What is the workup for hypocalcemia (low calcium levels)?

Workup for Hypocalcemia

The essential workup for hypocalcemia should include measurement of ionized calcium, albumin-corrected total calcium, parathyroid hormone (PTH), magnesium, phosphorus, 25-hydroxyvitamin D, and renal function tests to determine the underlying cause and guide appropriate treatment. 1

Initial Laboratory Assessment

• Ionized calcium: More accurate than total calcium for diagnosis (normal range: 1.1-1.3 mmol/L)
• Albumin-corrected total calcium: Useful when ionized calcium is unavailable
• Parathyroid hormone (PTH): Critical for determining if hypocalcemia is PTH-mediated
  • High PTH: Secondary hyperparathyroidism (vitamin D deficiency, renal disease)
  • Low/inappropriately normal PTH: Hypoparathyroidism
• Magnesium: Essential as hypomagnesemia can cause functional hypoparathyroidism
• Phosphorus: Often elevated in hypoparathyroidism and renal failure
• 25-hydroxyvitamin D: To identify vitamin D deficiency
• Renal function tests: To assess for chronic kidney disease

Diagnostic Algorithm

1. Confirm hypocalcemia:

   • Ionized calcium < 1.1 mmol/L or
   • Corrected total calcium < 8.5 mg/dL

2. Assess PTH levels:

   • Low/normal PTH with hypocalcemia:

     • Evaluate for hypoparathyroidism (post-surgical most common)
     • Check magnesium (hypomagnesemia can suppress PTH)

   • Elevated PTH with hypocalcemia:

     • Check vitamin D levels (deficiency common cause)
     • Assess renal function (CKD can cause secondary hyperparathyroidism)
     • Consider malabsorption (check for celiac disease, other GI disorders)

3. Additional tests based on clinical suspicion:

   • Urinary calcium excretion (low in vitamin D deficiency, high in some forms of hypoparathyroidism)
   • Alkaline phosphatase (elevated in vitamin D deficiency with secondary hyperparathyroidism)
   • Specific genetic testing if familial hypoparathyroidism is suspected

Special Considerations

• Acute vs. Chronic Presentation: Acute symptomatic hypocalcemia requires immediate IV calcium replacement 2, 3
• Monitoring Frequency:
  • During acute management: Monitor ionized calcium every 1-4 hours during continuous infusion 1
  • For chronic management: Monitor calcium every 2-4 weeks initially, then every 3-6 months once stable 1

Common Pitfalls to Avoid

1. Relying solely on total calcium: Total calcium can be misleading in patients with abnormal albumin levels; always check ionized calcium or calculate corrected calcium 4

2. Overlooking magnesium deficiency: Hypomagnesemia is a common cause of hypocalcemia that is resistant to calcium replacement; always check magnesium levels 1, 5

3. Missing vitamin D deficiency: A frequent cause of hypocalcemia, especially in elderly, institutionalized patients, and those with limited sun exposure 6, 5

4. Inadequate follow-up: Calcium levels should be monitored regularly during treatment to avoid complications of both under- and over-replacement 1

5. Failure to identify the underlying cause: Treating hypocalcemia without addressing the root cause leads to suboptimal outcomes 5

By following this systematic approach to hypocalcemia workup, clinicians can efficiently identify the underlying cause and implement appropriate treatment strategies to normalize calcium levels and prevent complications.