Normal Calcium Levels in Blood
Normal serum total calcium levels in adults range from 8.6 to 10.3 mg/dL (2.15 to 2.57 mmol/L), with ionized (free) calcium ranging from 4.65 to 5.28 mg/dL (1.16 to 1.32 mmol/L). 1
Total Calcium Reference Ranges
- Standard adult range: 8.6 to 10.3 mg/dL (2.15 to 2.57 mmol/L) 1
- UK population-based range: 2.19 to 2.56 mmol/L (8.76 to 10.24 mg/dL) for the general population 2
- Age and gender variations exist: Older women (55-69 years) have an upper limit of 2.59 mmol/L, younger women (40-55 years) 2.57 mmol/L, and men 2.55 mmol/L 2
Ionized (Free) Calcium Reference Range
- Normal ionized calcium: 4.65 to 5.28 mg/dL (1.16 to 1.32 mmol/L) 1
- This represents approximately 48% of total calcium, with 40% protein-bound and 12% complexed with anions 1
Important Clinical Considerations
When to Correct Calcium Levels
- Albumin correction is essential when serum albumin is abnormal, as low albumin falsely lowers total calcium measurements 1, 3
- Correction formula: Corrected total calcium (mg/dL) = Total calcium (mg/dL) + 0.8 [4 - Serum albumin (g/dL)] 3
- However, correction formulas have limitations and may introduce errors, particularly outside normal albumin ranges 1
Sampling Requirements for Accurate Measurement
- Fasting samples are critical for diagnostic purposes, as calcium supplements or calcium-containing foods cause transient elevations lasting several hours 4
- Prolonged venous stasis or standing causes hemoconcentration, artificially elevating the bound calcium fraction 4
- Preceding exercise can also affect blood calcium levels 4
- For diagnostic purposes, fasting ionized calcium is preferred over total calcium 4
When to Measure Ionized Calcium Directly
- Measure ionized calcium when:
Special Population Considerations
Chronic Kidney Disease Patients
- Target range for CKD patients: 8.4 to 9.5 mg/dL (2.10 to 2.37 mmol/L), preferably toward the lower end 3
- Total elemental calcium intake should not exceed 2,000 mg/day 1, 5
Monitoring Frequency
- For patients with chronic conditions requiring calcium monitoring, check serum calcium and phosphorus every 3 months 5
- In pediatric conditions like Williams syndrome, monitor every 4-6 months until age 2, then every 2 years 5
Common Pitfalls to Avoid
- Do not rely on single non-fasting measurements for diagnosis of calcium disorders 4
- Do not use total calcium alone when albumin is abnormal without correction 1, 3
- Recognize that ionized calcium measurement has worse reproducibility and is more expensive than total calcium, limiting routine use 1
- Be aware that all correction formulas have limitations and may not accurately reflect ionized calcium in all clinical situations 1