Electrolytes
Magnesium
Magnesium is one of the body's most abundant intracellular cations and acts as a cofactor for hundreds of enzyme systems, including those that govern neuromuscular excitability and cardiac rhythm. At or near the patient it is reported either as ionised (free) magnesium in whole blood by ion-selective electrode, or as total magnesium on benchtop disc chemistry analysers.
Why it is measured
Magnesium disturbance is common in critical care, cardiac, obstetric and alcohol-related presentations, where a rapid result can support timely replacement decisions and arrhythmia management. The ionised fraction is regarded as the biologically active form.
| Typical range | Total magnesium: indicatively about 0.70 to 1.00 mmol/L (roughly 1.7 to 2.4 mg/dL) in adults. Ionised (free) magnesium measured by point-of-care ion-selective electrodes runs lower, broadly 0.45 to 0.60 mmol/L. Ranges vary by method, analyser and population, and the lower limit for total magnesium is under active standardisation debate (some groups propose raising it toward 0.85 mmol/L), so always apply the issuing laboratory's stated reference interval. |
|---|---|
| Sample | Lithium-heparin whole blood for point-of-care ionised magnesium by ion-selective electrode. Whole blood, serum or lithium-heparin plasma for total magnesium on reagent-disc analysers. Avoid haemolysis and prolonged stasis, which can shift results. |
| Turnaround | Roughly 90 seconds for ionised magnesium on whole-blood critical-care analysers. About 12 minutes for disc-based total magnesium. |
Point of care devices that report it
- Nova Biomedical Stat Profile Prime Plus (ionised magnesium, ion-selective electrode, whole blood)
- Nova Biomedical Stat Profile pHOx Ultra (ionised magnesium, ion-selective electrode, whole blood)
- Abbott Abaxis Piccolo Xpress (total magnesium, reagent-disc panels such as Basic Metabolic Panel Plus and MetLac 12)
Questions, answered
Is point-of-care ionised magnesium the same as the total magnesium my laboratory reports?
No. Total magnesium includes protein-bound, complexed and free fractions, whereas point-of-care ion-selective electrodes measure only the biologically active ionised (free) portion, which has its own lower reference interval. The two are not interchangeable, so each result should be read against its matching method and range.
Why do many blood gas analysers report sodium, potassium and calcium but not magnesium?
Magnesium needs a dedicated, selective ion-selective electrode, and most compact blood gas devices do not include one. Whole-blood magnesium at the point of care is therefore limited to specific platforms such as the Nova Stat Profile Prime Plus or pHOx Ultra, or to disc-based chemistry analysers that report total magnesium.
Can sample handling affect a point-of-care magnesium result?
Yes. Magnesium is more concentrated inside red cells, so haemolysis can falsely elevate readings, and delayed analysis or prolonged tourniquet stasis can also shift values. Careful collection and prompt analysis support reliable results. This is general operational guidance, not patient-specific interpretation.