Calcium
Ca· also: Calcium, Total Calcium
Clinical Overview
Calcium is the most abundant mineral in the body, with 99% stored in bones and teeth and 1% in blood and soft tissues. In blood, calcium exists in three forms: ionized (free, 50% — biologically active), protein-bound (mainly albumin, 40%), and complexed (10%). Total serum calcium measures all three fractions and must be corrected for albumin levels.
Why This Test Matters
Calcium is critical for nerve conduction, muscle contraction, cardiac function, and blood clotting. Hypercalcemia (high calcium) is a common metabolic emergency — 90% of cases are due to primary hyperparathyroidism or malignancy. Hypocalcemia (low calcium) causes neuromuscular excitability and tetany. Calcium results must always be interpreted alongside albumin (for corrected calcium) or ionized calcium should be measured directly.
Reference RangesWHO/IFCC standards
| Age Group | Reference Range | Unit | Notes |
|---|---|---|---|
| Adults (18–64) | 8.5 – 10.5 | mg/dL | — |
Also reported in: mmol/L.Conversion factor: 1 mmol/L = 0.250 mg/dL.
Critical (Panic) Values
Critical Low: < 6.5 mg/dL. Critical High: > 13 mg/dL. Values outside these limits require immediate clinical attention.
What Causes Abnormal Results?
High Ca Causes
- Primary hyperparathyroidism (most common outpatient cause — usually from parathyroid adenoma)
- Malignancy with bone metastases or PTHrP secretion (most common inpatient cause)
- Vitamin D toxicity
- Sarcoidosis and other granulomatous diseases
- Immobilization (bone resorption)
- Thiazide diuretics (reduce renal calcium excretion)
- Milk-alkali syndrome (excess calcium carbonate intake)
Low Ca Causes
- Hypoparathyroidism (post-thyroid surgery most common)
- Vitamin D deficiency
- Hypomagnesemia (magnesium required for PTH secretion)
- Renal failure (impaired vitamin D activation)
- Acute pancreatitis (calcium chelated by fatty acids)
- Hypoalbuminemia (apparent low calcium — check ionized calcium)
- Respiratory alkalosis (shifts calcium into cells)
Signs & Symptoms to Watch For
How to Prepare for This Test
No fasting required. Calcium rises slightly after meals. Prolonged tourniquet use and hemolysis can falsely elevate calcium.
Factors That Can Affect Results
- Hypoalbuminemia (falsely lowers total calcium — use corrected calcium formula or measure ionized)
- Prolonged tourniquet (hemoconcentration raises calcium)
- Hemolysis (potassium and proteins released falsely affect some calcium assays)
- Alkalosis (more calcium binds to albumin, lowering ionized fraction even if total calcium is normal)
- Gadolinium contrast (interferes with colorimetric calcium assays — draw calcium before contrast)
Related Topics
Frequently Asked Questions
What is corrected calcium and when should I use it?
Corrected calcium adjusts for low albumin: Corrected Ca (mg/dL) = Measured Ca + 0.8 × (4.0 − albumin g/dL). When albumin is low (as in liver disease, malnutrition, or nephrotic syndrome), measured total calcium appears low even if ionized (biologically active) calcium is normal. Corrected calcium or directly measured ionized calcium gives a more accurate picture. In critically ill patients, ionized calcium should always be measured directly.
What should I do if my calcium is high on a routine blood test?
The first step is to confirm hypercalcemia on a repeat test (transient elevation can occur). If confirmed above 10.5 mg/dL, measure PTH simultaneously. An elevated PTH with high calcium indicates primary hyperparathyroidism (usually a benign parathyroid adenoma). A suppressed PTH with high calcium in the context of known or suspected cancer indicates malignancy-related hypercalcemia. A parathyroid nuclear scan (sestamibi) and DEXA bone scan are the next steps in confirmed primary hyperparathyroidism.