Test Description

What is Calcium?

Calcium is the most abundant mineral in the human body, with 99% stored in bones and teeth and only 1% in the bloodstream and soft tissues. Despite the small circulating amount, serum calcium is critical for numerous physiological functions and is tightly regulated by parathyroid hormone (PTH), vitamin D, and calcitonin.

How is Calcium Distributed?

Serum calcium exists in three forms:

  • Ionized calcium (50%): The physiologically active form, not bound to proteins. This is what actually matters for cellular function.
  • Protein-bound calcium (40%): Primarily bound to albumin. Inactive, serves as a reservoir.
  • Complexed calcium (10%): Bound to anions like citrate, phosphate, and bicarbonate.

Clinical Use

Serum calcium is measured to:

  • Evaluate parathyroid function: Screen for hyper- or hypoparathyroidism
  • Assess bone metabolism: Osteoporosis, bone metastases, Paget's disease
  • Investigate symptoms: Kidney stones, muscle cramps, tetany, confusion, arrhythmias
  • Monitor chronic kidney disease: CKD commonly causes hypocalcemia
  • Screen for malignancy: Hypercalcemia of malignancy is common in cancer patients
  • Monitor treatment: Track response to vitamin D, bisphosphonates, or calcium supplementation
Total vs. Ionized Calcium: Most labs measure total calcium (all three forms combined). However, only ionized calcium is physiologically active. When albumin is abnormal (hypoalbuminemia or hyperalbuminemia), total calcium is misleading. In these cases, either measure ionized calcium directly or calculate corrected calcium.
Quick Reference
  • Normal Range (Total Calcium): 8.5-10.5 mg/dL
  • Normal Range (Ionized Calcium): 4.5-5.3 mg/dL (1.12-1.32 mmol/L)
  • Critical Low: <7.0 mg/dL
  • Critical High: >12.0 mg/dL
  • Primary Use: Assessment of parathyroid function, bone metabolism, and neuromuscular disorders
  • Sample Type: Serum (fasting preferred)
  • Key Point: Always interpret total calcium in context of albumin level; consider ionized calcium when albumin is abnormal
Normal Ranges

Calcium reference ranges vary slightly by laboratory and measurement method. Age and albumin levels also affect interpretation.

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Measurement Type Normal Range (mg/dL) SI Units (mmol/L)
Total Calcium (Adults) 8.5-10.5 mg/dL 2.12-2.62 mmol/L
Ionized Calcium 4.5-5.3 mg/dL 1.12-1.32 mmol/L
Critical Low <7.0 mg/dL <1.75 mmol/L
Critical High >12.0 mg/dL >3.0 mmol/L

Corrected Calcium Formula

When albumin is abnormal, use the corrected calcium formula to estimate the true calcium level:

Corrected Calcium Formula
Corrected Calcium (mg/dL) = Measured Total Calcium + 0.8 × (4.0 - Serum Albumin)

Where albumin is in g/dL. Normal albumin is approximately 4.0 g/dL.

Example: Patient has calcium 7.8 mg/dL and albumin 2.5 g/dL
Corrected Calcium = 7.8 + 0.8 × (4.0 - 2.5) = 7.8 + 1.2 = 9.0 mg/dL (normal!)
Important Considerations:
  • Hypoalbuminemia is common: Malnutrition, liver disease, nephrotic syndrome, critical illness all lower albumin. This falsely lowers total calcium, but ionized calcium (the active form) may be normal.
  • Corrected calcium is an estimate: For critically ill patients or when precision matters, directly measure ionized calcium instead of relying on the correction formula.
  • pH affects ionized calcium: Alkalosis increases protein binding (lowers ionized Ca), acidosis decreases binding (raises ionized Ca). Corrected calcium doesn't account for pH.
Clinical Significance

Hypercalcemia (Calcium >10.5 mg/dL)

Hypercalcemia is common and usually discovered incidentally. The two most common causes account for 90% of cases: primary hyperparathyroidism (outpatient) and malignancy (inpatient).

Mnemonic for Hypercalcemia Causes: "CHIMPANZEES"

  • C – Calcium supplementation (excessive intake)
  • H – Hyperparathyroidism (primary or tertiary)
  • I – Iatrogenic (thiazide diuretics, lithium, vitamin A/D toxicity)
  • M – Malignancy (most common inpatient cause)
  • P – Paget's disease of bone (with immobilization)
  • A – Addison's disease (adrenal insufficiency)
  • N – Neoplasm (see malignancy)
  • Z – Zollinger-Ellison syndrome (MEN syndromes)
  • E – Excess vitamin D
  • E – Excess vitamin A
  • S – Sarcoidosis (and other granulomatous diseases)

