Hypomagnesaemia (Low Magnesium) Interpreter
Hypomagnesaemia (Low Magnesium) Interpreter
Grade a low magnesium, then use the fractional excretion of magnesium (FEMg) to decide whether it is being lost through the kidney or the gut, and check the drug history against that answer — PPIs, diuretics, alcohol, calcineurin inhibitors, cisplatin — and Gitelman syndrome. It also explains why a low magnesium keeps a low potassium or calcium from correcting.
How low is the magnesium, and where is it going?
Magnesium, FEMg, drugs → severity, renal or gut, causeA 71-year-old on omeprazole for six years has cramps. Magnesium 0.45 mmol/L, potassium and calcium both low. FEMg 1.1%, kidney function normal, no diarrhoea.
How the page reads it
FEMg (%) = (urine Mg × plasma creatinine) ÷ (0.7 × plasma Mg × urine creatinine) × 100
In hypomagnesaemia with normal kidney function: FEMg below 2% → gut or intake; above 4% → renal wasting
- 0.7
- about 70% of plasma magnesium is free to be filtered; the rest is bound to albumin. Multiplying by 0.7 turns total plasma magnesium into the filtered amount. The FEMg calculator on this site does include it
- 0.70 vs 0.85
- most laboratories set the lower limit at about 0.70 mmol/L (1.7 mg/dL); a 2022 proposal would raise it to 0.85 mmol/L (2.07 mg/dL)
- mmol/L to mg/dL
- × 2.4305; to mEq/L, × 2
Worked example
A 71-year-old on omeprazole for six years has cramps. Magnesium 0.45 mmol/L, potassium and calcium both low. FEMg 1.1%, kidney function normal, no diarrhoea.
Magnesium 0.45 mmol/L → moderate (0.40 to 0.56)
FEMg 1.1% is below 2% → the kidney is conserving: loss through the gut
A proton pump inhibitor reduces gut absorption → fits; stop or switch it
Low potassium and calcium will not correct until the magnesium is replaced
Three FEMg cut-offs are in print
| Source | Extrarenal (conserving) | Renal wasting |
|---|---|---|
| Elisaf 1997 | Mean 1.4% (range 0.5–2.7%) | More than 4% (renal cases 4–48%) |
| Viering 2017 review | Below 2% | More than 4% |
| Salinas 2024 review | — | More than 3% |
Expected FEMg by cause
| Cause | Route | FEMg expected |
|---|---|---|
| Proton pump inhibitor | Reduced gut absorption | Low |
| Diarrhoea, malabsorption, stoma, laxatives | Gut loss | Low |
| Loop or thiazide diuretic | Renal | High |
| Tacrolimus, ciclosporin | Renal | High |
| Cisplatin | Renal, can persist for years | High |
| Cetuximab, panitumumab | Renal (EGF-driven TRPM6 channel) | High |
| Gitelman syndrome | Renal, with low urine calcium | High |
| Alcohol | Intake, gut and kidney | Either |
Magnesium is the electrolyte that stops the others correcting
A low magnesium matters partly for itself — arrhythmias, cramps, tremor, seizures — but mostly because of what it does to potassium and calcium. Magnesium inside the cells of the distal nephron blocks the ROMK channel that secretes potassium into the urine. Take the magnesium away and the channel opens, so potassium given by mouth or by drip leaks straight back out. Magnesium is also needed for the parathyroid glands to release PTH and for bone and kidney to respond to it, so a low magnesium brings a low calcium with an inappropriately low PTH. In both cases the fix is the magnesium. A potassium or calcium that will not stay up is the commonest reason to check it.
Once the magnesium is known to be low, the next question is where it is going, and the fractional excretion of magnesium answers it. The formula carries a factor of 0.7 on the plasma magnesium, because only about 70% of plasma magnesium is free to be filtered; the FEMg calculator on this site includes it, and leaving it out makes every result about 30% too low. In a low-magnesium state the kidney should reabsorb almost everything filtered, so a FEMg below 2% says the kidney is doing its job and the loss is in the gut or the diet, and above 4% says the kidney is leaking. Between the two, the published cut-offs disagree.
The drug history then has to match the route. Proton pump inhibitors are the trap: they cause hypomagnesaemia by blocking absorption from the gut, so the kidney conserves and the FEMg is low. Diuretics, calcineurin inhibitors, cisplatin and EGFR antibodies act on the kidney and give a high FEMg. If the FEMg does not fit the drug, there is another cause. In a young adult with a high FEMg, a low potassium, a metabolic alkalosis and a low urine calcium, think of Gitelman syndrome.
Severity grades vary between sources; this page uses the grading in Salinas and colleagues’ 2024 review and shows the 2022 proposal to raise the lower limit to 0.85 mmol/L. Convert units with the magnesium unit converter. This page supports clinical judgement and does not replace it.
Frequently asked questions
Why won’t my patient’s potassium come up?
Check the magnesium. Low magnesium opens the ROMK potassium channel in the distal nephron, so replaced potassium is lost in the urine. Replace magnesium first or at the same time.
What FEMg means renal magnesium wasting?
In a patient with a low magnesium and normal kidney function, above 4% (Elisaf 1997; Viering 2017). Below 2% points to the gut or intake. Salinas’s 2024 review uses 3%.
Why does the FEMg formula multiply plasma magnesium by 0.7?
Only about 70% of plasma magnesium is free to be filtered; the rest is bound to protein. The 0.7 converts total plasma magnesium into the filtered amount.
Do proton pump inhibitors cause renal magnesium loss?
No. They reduce absorption of magnesium from the gut, so the kidney conserves and the FEMg is low. A high FEMg in a patient on a PPI means there is another, renal, cause.
What level of magnesium is severe?
Below 0.40 mmol/L on the grading used here (Salinas 2024), which calls 0.40 to 0.56 moderate and 0.57 to 0.70 mild.
Related calculators
References
- Viering DHHM, de Baaij JHF, Walsh SB, Kleta R, Bockenhauer D. Genetic causes of hypomagnesemia, a clinical overview. Pediatr Nephrol. 2017;32(7):1123–1135.
- Elisaf M, Panteli K, Theodorou J, Siamopoulos KC. Fractional excretion of magnesium in normal subjects and in patients with hypomagnesemia. Magnes Res. 1997;10(4):315–320.
- Salinas M, López-Garrigós M, Flores E, Leiva-Salinas C. Improving diagnosis and treatment of hypomagnesemia. Clin Chem Lab Med. 2024;62(2):234–248.
- Rosanoff A, West C, Elin RJ, et al. Recommendation on an updated standardization of serum magnesium reference ranges. Eur J Nutr. 2022;61(7):3697–3706.
- Huang CL, Kuo E. Mechanism of hypokalemia in magnesium deficiency. J Am Soc Nephrol. 2007;18(10):2649–2652.
- Rude RK, Oldham SB, Singer FR. Functional hypoparathyroidism and parathyroid hormone end-organ resistance in human magnesium deficiency. Clin Endocrinol (Oxf). 1976;5(3):209–224.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/