Adjustment of ionized calcium concentration for serum pH is not a valid marker of calcium homeostasis: implications for identifying individuals at risk of calcium metabolic disorders

Abstract

Background Ionized calcium (iCa) is the biologically active form of this micronutrient. Serum determination of iCa is measured via ion-electrode potentiometry (IEP) and reporting iCa relative to pH 7.4 is normally utilized to avoid the potential confounding effects of ex vivo changes to serum pH. Adjustment of iCa for pH has not been adequately justified. Methods In this study, utilizing carefully standardized protocols for blood collection, the preparation of serum and controlling time of collection-to-analysis, we determined serum iCa and pH utilizing an IEP-analyser hosted at an accredited diagnostic laboratory. Results Regression analysis of unadjusted-iCa (iCaraw) concentration versus pH was described by linear regression and accounted for 37% of serum iCaraw variability. iCaraw was then expressed at pH 7.4 by either adjusting iCaraw based on the linear regression equation describing the association of iCa with serum pH (iCaregr) or using IEP coded published normative equations (iCapub). iCaregr was comparable to iCaraw, indicating that blood collection and processing methodologies were sound. However, iCapub yielded values that were significantly lower than iCaraw. iCapub did not identify 15% subjects who had greater than desirable serum concentration of iCa based on iCaraw. Sixty percent of subjects with low levels of iCaraw were also not detected by iCapub. Determination of the kappa value measure of agreement for iCaraw versus iCapub showed relatively poor concordance ( κ = 0.42). Conclusions With simple protocols that avoid sampling artefacts, expressing iCaraw is likely to be a more valid and physiologically relevant marker of calcium homeostasis than is iCapub.