1. Primary Hyperparathyroidism

  • Mechanism: Parathyroid adenoma or hyperplasia secretes excessive PTH
  • Labs: Elevated calcium with elevated or "inappropriately normal" PTH
  • Presentation: Often asymptomatic; may have kidney stones, osteoporosis, fatigue
  • Treatment: Parathyroidectomy for symptomatic disease

2. Malignancy (Hypercalcemia of Malignancy)

  • PTHrP-mediated (most common): Squamous cell cancers (lung, head/neck), renal cell carcinoma, breast cancer secrete PTH-related peptide (PTHrP)
  • Osteolytic metastases: Breast cancer, multiple myeloma, lymphoma directly destroy bone, releasing calcium
  • Calcitriol production: Lymphomas (Hodgkin's, non-Hodgkin's) produce active vitamin D
  • Labs: Elevated calcium with suppressed PTH (key difference from hyperparathyroidism)
  • Presentation: Usually severe (>12 mg/dL), rapid onset, symptomatic

3. Other Causes

  • Granulomatous diseases: Sarcoidosis, tuberculosis, berylliosis (macrophages produce calcitriol)
  • Vitamin D intoxication: Excessive supplementation (>10,000 IU/day for prolonged periods)
  • Thiazide diuretics: Decrease renal calcium excretion
  • Milk-alkali syndrome: Excessive calcium and alkali intake (calcium carbonate antacids)
  • Immobilization: Increased bone resorption (especially with Paget's disease or adolescents)
  • Familial hypocalciuric hypercalcemia (FHH): Benign genetic condition; calcium is mildly elevated but asymptomatic

Symptoms of Hypercalcemia

Mnemonic: "Stones, Bones, Groans, Psychiatric Overtones"

  • Stones: Kidney stones (nephrolithiasis), nephrocalcinosis
  • Bones: Bone pain, osteoporosis, pathologic fractures
  • Groans: Abdominal pain, nausea, vomiting, constipation, peptic ulcers, pancreatitis
  • Psychiatric Overtones: Confusion, depression, fatigue, memory impairment, psychosis

Additional symptoms:

  • Polyuria and polydipsia: Nephrogenic diabetes insipidus (calcium impairs ADH action)
  • Cardiac: Shortened QT interval, arrhythmias, hypertension
  • Severe (>14 mg/dL): Altered mental status, coma, cardiac arrest

Hypocalcemia (Calcium <8.5 mg/dL)

Hypocalcemia increases neuromuscular excitability, leading to tetany, seizures, and cardiac arrhythmias. It is less common than hypercalcemia but can be life-threatening.

Causes of Hypocalcemia

1. Hypoparathyroidism (Low PTH)

  • Post-surgical: Most common cause. Parathyroid glands removed or damaged during thyroid/neck surgery
  • Autoimmune: Isolated or as part of polyglandular autoimmune syndrome
  • Genetic: DiGeorge syndrome (22q11 deletion), familial hypoparathyroidism
  • Infiltrative: Hemochromatosis, Wilson's disease, metastases
  • Radiation-induced: Neck radiation therapy

2. Vitamin D Deficiency

  • Nutritional deficiency: Inadequate sunlight exposure, poor dietary intake
  • Malabsorption: Celiac disease, Crohn's disease, post-gastric bypass
  • Liver disease: Impaired 25-hydroxylation of vitamin D
  • Chronic kidney disease: Impaired 1-alpha-hydroxylation (can't convert to active form)

3. Chronic Kidney Disease

  • Mechanism: Decreased calcitriol production + phosphate retention (binds calcium) + skeletal resistance to PTH
  • Result: Low calcium, high phosphate, high PTH (secondary hyperparathyroidism)

4. Hypomagnesemia

  • Mechanism: Magnesium is required for PTH secretion and action. Severe hypomagnesemia (<1.0 mg/dL) causes functional hypoparathyroidism
  • Result: Hypocalcemia refractory to calcium replacement until magnesium is corrected

5. Acute Pancreatitis

  • Mechanism: Calcium precipitates as "soap" with fatty acids in pancreatic necrosis
  • Severity marker: Hypocalcemia in pancreatitis indicates severe disease (Ranson's criteria)

6. Other Causes

  • Hyperphosphatemia: Tumor lysis syndrome, rhabdomyolysis, chronic kidney disease (phosphate binds calcium)
  • Medications: Bisphosphonates, denosumab, calcitonin, cinacalcet, foscarnet, loop diuretics
  • Massive blood transfusion: Citrate in stored blood binds calcium
  • Pseudohypoparathyroidism: End-organ resistance to PTH (PTH is elevated but ineffective)
  • Hungry bone syndrome: Rapid bone remineralization after parathyroidectomy

Symptoms of Hypocalcemia

Symptoms depend on the severity and rate of decline. Acute hypocalcemia is more symptomatic than chronic.

