Normal Serum Calcium and Phosphorus Levels in Healthy Adults
In healthy adults, normal serum total calcium ranges from 8.6 to 10.3 mg/dL (2.15 to 2.57 mmol/L), normal ionized calcium ranges from 4.65 to 5.28 mg/dL (1.16 to 1.32 mmol/L), and normal serum phosphorus ranges from 3.5 to 4.6 mg/dL (1.13 to 1.49 mmol/L). 1
Serum Calcium Reference Ranges
Total Calcium:
- The standard reference range for total serum calcium in healthy adults is 8.6 to 10.3 mg/dL (2.15 to 2.57 mmol/L) 2
- K/DOQI guidelines reference a typical healthy adult calcium level of approximately 9.5 mg/dL (2.37 mmol/L) as the midpoint of normal 1
Ionized (Free) Calcium:
- Normal ionized calcium levels range from 4.65 to 5.28 mg/dL (1.16 to 1.32 mmol/L) 2
- Ionized calcium represents approximately 48% of total calcium, with 40% protein-bound (primarily to albumin) and 12% complexed with anions 2
Serum Phosphorus Reference Ranges
- Normal serum phosphorus in healthy adults ranges from 3.5 to 4.6 mg/dL (1.13 to 1.49 mmol/L) 1
- The K/DOQI guidelines use 3.5 mg/dL (1.13 mmol/L) as the reference baseline for healthy individuals 1
Calcium-Phosphorus Product
- The calcium-phosphorus product (Ca × P) should be maintained below 55 mg²/dL² to minimize risk of soft tissue calcification 1
- When using ionized calcium measurements, the corresponding safe threshold is below 2.2 mmol²/L² 3
Age-Related Considerations
Elderly Adults (>65 years):
- Both serum calcium and phosphate levels decline slightly with normal aging in women over 65 years 4
- This decline is partly due to decreased protein-bound calcium but also includes a slight reduction in ionized calcium 4
- Recognition of these age-related changes is important to avoid missing clinically significant hypercalcemia in elderly patients, where even a calcium of 10.5 mg/dL may represent pathology 4
Critical Clinical Caveats
When Albumin is Abnormal:
- Total calcium measurements must be corrected for albumin levels using the formula: Corrected calcium (mg/dL) = Total calcium (mg/dL) + 0.8 × [4 – Serum albumin (g/dL)] 2, 5
- When albumin is below 3.0 g/dL, correction formulas become increasingly unreliable and direct ionized calcium measurement is strongly recommended 5
When to Measure Ionized Calcium Directly:
- Measure ionized calcium when subtle changes are expected, total calcium measurements are inadequate, plasma protein levels are abnormal, or acid-base disturbances are present 2
- In critically ill patients, massive transfusion scenarios, or those with severe acid-base disorders, ionized calcium provides superior accuracy compared to corrected total calcium 2, 5
pH Effects on Ionized Calcium: