ANALYZER-DEPENDENT DIFFERENCES IN RESULTS FOR IONIZED CALCIUM, IONIZED MAGNESIUM, SODIUM, AND PH

We compared two ion-selective analyzers (AVL 988-4 and NOVA CRT) for determining ionized calcium (iCa(2+)), ionized magnesium (iMg(2+)), sodium (Na+), and pH in serum specimens from healthy and diseased individuals. For assays of three levels of protein-based control materials, total imprecision (CV) was <3% for all analytes except iMg(2+) (less than or equal to 6.5% on NOVA, and less than or equal to 4.9% on AVL). We found a significant difference between the analyzers (P < 0.001) for the mean iMg(2+) concentration in patients but no significant correlation (r = 0.253) between the analyzers for iMg(2+) in specimens from healthy volunteers, even though the mean iMg(2+) concentration did not differ significantly between these groups. The reference interval (central 95 percentiles) for iMg(2+) with AVL (0.44-0.60 mmol/L) was contained within that of NOVA (0.39-0.64 mmol/L). The AVL gave higher values for iCa(2+) (P < 0.001) and lower values for pH (P < 0.001) in specimens from normal volunteers and patients. The mean value for Na+ in patients' samples was significantly higher by the NOVA (P < 0.01) than by the AVL analyzer. Thus, we found significant differences between these two analyzers for all four analytes.