Magnesium (Serum) Test
Serum magnesium measures total circulating magnesium and is the validated first-line test for detecting acute magnesium disturbance, investigating refractory hypokalaemia or hypocalcaemia, and monitoring magnesium-wasting medications. Because under 1% of body magnesium is extracellular and serum levels are defended by tissue stores, a normal result does not exclude chronic total body depletion.
1
Biomarkers
Blood (serum)
Sample
1–2 business days
Turnaround
No — fasting is not required, though consistent collection conditions improve comparability of serial results
Fasting
What is the Magnesium test?
The Magnesium Test measures total magnesium in serum. Magnesium is an obligatory cofactor for over 300 enzymatic reactions, including all reactions involving ATP, and is essential to neuromuscular function, cardiac conduction, and the regulation of potassium and calcium homeostasis. Serum magnesium is ordered to detect hypomagnesaemia and hypermagnesaemia, to investigate refractory hypokalaemia or hypocalcaemia, to assess arrhythmia and neuromuscular irritability, and to monitor patients on magnesium-wasting medications or receiving magnesium therapy. Within magnesium assessment, serum magnesium is the validated and appropriate first-line test, and it is the measure that appears in clinical guidelines and that acute management decisions rest upon. Its limitation is well recognised and specific rather than general: because under 1% of body magnesium is extracellular and serum levels are defended by release from bone and tissue stores, a normal serum magnesium does not exclude significant total body depletion, and the test is insensitive to the chronic depletion seen with prolonged proton pump inhibitor use, malabsorption, alcohol dependence, or poorly controlled diabetes. RBC magnesium assesses the intracellular compartment and is more informative in that specific setting, though it is not standardised and lacks outcome data, so the two tests answer different questions rather than competing. Two further points matter in practice. Magnesium depletion causes hypokalaemia and hypocalcaemia that characteristically will not correct until the magnesium deficit is replaced, so magnesium should be measured whenever those abnormalities prove refractory. And because roughly 30% of circulating magnesium is albumin-bound, total serum magnesium underestimates the active ionised fraction in hypoalbuminaemia, so results in patients with low albumin require careful interpretation.
What does the Magnesium test measure?
Magnesium is the second most abundant intracellular cation and an obligatory cofactor for over 300 enzymatic reactions, including all reactions involving ATP, which is biologically active as the Mg-ATP complex. Approximately 60% of body magnesium is in bone and most of the remainder is intracellular, leaving under 1% in the extracellular fluid that serum measurement samples. Of circulating magnesium, roughly 30% is protein-bound, principally to albumin, with the remainder ionised or complexed; the ionised fraction is the biologically active form. Homeostasis is maintained chiefly by renal reabsorption in the loop of Henle and distal tubule, together with intestinal absorption.
Serum magnesium is the validated, guideline-anchored test for detecting acute magnesium disturbance and is the appropriate measure in acute care, drug monitoring, and the investigation of refractory electrolyte abnormalities. Its recognised limitation is that it samples under 1% of body magnesium and is defended by release from bone and tissue stores, so it typically remains within the reference interval until depletion is substantial and is therefore insensitive to chronic depletion. Because roughly 30% is albumin-bound, total serum magnesium also underestimates the biologically active ionised fraction in hypoalbuminaemia.
Why is the Magnesium test ordered?
- Investigation of refractory hypokalaemia or hypocalcaemia, which characteristically will not correct until coexisting magnesium depletion is replaced
- Assessment of unexplained neuromuscular irritability, tetany, tremor, or seizures where hypomagnesaemia is in the differential
- Evaluation of arrhythmia, particularly torsades de pointes or ventricular ectopy, where magnesium status directly influences management
- Monitoring of patients on magnesium-wasting medications — loop and thiazide diuretics, proton pump inhibitors, aminoglycosides, amphotericin B, cisplatin, or calcineurin inhibitors
- Assessment of patients receiving magnesium therapy or at risk of hypermagnesaemia, particularly those with renal impairment or receiving magnesium-containing antacids or laxatives
- Evaluation of patients with malabsorption, chronic diarrhoea, alcohol dependence, poorly controlled diabetes, or a history of bariatric surgery
- Assessment of patients at risk of refeeding syndrome, alongside phosphate and potassium, during nutritional replenishment
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
No — fasting is not required, though consistent collection conditions improve comparability of serial results
Collection method
Venipuncture at patient service centre
Turnaround
1–2 business days
Collection notes
Avoid haemolysis, which falsely elevates results and warrants recollection rather than a caveated report. Order alongside albumin so that total magnesium can be interpreted against protein binding, and alongside potassium and calcium where refractory electrolyte abnormalities are being investigated. Document magnesium supplementation, magnesium-containing antacids or laxatives, magnesium-wasting medications, renal function, and refeeding or insulin administration.
