Calcium Level

Total serum calcium assays measure both the unbound and bound portions of calcium. However, it is the ionised portion that is responsible for the calcium effects seen clinically in hypercalcaemia, so ionised calcium (which is typically measured on a blood gas) is the best measured indicator of active calcium.

As calcium is heavily protein bound in blood, if albumin levels are low, total calcium levels may appear artificially lowered (pseudohypocalcaemia). Conversely, with elevated albumin, they may appear artificially elevated (pseudohypercalcaemia). A corrected calcium level may be calculated as follows:

Method for calculating correction of calcium level to reflect albumin level:

• If serum albumin is less than 40 g/L, increase measured calcium by 0.20 mmol/L for every 10 g of albumin below 40 g/L
• If serum albumin is greater than 40 g/L, reduce measured calcium by 0.20 mmol/L for every 10 g of albumin over 40 g/L

Alternatively:

• Corrected calcium (mmol/L) = Measured calcium (mmol/L) + [0.02 x (40 – measured albumin g/L)]

A calculator is available at MDCalc. [4]

Although there is not a universal definition of the severity of hypercalcaemia, it is commonly graded as mild, moderate or severe [5]:

• Mild hypercalcaemia: total calcium of less than 3 mmol/L (<12 mg/dL) or ionised calcium of 1.4 to 2.0 mmol/L (5.6 to 8.0 mg/dL)
• Moderate hypercalcaemia: total calcium of 3.0 to 3.5 mmol/L (12.0 to 13.9 mg/dL) or ionised calcium of 2.5 mmol/L or greater (≥10 mg/dL)
• Severe hypercalcaemia: 3.5 mmol/L or greater (≥14 mg/dL) or ionised calcium of 2.5 to 3.0 mmol/L (10-12 mg/dL).

ECG

Hypercalcaemia may produce ECG abnormalities related to altered trans-membrane potentials that affect conduction time. QT interval shortening is common and, in some cases, the PR interval is prolonged. At very high levels, the QRS interval may lengthen, T waves may flatten or invert, and a variable degree of heart block may develop. Digoxin effects are amplified.