What is the appropriate workup for a patient with hypomagnesemia?

Workup for Hypomagnesemia

The appropriate workup for hypomagnesemia should begin with measuring fractional excretion of magnesium (FEMg) and urinary calcium-creatinine ratio to determine the underlying cause, followed by evaluation of associated electrolyte abnormalities. 1

Initial Assessment

Laboratory Evaluation

• Confirm hypomagnesemia with serum magnesium level

  • Mild: 1.2-1.7 mg/dL
  • Moderate: 0.8-1.2 mg/dL
  • Severe: <0.8 mg/dL 2

• Essential concurrent electrolyte testing:

  • Serum potassium (hypokalemia commonly coexists)
  • Serum calcium (hypocalcemia often present)
  • Serum parathyroid hormone (may be suppressed) 3
  • Renal function tests (BUN, creatinine)

Urinary Studies

• Calculate fractional excretion of magnesium (FEMg):

  • FEMg = [(UMg × SCr) / (0.7 × SMg × UCr)] × 100
  • FEMg <2%: suggests extrarenal loss
  • FEMg >2%: indicates renal magnesium wasting 1

• Measure urinary calcium-creatinine ratio:

  • Helps differentiate between causes of renal magnesium wasting
  • High ratio (hypercalciuria): suggests loop diuretic effect or familial renal magnesium wasting
  • Low ratio (hypocalciuria): suggests thiazide-like effect or Gitelman syndrome 1

Diagnostic Algorithm

1. Step 1: Determine if renal or extrarenal cause

   • FEMg <2% → Extrarenal causes
   • FEMg >2% → Renal causes

2. Step 2: For extrarenal causes, evaluate:

   • Dietary intake (malnutrition, alcoholism)
   • Gastrointestinal losses (diarrhea, malabsorption, vomiting)
   • Medication history (PPIs, antibiotics)
   • Redistribution (hungry bone syndrome, refeeding)

3. Step 3: For renal causes, evaluate:

   • Medication review (diuretics, cisplatin, cetuximab, aminoglycosides)
   • Urinary calcium excretion pattern
   • Acid-base status (metabolic alkalosis often present)
   • Volume status

Specific Scenarios to Consider

Cancer Patients

• Review chemotherapy history, particularly:
  • Cisplatin therapy
  • Cetuximab treatment 4
  • Aminoglycoside antibiotics 5

Critical Illness

• More aggressive workup in ICU patients as hypomagnesemia is present in up to 65% of severely ill patients 6
• Evaluate for concurrent conditions that may worsen outcomes

Symptomatic Patients

• For patients with neuromuscular symptoms (tetany, seizures, arrhythmias):
  • Expedite workup and treatment
  • Check for Chvostek and Trousseau signs 6

Common Pitfalls to Avoid

1. Failing to check magnesium levels in patients with unexplained hypokalemia or hypocalcemia 3

2. Overlooking concurrent electrolyte abnormalities - hypomagnesemia often causes secondary hypokalemia and hypocalcemia that won't correct until magnesium is replaced 3

3. Treating calcium deficiency before magnesium - hypocalcemia due to hypomagnesemia won't respond to calcium supplementation alone 3

4. Missing medication-induced causes - particularly in cancer patients on chemotherapy 4

5. Ignoring renal function - impaired renal function affects both diagnosis and treatment approach 1

By following this systematic approach to hypomagnesemia workup, clinicians can identify the underlying cause and implement appropriate treatment strategies to prevent potentially serious complications including ventricular arrhythmias and seizures.