Definition of Hypomagnesemia
Hypomagnesemia is defined as a serum magnesium level less than 0.70 mmol/L (equivalent to 1.4 mEq/L or 1.7 mg/dL). 1, 2
Standard Diagnostic Threshold
- The most recent guideline consensus establishes serum magnesium <0.70 mmol/L as the diagnostic cutoff for hypomagnesemia 1, 2
- This threshold is consistently referenced across both European and American clinical nutrition societies 2
- The FDA drug label for magnesium sulfate notes that normal plasma magnesium levels range from 1.5 to 2.5 mEq/L, making values below this range abnormal 3
Severity Classification
Hypomagnesemia is commonly stratified into three severity categories based on serum concentration 1:
- Mild: 0.64-0.76 mmol/L (approximately 1.3-1.5 mg/dL)
- Moderate: 0.40-0.63 mmol/L (approximately 0.8-1.3 mg/dL)
- Severe: <0.40 mmol/L (<0.8 mg/dL)
Clinical Significance of Different Thresholds
- Symptomatic threshold: Most patients remain asymptomatic until serum magnesium falls below 1.2 mg/dL (0.50 mmol/L) 4
- Treatment threshold for parenteral therapy: Severe or symptomatic cases, particularly those <0.50 mmol/L, warrant intravenous magnesium replacement 2
- Acute hypomagnesemia with tetany: The FDA defines this as serum magnesium "usually below the lower limit of normal (1.5 to 2.5 mEq/L)" with accompanying tetanic signs 3
Important Clinical Context
- Early symptoms of hypomagnesemia may develop within 3-4 days to weeks of magnesium depletion 3
- Serum magnesium can appear normal despite significant intracellular magnesium depletion, so a low measured level typically indicates substantial total body deficiency 5
- In hospitalized patients, particularly those in intensive care units, the prevalence of hypomagnesemia can reach 60-65% 2
- Among patients undergoing prolonged kidney replacement therapy with intensive dialysis strategies, prevalence can rise to 80% 1
Common Pitfall
The most critical pitfall is assuming normal serum magnesium excludes magnesium deficiency—intracellular stores may be depleted while serum levels remain within normal range, requiring additional testing such as 24-hour urinary magnesium excretion or parenteral magnesium load testing for definitive diagnosis 5, 6