Context

The adult body contains approximately 1kg of Ca2+ ions (Ca2+) (25,000mmol) with over 99% bound in the skeleton. The 70 kg male contains approximately 22.5mmol of Ca2+ in extracellular fluid, of this only 9% is in plasma.

Normal range

Measurement

The measurement of ionised Ca2+ in plasma is difficult and not routine practice.

Formula

Total plasma Ca2+ is corrected for protein binding by adding or subtracting 0.02mmol/l for every gram of albumin measured concurrently above or below 40g/l. Formula: [Ca] +/- 0.02 (40 albumin)

Corrected calcium (mmol/L) = measured total calcium (mmol/L) + 0.02 (40 – serum albumin [g/L]), where 40 represents the average albumin level in g/L.
In other words, each 1 g/L decrease of albumin, will raise 0.02 mmol/L in serum Ca.

When there is hypoalbuminemia (a lower than normal albumin), the corrected calcium level is higher than the total calcium. The formula is shown below:

[Ca] + 0.02 (40 albumin)

Thus, when there is a higher than normal albumin, the corrected calcium level is lower than the total calcium. The formula is shown below:

[Ca] – 0.02 (40 albumin)

Learning Bite:

Calcium samples are ideally taken when the patient is fasting. Samples should always be taken without the tourniquet as this increases plasma protein measurement and alters values.