I heard that ionized calcium should be measured anaerobically. Why do you measure it aerobically?
Ionized calcium can be accurately measured even after the sample has been exposed to air, as long as some technical issues have been addressed. Exposure to air increases the pH of serum, which causes proteins to bind to calcium, resulting in low ionized calcium concentrations. This protein-binding/pH interaction behaves in a linear fashion, and thus a correction formula can be developed to adjust the measured ionized calcium of a sample at a known pH to the ionized calcium concentration at a standard pH (7.4). This is routinely done in human medicine. In our laboratory, we have developed formulas to correct the ionized calcium concentration to a pH of 7.4, and these formulas have been verified by comparison to samples handled without exposure to air.
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