Specimen requirements
Minimum 0.5 mL serum. Separate serum from cells per LabCorp protocol. Avoid haemolysis. Specimen stable refrigerated; follow LabCorp transport guidelines.
What can affect Magnesium results?
Magnesium is predominantly intracellular, so red cell lysis releases magnesium into the sample and falsely elevates results. A haemolysed specimen should be recollected rather than reported with a caveat.
Roughly 30% of circulating magnesium is albumin-bound, so total serum magnesium falls with low albumin while the biologically active ionised fraction may be preserved. In hypoalbuminaemic patients, total magnesium underestimates functional status.
Oral or intravenous magnesium, and magnesium-containing antacids and laxatives, raise serum levels and may mask underlying depletion. Supplement type, dose, and timing relative to collection should be documented.
Magnesium is excreted renally, so declining glomerular filtration causes retention. Hypermagnesaemia is largely confined to patients with renal impairment, particularly those also receiving magnesium-containing preparations.
Prolonged proton pump inhibitor use impairs intestinal absorption, while loop and thiazide diuretics, aminoglycosides, amphotericin B, cisplatin, and calcineurin inhibitors increase renal losses. Medication history should be reviewed before interpreting a low result.
Insulin drives magnesium into cells alongside potassium and phosphate, so serum levels fall acutely during refeeding and treatment of diabetic ketoacidosis without a change in total body stores. Acute illness and catecholamine surges have a similar transcellular effect.
What do Magnesium results mean?
Elevated
Hypermagnesaemia is uncommon and is almost always associated with renal impairment, frequently compounded by magnesium-containing antacids, laxatives, or therapeutic magnesium. Other causes include adrenal insufficiency, tumour lysis syndrome, and, in obstetric practice, magnesium therapy for pre-eclampsia. Progressive elevation causes loss of deep tendon reflexes, hypotension, respiratory depression, and cardiac conduction abnormalities, so a markedly raised result in a symptomatic patient requires urgent assessment. Spurious elevation from haemolysis should be excluded.
Normal
A normal serum magnesium is reassuring for acute disturbance but does not exclude significant total body depletion, since under 1% of body magnesium is extracellular and serum levels are defended by release from bone and tissue stores. Where chronic depletion is suspected — prolonged proton pump inhibitor use, malabsorption, alcohol dependence — a normal serum result should not close the question, and RBC magnesium or a supervised repletion trial may be considered alongside the clinical assessment.
Reduced
Hypomagnesaemia arises from reduced intake or absorption, renal wasting, or transcellular shift. Common contributors are prolonged proton pump inhibitor use, diuretics, alcohol dependence, malabsorption, chronic diarrhoea, poorly controlled diabetes, and refeeding. Clinically it manifests as neuromuscular irritability, tetany, arrhythmia including torsades de pointes, and — importantly — refractory hypokalaemia and hypocalcaemia that will not correct until magnesium is replaced. Total magnesium should be interpreted against albumin, since hypoalbuminaemia lowers the total while the ionised fraction may be preserved.
Note: Serum magnesium is the validated test for acute disturbance but is insensitive to chronic depletion, so a normal value does not exclude deficiency. Total magnesium underestimates the active ionised fraction in hypoalbuminaemia. Haemolysis falsely elevates results. Levels shift acutely with insulin and refeeding independent of body stores, and recent supplementation or magnesium-containing preparations can mask depletion. It is interpreted alongside potassium, calcium, albumin, and renal function rather than in isolation.
Frequently asked questions
It measures total magnesium in serum, including the albumin-bound and ionised fractions. Magnesium is an obligatory cofactor for over 300 enzymatic reactions and is essential to neuromuscular function, cardiac conduction, and potassium and calcium regulation.
How to access the Magnesium test
The Magnesium reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
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Published: July 16, 2026
Last reviewed: July 16, 2026
- Ayuk J, Gittoes NJ. Contemporary view of the clinical relevance of magnesium homeostasis. Ann Clin Biochem. (2014)
- Agus ZS. Hypomagnesemia. J Am Soc Nephrol. (1999)
- Elin RJ. Assessment of magnesium status for diagnosis and therapy. Magnes Res. (2010)
- Huang CL, Kuo E. Mechanism of hypokalemia in magnesium deficiency. J Am Soc Nephrol. (2007)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.
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