Unit Converter
Calcium (Ca)
(Total Serum Calcium – Major Mineral for Bone, Nerve, and Muscle Function)
Synonyms
- Serum calcium
- Total calcium
- Ca²⁺
- Plasma calcium
- Bound + ionized calcium
Units of Measurement
- mmol/L
- µmol/L
- mg/dL
- mg/100 mL
- mg%
- mg/L
- µg/mL
- mEq/L
1 mmol/L Ca = 2 mEq/L (valency = 2)
Description
Calcium is the most abundant mineral in the body, essential for:
- Bone formation
- Muscle contraction
- Nerve conduction
- Blood clotting (factor activation)
- Enzyme regulation
- Hormonal secretion
Serum calcium is present in three forms:
- Free/Ionized Ca²⁺ (~50%) – biologically active
- Protein-bound (mainly albumin) (~40%)
- Complexed (~10%) – with phosphate, citrate, bicarbonate
Clinical calcium measurement typically refers to total calcium, though ionized calcium is more accurate in critical illness.
Physiological Regulation
Regulated by:
- PTH (Parathyroid Hormone)
- Vitamin D (1,25-dihydroxy)
- Calcitonin (minor role)
Main target organs:
- Bone (release calcium)
- Kidney (reabsorption)
- Gut (absorption)
Clinical Significance
Hypercalcemia (High Calcium)
1. Primary Hyperparathyroidism (Most common)
- Elevated Ca + elevated/inappropriately normal PTH
2. Malignancy (Common in hospitalized patients)
- PTHrP-producing tumors
- Osteolytic metastases
3. Vitamin D Excess
- Supplements
- Granulomatous diseases (TB, sarcoidosis)
4. Medications
- Thiazides
- Lithium
- Excess calcium carbonate (milk-alkali syndrome)
5. Endocrine Disorders
- Hyperthyroidism
- Adrenal insufficiency
Symptoms:
“Stones, bones, groans, moans, and psychiatric overtones.”
Hypocalcemia (Low Calcium)
1. Hypoparathyroidism
- Postsurgical
- Autoimmune
- DiGeorge syndrome
2. Vitamin D Deficiency
- Nutritional
- CKD
- Liver disease
3. Hypoalbuminemia
- Total Ca ↓ but Ionized Ca normal
4. Acute Pancreatitis
Calcium deposition in fat.
5. Renal Failure
Hyperphosphatemia → binds calcium.
6. Drugs
- Bisphosphonates
- Calcitonin
- Chemotherapy agents
- Loop diuretics
Symptoms:
Tetany, perioral numbness, seizures, QT prolongation.
Reference Intervals
(Tietz 8E + Endocrine Society + Mayo + ARUP)
Total Serum Calcium
- 2.15 – 2.55 mmol/L
- 8.6 – 10.2 mg/dL
Corrected Calcium (for albumin)
Corrected Ca (mg/dL)=Measured Ca+0.8(4−albumin)\text{Corrected Ca (mg/dL)} = \text{Measured Ca} + 0.8(4 - \text{albumin})Corrected Ca (mg/dL)=Measured Ca+0.8(4−albumin)
Ionized Calcium
- 1.10 – 1.30 mmol/L
Critical Values
- < 6.5 mg/dL (1.6 mmol/L) → severe hypocalcemia
- > 13 mg/dL (3.25 mmol/L) → severe hypercalcemia
Unit Meanings
| Unit | Description |
| mmol/L | millimole per liter |
| µmol/L | micromole per liter |
| mg/dL | milligram per deciliter |
| mg% | mg per 100 mL (same as mg/dL) |
| mg/L | milligram per liter |
| µg/mL | microgram per milliliter |
| mEq/L | milliequivalent per liter |
Diagnostic Uses
1. Parathyroid Disorders
- Hyperparathyroidism
- Hypoparathyroidism
2. Bone Health
- Osteoporosis
- Osteomalacia
- CKD mineral-bone disorders
3. Malignancy
- PTHrP-mediated hypercalcemia
- Bone metastases
4. Critical Care
- Pancreatitis
- Sepsis
- Massive transfusions (citrate binding calcium)
5. Endocrine & Vitamin D Evaluation
- Thyroid dysfunction
- Adrenal insufficiency
- Vitamin D deficiency/excess
Analytical Notes
- Serum preferred; avoid EDTA or citrate tubes (bind calcium).
- Ionized calcium requires anaerobic collection and immediate analysis.
- Hemolysis minimal effect.
- Albumin-adjusted calcium needed when albumin is low.
Clinical Pearls
- Total calcium can be misleading in hypoalbuminemia - always correct or measure ionized Ca.
- Hypercalcemia + high PTH → primary hyperparathyroidism.
- Hypercalcemia + low PTH → malignancy until proven otherwise.
- Hypocalcemia with tetany → check Mg (often low and needs correction).
- Vitamin D deficiency is a common cause of low calcium worldwide.
Interesting Fact
Calcium constitutes 99% of the body’s mineral content, primarily in bones and teeth, but the tiny 1% in serum is what maintains nerve and cardiac function.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Minerals & Electrolytes.
- Endocrine Society - Hypercalcemia & Hypocalcemia Guidelines.
- KDIGO - CKD-Mineral Bone Disorder Guidelines.
- Mayo Clinic Laboratories - Serum Calcium.
- ARUP Consult - Calcium Disorders.
- MedlinePlus / NIH - Calcium Test