What is a normal calcium level?

Normal Calcium Levels

Normal serum total calcium levels in adults range from 8.6 to 10.3 mg/dL (2.15 to 2.57 mmol/L), while normal ionized calcium ranges from 4.65 to 5.28 mg/dL (1.16 to 1.32 mmol/L). 1

Total Calcium Reference Range

• Standard adult range: 8.6 to 10.3 mg/dL (2.15 to 2.57 mmol/L) 1
• For patients with chronic kidney disease (CKD), serum calcium should be maintained within the normal range, preferably toward the lower end at 8.4 to 9.5 mg/dL 1
• Total calcium measurements must be interpreted in the context of serum albumin levels, as approximately 40% of total calcium is protein-bound 1

Ionized (Free) Calcium Reference Range

• Normal ionized calcium: 4.65 to 5.28 mg/dL (1.16 to 1.32 mmol/L) 1
• Alternative reference range: 1.1-1.3 mmol/L (or 1.15-1.36 mmol/L) 2
• Ionized calcium represents approximately 45-50% of total calcium and is the physiologically active form 1, 2
• The remaining calcium is 40% protein-bound and 12% complexed with anions 1

When to Correct Total Calcium for Albumin

You must correct total calcium when serum albumin is abnormal, as low albumin falsely lowers total calcium measurements. 1

Correction Formula

• Corrected total calcium (mg/dL) = Total calcium (mg/dL) + 0.8 [4 - Serum albumin (g/dL)] 1
• For CKD patients, a more precise formula exists: Corrected calcium (mg/dL) = Total calcium (mg/dL) - 0.0704 [34 - Serum albumin (g/L)] 1
• The corrected calcium value helps reflect the physiologically active free calcium, which is the clinically relevant parameter 1

Important Limitations of Correction Formulas

• All correction formulas have limitations and may introduce errors, particularly outside normal albumin ranges 1
• Correction formulas may not accurately reflect ionized calcium in all clinical situations 1
• High albumin levels can cause total calcium to appear falsely elevated, emphasizing the importance of correction 1

When to Measure Ionized Calcium Directly

Measure ionized calcium directly rather than relying on corrected total calcium in these specific scenarios: 1

• When subtle changes in calcium are expected
• When total calcium measurements are inadequate
• When plasma protein levels are abnormal
• When acid-base disturbances are present (pH affects ionized calcium, with each 0.1 unit increase in pH decreasing ionized calcium by approximately 0.05 mmol/L) 2
• In critically ill patients, according to the American Society of Critical Care 1
• During massive transfusion protocols, where citrate binding affects calcium availability 1
• In calcium channel blocker toxicity to guide treatment decisions 2

Trade-offs of Ionized Calcium Measurement

• Ionized calcium measurement has worse reproducibility and is more expensive than total calcium, limiting routine use 1
• Despite these limitations, direct measurement is superior to calculating ratios or using correction formulas in the above clinical scenarios 1

Clinical Significance and Safety Thresholds

Hypocalcemia Definitions and Consequences

• Chronic hypocalcemia can cause secondary hyperparathyroidism, adverse effects on bone mineralization, and increased mortality 1
• Symptomatic hypocalcemia or ionized calcium below 0.8-0.9 mmol/L requires treatment 2

Hypercalcemia Definitions

• Mild hypercalcemia: total calcium <12 mg/dL (<3 mmol/L) or ionized calcium 5.6 to 8.0 mg/dL (1.4-2 mmol/L) 3
• Severe hypercalcemia: total calcium ≥14 mg/dL (≥3.5 mmol/L) or ionized calcium ≥10 mg/dL (≥2.5 mmol/L) 3
• Mild hypercalcemia is usually asymptomatic but may cause fatigue and constipation in approximately 20% of people 3
• Severe or rapidly developing hypercalcemia causes nausea, vomiting, dehydration, confusion, somnolence, and coma 3

Safe Upper Limits for Calcium Intake

• Total elemental calcium intake should not exceed 2,000 mg/day 1, 4
• This limit applies to both dietary and supplemental calcium combined 4, 5
• Exceeding this amount may cause inadvertent mineral deficiencies 4

Special Population Considerations

CKD Patients

• Maintain calcium-phosphorus product at <55 mg²/dL² to prevent soft tissue calcification 1
• Target serum calcium toward the lower end of normal (8.4 to 9.5 mg/dL) 1

Cardiac Disease Patients

• Monitor ionized calcium selectively rather than routinely 2
• Maintain ionized calcium >0.9 mmol/L in massive transfusion scenarios to prevent impaired coagulation and cardiovascular function 1

Dialysis Patients

• The most logical dialysate calcium concentration is 2.5 mEq/L 6
• Lower calcium dialysate (1.5 to 2.0 mEq/L) may be considered when low PTH is associated with adynamic bone disease 6