in deficiency?
Yes — only 1% of body magnesium is in serum. Total-body depletion can exist with a normal serum value. Clinical suspicion + a urinary magnesium fractional excretion can help.
How is low magnesium treated?
Mild deficiency — oral magnesium oxide, citrate or glycinate. Moderate / severe — IV magnesium sulphate. Always correct the cause (alcohol, PPI, diuretic, diarrhoea).
Is high magnesium ever dangerous?
Yes — values >5 mg/dL cause loss of reflexes, hypotension, respiratory depression and ultimately cardiac arrest. It is mainly seen in chronic kidney disease and in patients getting magnesium sulphate (eclampsia, severe asthma) — close monitoring is essential.
Does magnesium help with leg cramps?
Mixed evidence. In documented deficiency, replacement helps. In people with normal magnesium and idiopathic cramps, evidence for supplementation is weak. Trial under medical advice is reasonable.
Related Kidney / Electrolytes tests
Tests commonly ordered alongside MAGNESIUM, or that help interpret an unexpected result.
Sources & references
- NIH MedlinePlus — Magnesium Blood Test · accessed 2026-05-30T00:00:00.000Z
- NCBI StatPearls — Hypomagnesemia · accessed 2026-05-30T00:00:00.000Z
- NIH Office of Dietary Supplements — Magnesium · accessed 2026-05-30T00:00:00.000Z
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