Calcium is traditionally reported as a total serum concentration (i.e., complete metabolic profiles) and ionized calcium concentration (i.e., blood gases). To interpret these labs, one must understand what they represent. Under normal physiologic conditions, roughly 50% of calcium is bound (primarily to albumin), and 50% is ionized (the biologically active component). Therefore, total calcium = bound + ionized.
Based on this relationship, it’s possible to see an increase/decrease in total calcium WITHOUT a change in ionized. This is the premise behind “pseudo-hyper/hypo-calcemia,” which is largely driven by changes in albumin concentration. More recent literature has shown that the “albumin correction formula” (corrected [Ca] = 0.8 * (4 – [albumin]) + measured [Ca]) provides poor estimates in the critically ill/renally compromised patient populations, so use it mindfully!
Ionized fractions can also change without affecting the total calcium concentration! In the OR and ICU, I see this in the context of acid-base derangements. Albumin is a negatively charged protein that attracts positively charged calcium ions AND H+ ions (pH). In acidotic states, [H+] increases and competes with calcium to bind albumin. Thus, less calcium has a home on albumin, and more is left ionized in its biologically active state. Since this represents a shift between bound and unbound states, the TOTAL remains relatively UNCHANGED.
As a cardiac anesthesiologist and intensivist, I’m almost invariably looking for ionized calcium to guide therapy, electrolyte repletion, etc. Why? Because this tells me what I need to know – the biologically active fraction! In lower-risk patients… well… please don’t send extra labs unless you’re going to DO something with them!
Drop me a comment below with questions!