The use of aqueous standards to check routinely the linearity of SMA chemistries

Abstract

This paper presents an investigation of the linearity of plasma chemistry results from the Technicon SMA 6/60 and 12/60 instruments. We assay both plasma and urine electrolytes on the SMA 6/60, this investigation being prompted by the observation of non-linearity of urinary potassium for results that were greater than the reference material concentration. Current Technicon literature suggests no means of testing linearity beyond the values of their top serum standard. Scale II, though a number of users have top scale values significantly greater than the stated value of this material. To check the linearity from zero to the top scale value for each chemical analysis, we prepared a series of aqueous standards for sodium, potassium, bicarbonate, creatinine, urea, uric acid, calcium and inorganic phosphate. Bilirubin standards were prepared using lyophilized serum with a high bilirubin value. Albumin standards were assayed using Technicon SMA reference serum for calibration. As a result of these investigations non-rectifiable deviations from linearity were detected in two results. These results, viz, for urea and inorganic phosphate, have consequently had their top of scale values reduced. Linearity characteristics, particularly in the high scale regions of an SMA system, can alter significantly with changes in reagent batch, or mechanical or electrical deterioration of the instrument. We therefore recommend regular linearity checks encompassing the complete range of each chemical analysis even though there is no reason to suspect deviations from the assumed linear response.