  • Neuromuscular irritability: Paresthesias (perioral, fingertips), muscle cramps, tetany, carpopedal spasm
  • Chvostek sign: Tapping facial nerve anterior to ear causes facial twitching (not specific, present in 10% of normal people)
  • Trousseau sign: Carpopedal spasm induced by inflating BP cuff above systolic pressure for 3 minutes (more specific than Chvostek)
  • Seizures: Generalized tonic-clonic seizures (more common in children)
  • Cardiac: Prolonged QT interval, torsades de pointes, heart failure (chronic hypocalcemia)
  • CNS: Confusion, irritability, depression, psychosis, extrapyramidal symptoms, papilledema
  • Laryngospasm/bronchospasm: Life-threatening in severe cases
Interpretation Guidelines

Diagnostic Algorithm for Hypercalcemia

Step 1: Confirm true hypercalcemia

  • Check albumin and calculate corrected calcium OR measure ionized calcium directly
  • Repeat measurement to confirm (transient elevations occur with dehydration, tourniquets)

Step 2: Measure PTH

  • PTH elevated or inappropriately normal (>20 pg/mL): PTH-mediated hypercalcemia
    • Primary hyperparathyroidism (most common)
    • Tertiary hyperparathyroidism (CKD)
    • Familial hypocalciuric hypercalcemia (FHH)
    • Lithium use
  • PTH suppressed (<20 pg/mL): Non-PTH-mediated hypercalcemia
    • Malignancy (check PTHrP, consider imaging)
    • Vitamin D intoxication (check 25-OH vitamin D)
    • Granulomatous disease (check 1,25-OH vitamin D, ACE level)
    • Thiazide diuretics, milk-alkali syndrome

Diagnostic Algorithm for Hypocalcemia

Step 1: Confirm true hypocalcemia

  • Check albumin and calculate corrected calcium OR measure ionized calcium
  • Rule out pseudohypocalcemia (hypoalbuminemia with normal ionized calcium)

Step 2: Check PTH, magnesium, phosphate, vitamin D, and renal function

  • Low PTH: Hypoparathyroidism (post-surgical, autoimmune, genetic)
  • High PTH + high phosphate + low vitamin D: Vitamin D deficiency or chronic kidney disease
  • High PTH + low phosphate: Vitamin D deficiency (PTH is appropriately elevated)
  • Low magnesium (<1.0 mg/dL): Functional hypoparathyroidism (correct Mg first)

ECG Changes

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Condition ECG Findings
Hypercalcemia • Shortened QT interval
• Osborn waves (rare)
• Bradycardia
• First-degree AV block
• Arrhythmias at very high levels
Hypocalcemia • Prolonged QT interval (corrected QTc)
• Risk of torsades de pointes
• T wave flattening or inversion

Treatment of Hypercalcemia

Severe Hypercalcemia (>14 mg/dL or symptomatic):
  1. IV hydration: 0.9% normal saline 200-300 mL/hour (first-line). Expands volume and promotes renal calcium excretion.
  2. Calcitonin: 4 IU/kg SC/IM q12h. Rapid onset (4-6 hours) but tachyphylaxis develops after 48 hours.
  3. Bisphosphonates: Zoledronic acid 4 mg IV over 15 min (or pamidronate 60-90 mg IV over 2-4 hours). Onset 2-4 days, duration 2-4 weeks. Most effective long-term therapy.
  4. Denosumab: 120 mg SC for bisphosphonate-refractory hypercalcemia of malignancy.
  5. Glucocorticoids: Prednisone 40-60 mg/day PO for vitamin D intoxication, granulomatous disease, or hematologic malignancies (lymphoma, myeloma).
  6. Hemodialysis: Reserved for severe, refractory cases or acute kidney injury.

Treatment of Hypocalcemia

Severe Hypocalcemia (<7.0 mg/dL or symptomatic with tetany, seizures, QT prolongation):
  1. IV calcium gluconate 10%: 1-2 grams (10-20 mL) IV over 10-20 minutes, then continuous infusion 0.5-1.5 mg/kg/hour
  2. Continuous cardiac monitoring: Watch for QT interval normalization
  3. Check and correct magnesium: If Mg <1.0 mg/dL, give magnesium sulfate 2g IV over 15 min, then 1-2 g/hour infusion
  4. Avoid bicarbonate: Alkalosis worsens ionized hypocalcemia
Chronic hypocalcemia:
  • Oral calcium: Calcium carbonate 1-2g elemental calcium daily in divided doses (with meals for absorption)
  • Vitamin D: Calcitriol 0.25-2 mcg/day (active form) or ergocalciferol 50,000 IU weekly (if vitamin D deficient)
  • Correct hypomagnesemia: Magnesium oxide 400-800 mg/day
Interfering Factors

