The Influence of Albumin and pH on Total and Ionized Calcium and Magnesium

Objectives: The objectives of this study were to demonstrate the effect of albumin and pH on total and ionized calcium (TCa, iCa) and total and ionized magnesium (TMg, iMg) in a tertiary level hospital. The ionized fraction was tested on 2 different blood gas analyzers (Nova Prime Plus and Siemens RapidPoint 500), whereas iMg was only available on the Prime Plus. Methods: The study consisted of 3 parts. Part 1 was to investigate the effects of increasing albumin by using a pooled serum sample and then spiking aliquots with Albumix to provide a wide concentration range. Part 2 investigated the effects of pH in fresh whole blood and pooled serum by spiking the aliquots with sodium hydroxide (NaOH) or hydrochloric acid (HCl) to provide a wide concentration range. The final part was to determine the percentage range of the ionized fractions in normal samples. Results: Total calcium and iCa decreased by 0.01 mmol/L per 1 g/L with increasing albumin at a similar rate in response to increasing albumin concentration. Total magnesium and iMg also decreased but to a lesser degree by 0.003 and 0.005 mmol/L, respectively. As the pH increased, iCa decreased by approximately 0.04 mmol/L per 0.1 pH units in both sample types. The iMg decreased at a much smaller magnitude, by 0.002 mmol/L in whole blood and 0.001 mmol/L in serum per 0.1 pH unit. The ionized fraction percentages in the normal samples were as follows: iCa, 51% (44-57), and iMg, 69% (61-77). Conclusions: The iCa and iMg should be routinely measured, owing to their clear inverse relationship with albumin and pH. Routine measurement of the ionized fractions would avoid the need for correction factors and would highlight the physiologically active ion concentration for better patient care decisions.