Ionised calcium can be measured with an ion specific electrode. The specimen tube must be filled completely and handled promptly to minimise pCO2 or pH changes. Some machines may correct to a normal pH or a normal CO2 but this can be undesirable since it may mask abnormalities in the patient due solely to pH or pCO2.
Most laboratories quote the total calcium concentration. The biologically-relevant form of calcium is the free ion. The proportion of the total calcium which is free in solution depends on the concentration of binding proteins such as albumin. For example, a low total calcium when combined with a low protein level may yield a normal free calcium concentration.
Concentration in a sample is increased by venous stasis.
Corrected calcium concentration estimates the total concentration as if the albumin concentration was normal - usually taken as 40 g/l. A typical correction is that for every 1 g/l that the albumin concentration is below this mean, the calcium concentration is 0.02 mmol/l below what it would be if the albumin concentration was normal; i.e.
corrected [Ca] = measured [Ca] + {(40 - [albumin]) * 0.02}
However, in interpreting values adequate consideration must be paid to other factors which may affect albumin binding, for example, other proteins in myeloma, individual variation, cirrhosis.
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