Calcium-Phosphorus (Ca:P) Product Calculation
The calcium-phosphorus product is calculated by multiplying the corrected serum calcium (mg/dL) by the serum phosphorus (mg/dL), with the result expressed as mg²/dL². 1
Calculation Formula
Ca × P Product = Corrected Calcium (mg/dL) × Serum Phosphorus (mg/dL)
The result is expressed in units of mg²/dL². 1
Step-by-Step Calculation Process
Step 1: Correct Total Calcium for Albumin
Before calculating the Ca:P product, you must first correct the total serum calcium for albumin levels, as approximately 40% of total serum calcium is bound to albumin. 1, 2
Use this simplified formula for clinical practice:
Corrected Calcium (mg/dL) = Total Calcium (mg/dL) + 0.8 × [4 - Serum Albumin (g/dL)] 1, 2
For patients with CKD requiring more precision, an alternative formula exists:
Corrected Calcium (mg/dL) = Total Calcium (mg/dL) - 0.0704 × [34 - Serum Albumin (g/L)] 1, 2
Step 2: Multiply Corrected Calcium by Phosphorus
Once you have the corrected calcium value, multiply it directly by the serum phosphorus level (no correction needed for phosphorus). 1
Clinical Example
If a patient has:
- Total calcium = 9.0 mg/dL
- Serum albumin = 3.0 g/dL
- Serum phosphorus = 6.0 mg/dL
Calculation:
- Corrected Calcium = 9.0 + 0.8 × [4 - 3.0] = 9.0 + 0.8 = 9.8 mg/dL
- Ca × P Product = 9.8 × 6.0 = 58.8 mg²/dL²
Target Range and Clinical Significance
The K/DOQI guidelines recommend maintaining the Ca × P product below 55 mg²/dL² in patients with CKD Stage 5 and those on dialysis. 1, 2
Why This Matters
Phosphorus contributes more to elevated Ca × P product than calcium does. When phosphorus rises from 3.5 to 7.0 mg/dL (a factor of 2), compared to calcium rising from 9.5 to 11.0 mg/dL (a factor of 1.2), the relative importance of controlling phosphorus is greater. 1
Ca × P products exceeding 55 mg²/dL² are associated with increased risk of soft-tissue calcification, vascular calcification, cardiovascular disease, and increased mortality. 1, 3
Even values previously considered safe (Ca × P product in the 50s) are now recognized as significant predictors of cardiovascular mortality. 3
Important Clinical Caveats
When Albumin Correction May Be Insufficient
In patients with ESKD, approximately 12% of bound calcium is linked to phosphate and other anions. 4 Some evidence suggests that a phosphate-adjusted calcium formula may be more accurate:
Ca(albPh) = Ca(tot) + (0.015 × [40 - albumin]) + 0.07 × [1.5 - phosphate]) 4
However, the standard albumin-corrected formula remains the guideline-recommended approach for routine clinical practice. 1, 2
When to Measure Ionized Calcium Directly
Consider direct measurement of ionized calcium if the patient has:
- Severe albumin abnormalities
- Acid-base disturbances (acidosis increases free calcium despite normal total calcium) 1
- Critical illness
- Clinical picture that doesn't match the corrected calcium value 5
Management Based on Ca × P Product
If Ca × P product exceeds 55 mg²/dL²:
- Immediately reduce or discontinue calcium-based phosphate binders 5
- Hold or reduce vitamin D therapy 1, 5
- Intensify phosphate control (dietary restriction to 800-1,000 mg/day, optimize phosphate binder therapy) 1
- Ensure total elemental calcium intake does not exceed 2,000 mg/day 1, 2
- Monitor corrected calcium and phosphorus at least every 2 weeks initially, then monthly once stable 2
The primary strategy should focus on lowering phosphorus rather than calcium, given phosphorus's greater contribution to the product. 1