What is the workup for hypocalcemia?

Workup for Hypocalcemia

The diagnostic workup for hypocalcemia should include measurement of serum calcium with albumin correction or ionized calcium, parathyroid hormone (PTH) levels, magnesium, phosphorus, vitamin D levels, and assessment of renal function to determine the underlying cause. 1

Initial Laboratory Assessment

• Calcium measurement:

  • Total serum calcium with albumin correction
  • Ionized calcium (more accurate, especially in critically ill patients)
  • Levels below 0.9 mmol/L require prompt correction; levels below 0.8 mmol/L can cause cardiac dysrhythmias 1

• Essential accompanying tests:

  • Parathyroid hormone (PTH) levels
  • Magnesium levels
  • Phosphorus levels
  • 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D levels
  • Renal function tests (BUN, creatinine)
  • Albumin levels (for interpretation of total calcium)

Diagnostic Algorithm

1. Confirm true hypocalcemia:

   • Correct total calcium for albumin or measure ionized calcium
   • Formula: Corrected calcium = measured calcium + 0.8 × (4.0 - albumin)

2. Assess PTH levels:

   • High PTH → Secondary hyperparathyroidism

     • Check vitamin D levels
     • Assess renal function
     • Evaluate for calcium malabsorption

   • Low/inappropriately normal PTH → Hypoparathyroidism

     • Review surgical history (neck surgery)
     • Consider autoimmune disorders
     • Evaluate for infiltrative diseases
     • Consider genetic causes

3. Check magnesium levels:

   • Hypomagnesemia can cause functional hypoparathyroidism
   • Must be corrected before calcium levels will normalize

4. Evaluate phosphate levels:

   • High phosphate + low calcium suggests renal failure or hypoparathyroidism
   • Low phosphate + low calcium suggests vitamin D deficiency or malabsorption

Common Etiologies to Consider

• Hypoparathyroidism: Post-surgical, autoimmune, genetic
• Vitamin D disorders: Deficiency, impaired activation in kidney disease
• Renal failure: Decreased vitamin D activation, phosphate retention
• Medication-induced: Bisphosphonates, anticonvulsants, proton pump inhibitors
• Critical illness: Pancreatitis, sepsis, massive transfusion
• Malabsorption: Celiac disease, inflammatory bowel disease, bariatric surgery

Special Considerations

• In CKD patients: Regular monitoring of calcium, phosphorus, vitamin D, and bone mineral status is recommended 2, 1

• Post-parathyroidectomy: High risk for hungry bone syndrome; preoperative and postoperative use of active vitamin D derivatives may reduce the incidence of severe hypocalcemia 2

• Medication review: Evaluate for drugs that can cause hypocalcemia (bisphosphonates, cisplatin, antiepileptics, aminoglycosides, proton pump inhibitors) 3

• Transfusion-related hypocalcemia: Consider in patients receiving massive blood transfusions 1

Management of Acute Symptomatic Hypocalcemia

For patients with severe symptomatic hypocalcemia identified during workup:

• Immediate treatment: 1-2 g IV calcium gluconate (100-200 mg/kg for pediatric patients), administered slowly with ECG monitoring 1, 4

• For transfusion-related hypocalcemia: Calcium chloride is preferred over calcium gluconate due to higher elemental calcium content 1, 5

• Monitoring: Measure serum calcium during intermittent infusions every 4-6 hours and during continuous infusion every 1-4 hours 1, 4

Pitfalls to Avoid

• Failure to correct calcium for albumin levels can lead to misdiagnosis
• Overlooking hypomagnesemia, which must be corrected for calcium levels to normalize
• Not considering medication-induced causes of hypocalcemia
• Overcorrection of hypocalcemia, which can lead to hypercalcemia, renal calculi, and renal failure 1
• Administering calcium without monitoring in patients on cardiac glycosides (risk of arrhythmias) 4

By systematically following this diagnostic approach, the underlying cause of hypocalcemia can be identified and appropriate treatment initiated promptly.