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Hypocalcemia is defined as low serum calcium levels (total Ca²⁺ < 8.5 mg/dL or ionized Ca²⁺ < 4.65 mg/dL). Calcium homeostasis is primarily regulated by parathyroid hormone (PTH), vitamin D, and the calcium-sensing receptor. The condition can result from various disorders affecting these regulatory mechanisms or other pathological processes.

Last updated: December 3, 2024 737 views

Types of Calcium Measurement

  1. Total Calcium (Normal: 8.5-10.5 mg/dL)
    • Includes both bound and free calcium
    • Affected by albumin levels
    • Most commonly measured
  2. Ionized Calcium (Normal: 4.65-5.25 mg/dL)
    • Physiologically active form
    • Not affected by albumin
    • More accurate but less available

 

When to Correct for Albumin

  • Important: Always correct total calcium when albumin is abnormal
  • Why: 40% of calcium is bound to albumin
  • When: Any condition affecting albumin (liver disease, malnutrition, nephrotic syndrome)

 

Calcium Correction Formula and Examples

  • Corrected Ca = Measured Ca + 0.8 × (4.0 - albumin)
  • Example 1: Low Albumin
    • Measured Ca: 7.8 mg/dL
    • Albumin: 2.5 g/dL
    • Calculation: 7.8 + 0.8 × (4.0 - 2.5) = 9.0 mg/dL
    • Interpretation: Normal calcium (Pseudohypocalcemia)
  • Example 2: High Albumin
    • Measured Ca: 9.5 mg/dL
    • Albumin: 5.0 g/dL
    • Calculation: 9.5 + 0.8 × (4.0 - 5.0) = 8.7 mg/dL
    • Interpretation: Lower than measured

Understanding PTH's Role

PTH is the main regulator of calcium homeostasis. Causes of hypocalcemia can be categorized based on PTH levels:

PTH Status Causes Mechanism Key Features
Low PTH
  • Post-surgical
  • Autoimmune
  • DiGeorge syndrome
  • Insufficient PTH production
  • High phosphate
  • Low calcium
  • Low 1,25-vitamin D
High PTH
  • Vitamin D deficiency
  • Chronic kidney disease
  • PTH resistance
  • PTH unable to maintain calcium
  • Variable phosphate
  • Low calcium
  • Often chronic
Normal/Low PTH
  • Hypomagnesemia
  • Acute illness
  • Impaired PTH action
  • Often acute
  • May be severe

 

Medication-Induced Hypocalcemia

Important medications to remember:

  • Bisphosphonates: Inhibit bone resorption
  • Loop diuretics: Increase calcium excretion
  • Denosumab: Blocks bone resorption
  • Anticonvulsants: Affect vitamin D metabolism
  • Multiple transfusions: Citrate binding

Severity Classification
Severity Calcium Level Typical Symptoms
Mild 7.5-8.4 mg/dL Often asymptomatic
Moderate 7.0-7.4 mg/dL Perioral numbness, tingling
Severe < 7.0 mg/dL Tetany, seizures, cardiac issues

 

Key Clinical Signs
  1. Neuromuscular Signs
    • Chvostek's Sign
      • How to test: Tap facial nerve anterior to ear
      • Positive: Facial muscle twitching
      • Significance: 15% false positive in normal people
    • Trousseau's Sign
      • How to test: Inflate BP cuff above systolic for 3 minutes
      • Positive: Carpopedal spasm
      • More specific than Chvostek's
  2. Cardiac Manifestations
    • QT prolongation (calculate corrected QT)
    • T wave changes
    • Heart blocks
    • Monitor closely if QTc > 500ms

Step-by-Step Evaluation

  1. Confirm True Hypocalcemia
    • Check total calcium and albumin
    • Calculate corrected calcium
    • Consider ionized calcium if available
  2. Assess Severity
    • Review symptoms
    • Check ECG
    • Evaluate for tetany
  3. Determine Cause
    • Check PTH level
    • Basic metabolic panel
    • Magnesium level
    • Vitamin D if appropriate

Required Lab Tests

 

Test Rationale Key Findings
PTH Primary diagnostic test Guides cause
Magnesium Required for PTH function Often low
Phosphate Inversely related to calcium Usually high in hypoparathyroidism
Vitamin D Common deficiency May be low
Creatinine Assess kidney function Affects treatment

 

Emergency Management (Ca < 7.0 mg/dL or Symptomatic)

  • Immediate Actions
    • IV calcium gluconate 1-2g
    • Cardiac monitoring
    • Repeat calcium every 1-2 hours
    • Consider ICU admission
  • Concurrent Steps
    • Check magnesium
    • Start vitamin D if needed
    • Identify and treat cause

 

Non-Emergency Management

  • Oral Supplementation
    • Calcium carbonate 1-2g TID
    • Calcium citrate if on PPI
    • Vitamin D supplementation
  • Monitoring
    • Check calcium weekly until stable
    • Monitor symptoms
    • Adjust dose based on levels

 

Special Situations

  • Hypomagnesemia
    • Correct magnesium first
    • Then address calcium
  • Chronic Kidney Disease
    • Use activated vitamin D
    • Monitor phosphate
    • Consider calcimimetics