Only a small portion of total calcium measured in the blood, called “free calcium” or “ionized calcium,” is active in the body tissues. Generally speaking, total calcium levels are a good indicator of the proportion of free or ionized calcium. But this is not always the case. Low levels of albumin in the blood push down total calcium measurements, but not free calcium measurements. Albumin-adjusted calcium levels can therefore be calculated to give a more accurate picture of the proportion of free calcium.
Sometimes the proportion of free calcium needs to be directly measured at the body’s normal pH, due to changes in blood pH or the presence of calcium-binding agents, such as lactates. This measurement is called “adjusted ionized calcium (at pH 7.4).”
Albumin-adjusted total calcium levels and adjusted ionized calcium levels at pH 7.4 are interpreted in the same way as total calcium levels (see those headings).