What is the primary indicator of physiologic blood calcium activity?

Ionized calcium is the primary indicator of physiologic blood calcium activity because it represents the biologically active form of calcium in the bloodstream that is free, not bound to proteins or other substances. This form of calcium is crucial for various physiological processes, including muscle contraction, nerve transmission, and blood coagulation.

Total calcium measures both the ionized and bound fractions, but it does not accurately reflect the active calcium available for physiological functions. Similarly, serum calcium encompasses total calcium without distinguishing between ionized and non-ionized forms. Calcium phosphate pertains to the relationship between calcium and phosphate ions in the body but does not directly indicate calcium's physiological activity.

Thus, monitoring ionized calcium provides the most relevant and clinically significant insight into calcium's role in bodily functions.