Comparison of precision and effect of pH and calcium on the AVL and NOVA magnesium ion-selective electrodes

We compared the precision of the AVL 988-4 and NOVA CRT instruments for determining ionized magnesium (iMg) and assessed the effect of pH and ionized calcium (iCa) concentration on the results. Within-run and day-to-day precision for the iMg electrodes were determined using three levels of control material supplied by each manufacturer. The effect of pH on iMg results was assessed by analyzing anaerobic serum samples from patients, reanalyzing those same samples after pH was increased by in vitro loss of CO2 and comparing the results. To assess the effect of iCa concentration on the iMg results, we added CaCl2 to aqueous standards from both manufacturers and to a normal serum pool. The results show comparable coefficients of variation for the two iMg electrodes both within-run (0.68-2.05 for NOVA; 0.77-2.60 for AVL) and day-to-day (2.90-6.48 for NOVA; 1.71-4.93 for AVL). The AVL results were not affected by the increase in serum pH and agreed with the NOVA results that were adjusted to a pH of 7.4 (paired t-test; p > 0.2). There was a significant direct relationship between the iCa and iMg results for both analyzers, but the AVL slopes were smaller (0.026, 0.083) than the NOVA slopes (0.129, 0.165). Thus, these two iMg electrodes have comparable precision but differ in response to an increase in pH and iCa.