Analysis of variation in automated determination of sodium, potassium and calcium ions in human serum.

The concentration of sodium ion (Na+), potassium ion (K+) and calcium ion (Ca2+) in patient sera were assayed simultaneously on two separate flame photometers and then evaluated by analysis of variance (ANOVA). Two different experimental designs were devised in order to separate the error components assignable to pipetting, dilution, and instrumental measurement. Within-series variability was compared with the between-series variability. Similar statistical treatment of results from quality control samples gave estimates of variances between days, between dilutions, between instruments, and between channels, as well as the interaction terms and instrumental measurement errors. The total (overall) coefficients of variation (CV) were 1.15, 1.60, and 1.38% for Na+, K+, and Ca2+, respectively. The adequacy of the quality-control system to predict expected variation in patient specimens was verified by comparing errors from patient-sample duplicates and from control samples.