From the Guidelines
To calculate ionized calcium from total calcium and albumin in SI units, use the formula: Corrected calcium (mmol/L) = Measured total calcium (mmol/L) + 0.02 × [40 - Albumin (g/L)], as this provides a reliable estimate of ionized calcium levels, taking into account the binding of calcium to albumin 1. When considering the relationship between total calcium, albumin, and ionized calcium, it's crucial to adjust total calcium levels for albumin to accurately reflect the free calcium, especially in patients with chronic kidney disease (CKD) 1. The formula derived from a study using preferable statistical methods and strict control of blood drawing and handling is: Corrected calcium (mg/dL) = Total calcium (mg/dL) + 0.0704 [34 Serum albumin (g/L)], but for simplicity and clinical utility, a more straightforward formula can be applied: Corrected total calcium (mg/dL) = Total calcium (mg/dL) + 0.8 [4 Serum albumin (g/dL)] 1. However, for calculations in SI units, the formula Corrected calcium (mmol/L) = Measured total calcium (mmol/L) + 0.02 × [40 - Albumin (g/L)] is more appropriate, considering the standard units used in clinical practice. Key points to consider include:
- Approximately 40% of calcium in blood is bound to albumin.
- Ionized calcium represents about 50% of total calcium.
- Low albumin levels can make total calcium appear decreased even when ionized calcium is normal.
- The correction factor compensates for the effect of low albumin on total calcium measurements.
- Direct measurement of ionized calcium is more accurate, especially in critically ill patients or those with acid-base disturbances.
From the Research
Calculation of Ionized Calcium
The calculation of ionized calcium from total calcium and albumin in Systeme International (SI) units can be determined using various formulas.
- The formula for "revised corrected calcium" is [Calcium (revised)] = plasma calcium (mmol/L) + [(4- Albumin (g/L)]*(plasma calcium (mmol/L)0.025)] is not directly provided, however, a similar formula [Calcium (revised)] = plasma calcium (mg/dl) + [(4- Albumin (g/dL)](plasma calcium (mg/dl)*0.052)] was derived from a linear regression of 468 laboratory tests 2.
- Another formula for corrected calcium is [corrected Ca (mmol/L) = Ca measured (mmol/L) + 0.020 or 0.025 (40 - albumin (g/L))] 3.
- A study also derived equations for calculating ionized calcium concentration from total calcium, albumin, total protein, and inorganic phosphate, but the exact formula is not provided in the given text 4.
- Other formulas for corrected calcium are [corrected Ca (mg/dL) = total Ca-0.76ALB + 3.2] and [corrected Ca = total Ca-0.7ALB + 2.6], which can be simplified to [corrected Ca = total Ca + 0.8(4-ALB] and [corrected Ca = total Ca + 0.7 (4-ALB)] respectively 5.
Limitations of Formulas
- The use of these formulas may lead to underestimation of calcium status in non-hypoalbuminemic patients and in hypercalcemic patients 3.
- The albumin-corrected total/ionized calcium ratio is not superior to total/ionized calcium ratio as an indicator of citrate accumulation 6.
- The formula for albumin corrected calcium may not accurately depict ionized calcium 2.