When is it particularly important to measure ionized calcium rather than total calcium?

Measuring ionized calcium is particularly important when albumin levels are low because calcium in the blood exists in two forms: bound to proteins (mainly albumin) and free or ionized. Total calcium measurement reflects both the bound and ionized components, but in cases of low albumin, the total calcium level may appear falsely low or normal, despite the biologically active ionized calcium actually being low. Since ionized calcium is the physiologically active form that regulates various metabolic processes, accurately assessing it in the context of low albumin ensures that any latent hypocalcemia is identified and treated appropriately.

When dealing with dehydration, cardiac arrest, or liver disease, it remains essential to monitor calcium levels, but these scenarios often involve different mechanisms affecting calcium metabolism and wouldn't specifically underscore the distinction between total and ionized calcium in the same way as low albumin does. In cases of dehydration, for instance, the total calcium can still be misleading if the protein levels are normal. Similarly, during cardiac arrest, the main focus would be on immediate interventions rather than differentiating calcium forms, while liver disease may influence protein binding but does not equally emphasize the critical need for measuring ionized calcium as low albumin.