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Almost 99% of the bodies supply of calcium is found in bone. The routine calcium level reported by our lab is a total calcium, representing the small portion found in plasma. The total calcium is the sum of all plasma calcium, including the calcium that is bound to proteins and anions, and the ionized component.
Approximately 50% of the total plasma calcium is bound to protein, with albumin being the most abundant binding protein. As a positively charged electrolyte (cation), another 5 - 10 % of the total plasma calcium can be found attached to negatively charged electrolytes (anions) such as sulphates and phosphates. The remaining 45-50% of the total plasma calcium circulates freely. This free calcium is referred to as the ionized calcium and represents the physiologically active portion of the salt, or the most clinically significant form.
Because we do not routinely measure the ionized or important portion of the calcium, we use the total plasma calcium to reflect the ionized portion. One limitation of this value is that the total plasma calcium and the ionized calcium do not rise and fall proportionately.
If there is a reduction in the serum albumin, the portion of the calcium bound to protein will fall. This will reduce the total plasma calcium. This reduction in the portion of calcium bound to protein will occur even if the ionized calcium (the physiologically important portion) is normal or elevated. An elevated ionized calcium (true hypercalcemia) can be masked by the presence of a low serum albumin level.
If both the albumin and total plasma calcium levels are below normal, the low calcium level may be clinically unimportant (i.e. the ionized calcium may be normal). One formula to determine the significance of a low total calcium is to correct the calcium upward by .2 mmol/l for every 10 g/dl fall in albumin.
For example, if the albumin level is 30 g/dl (decreased by 10), and the calcium level is 2.06, the corrected calcium would be 2.26 mmol/l.
While this provides some general direction for assessing calcium levels, the only way to know for certain whether true hypocalcemia or hypercalcemia exists is to measure an ionized calcium.