From the Guidelines
The corrected calcium level is 9.18 mg/dL when the total calcium is 9 mg/dL and serum albumin is 3.1 g/dL. This calculation is based on the formula: Corrected calcium = Total calcium + 0.8 × (4 - Albumin) 1. Since the normal albumin level is considered to be 4 g/dL, and the patient's albumin is slightly lower at 3.1 g/dL, a small adjustment is needed. The calculation is: 9 + 0.8 × (4 - 3.1) = 9 + 0.8 × 0.9 = 9 + 0.72 = 9.18 mg/dL. This correction is important because calcium in the blood is partially bound to albumin, so low albumin levels can falsely lower the total calcium measurement 1. The corrected value provides a more accurate assessment of the physiologically active calcium in the body. A corrected calcium of 9.18 mg/dL falls within the normal range (typically 8.5-10.5 mg/dL), suggesting normal calcium homeostasis in this patient. Key points to consider in this calculation include:
- The use of a simple formula for adjusting total serum calcium concentration for changes in plasma albumin concentration 1
- The importance of correcting for albumin levels to accurately assess calcium homeostasis in patients with kidney diseases 1
- The potential for low albumin levels to falsely lower total calcium measurements, highlighting the need for correction 1
From the Research
Corrected Calcium Calculation
To calculate the corrected calcium level, we can use the conventional calcium correction formula (Payne) which is:
- corrected total calcium (TCa) (mg/dL) = TCa (mg/dL) + (4 - albumin (g/dL)) 2, 3 Given the total calcium is 9 mg/dL and albumin is 3.1 g/dL, we can plug these values into the formula:
- corrected total calcium (TCa) (mg/dL) = 9 + (4 - 3.1)
- corrected total calcium (TCa) (mg/dL) = 9 + 0.9
- corrected total calcium (TCa) (mg/dL) = 9.9
Alternative Formulas
However, it's worth noting that there are alternative formulas that have been proposed for calculating corrected calcium levels, particularly in patients with chronic kidney disease (CKD) 2, 3. These formulas take into account additional factors such as pH and phosphate levels.
- For example, one study proposed the following formula: approximated corrected TCa (mg/dL) = TCa + 0.25 × (4 - albumin) + 4 × (7.4 - pH) + 0.1 × (6 - phosphate) + 0.3 2
- Another study proposed a similar formula: approximated corrected TCa (mg/dL) = TCa + 0.25 × (4 - albumin) + 4 × (7.4 - pH) + 0.1 × (6 - P) + 0.22 3
Limitations of Conventional Formula
It's also important to note that the conventional Payne formula may not be accurate in all situations, particularly in patients with hypoalbuminemia or hypercalcemia 4. In these cases, alternative formulas or direct measurement of ionized calcium may be necessary to get an accurate estimate of calcium levels.
- One study found that the conventional formula led to an underestimation of calcium levels in patients with albumin values greater than 40 g/L 4
- Another study found that correcting total serum calcium values for serum albumin concentration improved discrimination of borderline hyperparathyroid patients, but that measurement of ionized calcium in serum discriminated better 5