Factors That Increase Calcium

  • Medications: Thiazide diuretics, lithium, vitamin D supplements, vitamin A, calcium supplements, antacids (milk-alkali syndrome)
  • Prolonged tourniquet time: Hemoconcentration can falsely elevate total calcium
  • Dehydration: Increases protein-bound calcium (and total calcium)
  • Hyperalbuminemia: Rare, but increases protein-bound calcium

Factors That Decrease Calcium

  • Hypoalbuminemia: Most common cause of falsely low total calcium. Liver disease, malnutrition, nephrotic syndrome, critical illness. Ionized calcium is normal.
  • Medications: Loop diuretics (furosemide), bisphosphonates, denosumab, calcitonin, cinacalcet, foscarnet, phenytoin, phenobarbital
  • EDTA contamination: Purple-top tubes contain EDTA (calcium chelator). Using wrong tube will cause falsely low calcium.
  • Citrate anticoagulation: Massive transfusion or plasmapheresis (citrate binds calcium)
  • Alkalosis: Increases protein binding, decreases ionized calcium (total calcium may be normal)

Pseudohypocalcemia

Low total calcium with normal ionized calcium. Most commonly due to hypoalbuminemia. Patient is asymptomatic and does not require treatment. Always correct calcium for albumin or measure ionized calcium directly.

Clinical Pearls
"Calcium rules the heart": Both hypercalcemia and hypocalcemia cause serious cardiac arrhythmias. Shortened QT (high calcium) and prolonged QT (low calcium) are key ECG findings that can lead to torsades de pointes and sudden cardiac death.
Always correct calcium for albumin: Hypoalbuminemia is extremely common (malnutrition, liver disease, critical illness). Use the formula: Corrected Ca = Measured Ca + 0.8 × (4.0 - Albumin). Better yet, directly measure ionized calcium if the patient is critically ill.
PTH is the key to diagnosing hypercalcemia: Elevated/normal PTH = primary hyperparathyroidism. Suppressed PTH = malignancy or vitamin D excess. PTH should be suppressed when calcium is high; if it's not, suspect hyperparathyroidism.
"Stones, Bones, Groans, Psychiatric Overtones": Classic mnemonic for hypercalcemia symptoms. Kidney stones, bone pain, abdominal groans (constipation, nausea, pancreatitis), and psychiatric symptoms (confusion, depression).
Check magnesium in every hypocalcemia patient: Severe hypomagnesemia (<1.0 mg/dL) causes functional hypoparathyroidism. You won't be able to correct calcium until you replace magnesium first. "No mag, no calc."
Trousseau sign is more specific than Chvostek: Chvostek (facial twitch when tapping facial nerve) is present in 10% of normal people. Trousseau (carpopedal spasm with BP cuff inflation) is much more specific for hypocalcemia.
Hypercalcemia in a hospitalized patient = think malignancy first: While primary hyperparathyroidism is the most common outpatient cause, hypercalcemia of malignancy is the most common cause in hospitalized patients. Check for known cancer history and measure PTH (should be suppressed).
Calcium and phosphate have an inverse relationship: High calcium → low phosphate (hyperparathyroidism). Low calcium → high phosphate (hypoparathyroidism, CKD). The product of Ca × PO4 is tightly regulated; if it exceeds 55-60, calcium-phosphate crystals can deposit in tissues (metastatic calcification).
Hungry bone syndrome after parathyroidectomy: Removing a parathyroid adenoma causes rapid bone remineralization, leading to severe hypocalcemia and hypophosphatemia. Requires aggressive calcium and vitamin D supplementation for weeks to months.
IV calcium: gluconate vs. chloride: Calcium gluconate is preferred for peripheral IV (less tissue necrosis if extravasated). Calcium chloride has 3× more elemental calcium per mL but is more caustic and should only be given via central line in emergencies.
Don't give IV calcium through the same line as bicarbonate: Calcium will precipitate when mixed with bicarbonate. Use separate IV access.
Hypercalcemia causes nephrogenic diabetes insipidus: High calcium impairs the kidney's response to ADH, leading to polyuria and polydipsia. This further concentrates calcium (dehydration worsens hypercalcemia). Aggressive IV hydration is first-line treatment.
References
  1. Kratz, A., Ferraro, M., Sluss, P. M., & Lewandrowski, K. B. (2004). Laboratory reference values. New England Journal of Medicine, 351, 1548-1564.
  2. Lee, M. (Ed.). (2009). Basic skills in interpreting laboratory data. Ashp.
  3. Farinde, A. (2021). Lab values, normal adult: Laboratory reference ranges in healthy adults. Medscape. https://emedicine.medscape.com/article/2172316-overview?form=fpf
  4. Nickson, C. (n.d.). Critical Care Compendium. Life in the Fast Lane • LITFL. https://litfl.com/ccc-critical-care-compendium/
  5. Farkas, Josh MD. (2015). Table of Contents - EMCrit Project. EMCrit Project. https://emcrit.org/ibcc/toc/
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