Magnesium (serum magnesium)

Pick Ordered or Not ordered to see the note.

Indicated when

• —Long-term use of a proton pump inhibitor, particularly beyond one year or alongside a diuretic [1]
• —Diuretic use, especially with unexplained hypokalemia or hypocalcemia [1]
• —Unexplained cardiac arrhythmia, tetany, or seizures, or hypokalemia/hypocalcemia that does not correct as expected with treatment [1]

Not indicated when

• —Routine 'wellness panel' magnesium testing with no medication, GI, renal, or symptom-based risk factor [2]
• —Ordering 'RBC magnesium' as a routine alternative to serum magnesium: it has no OHIP code and no validated reference standard identified in this session

Why not

Magnesium is sometimes called the 'forgotten ion,' and drug-induced hypomagnesemia is under-recognized rather than over-tested; the harm of testing without any risk factor is mainly low-value cost, since a normal result in a person with no risk factor is unlikely to change management [1]. Serum magnesium, despite recognized limitations as a marker of total-body magnesium, remains the recommended first test when a real risk factor for drug-induced hypomagnesemia is present.

Better first step

For fatigue or muscle cramps with no drug, GI, or renal risk factor, look for a more likely cause before ordering magnesium. When a real risk factor for drug-induced hypomagnesemia is present, chronic PPI or diuretic use, or unexplained hypokalemia, hypocalcemia, or arrhythmia, serum magnesium is itself the appropriate first test [1].

Typical ND rationale

An ND may order magnesium, sometimes specifically requesting 'RBC magnesium,' in a patient with fatigue, muscle cramps, or stress, reasoning that magnesium is involved in hundreds of enzymatic reactions and that a standard serum level can miss a true tissue-level depletion.

Ontario coverage & CONO orderability

OHIP status

insured

L165 Magnesium is listed in the 2026 Schedule of Benefits for Laboratory Services with no restricting condition, so it is insured when a physician orders it after clinically assessing the patient ([3]; Reg. 552 s.22). 'RBC magnesium' by that specific name was not found in the 2026 Schedule during this session's search, so its coverage status is unverified rather than asserted as uninsured. ND-ordered testing of either is patient-paid regardless [6].

CONO orderable

Yes — CONO list #91

CONO list item 91, blood: 'Magnesium.' No separate 'RBC magnesium' line item is listed on the CONO list; an Ontario ND clinic offering 'RBC magnesium' by that name is either billing it under this same generic item or sending it to a lab not on this list.

Linked conditions

• tired ↗

Counselling script

“If you're on a long-term acid-reducer or water pill, or you've had an unexplained low potassium, low calcium, or irregular heartbeat, checking magnesium makes sense, since those medicines can genuinely lower it over time. Without one of those, I wouldn't add magnesium to routine bloodwork, since a normal result there usually wouldn't change anything. The 'RBC magnesium' version specifically isn't a test with clear evidence behind it, so I'd use the standard magnesium test if we're checking.”

Revisit if

• —Long-term proton pump inhibitor or diuretic use is started
• —Unexplained hypokalemia, hypocalcemia, arrhythmia, tetany, or seizures occur

References

1. [1]Pharmacology Research & Perspectives (2021). An overview of diagnosis and management of drug-induced hypomagnesemia. linkDrug-induced hypomagnesemia is linked to diuretics and proton pump inhibitors among other medications, and should be suspected in patients with relevant symptoms
2. [2]College of Family Physicians of Canada / Choosing Wisely Canada (2026). Family Medicine: Fifteen Tests and Treatments to Question. linkDon't do annual screening blood tests unless directly indicated by the patient's risk profile
3. [3]Ontario Ministry of Health (2026). Ontario Health Insurance Plan: Schedule of Benefits for Laboratory Services (effective April 1, 2026) - L165 Magnesium. linkL165 Magnesium is listed in the 2026 OHIP Schedule with no restricting condition
4. [4]Ontario Ministry of Health (2026). Ontario Health Insurance Plan: Schedule of Benefits for Laboratory Services (effective April 1, 2026). linkCommunity lab tests are insured only when ordered under the Schedule by an authorized prescriber
5. [5]Government of Ontario (e-Laws) (2026). Health Insurance Act, R.R.O. 1990, Reg. 552: General, s. 22 (insured laboratory services). linkA lab test is insured only when ordered by a physician, midwife, or NP who has clinically assessed the patient
6. [6]Ontario Ministry of Health (2026). Ontario Health Insurance Plan: Schedule of Benefits for Laboratory Services (effective April 1, 2026). linkTests ordered by anyone other than an authorized provider, including NDs, are not insured services