Intermethod Variability of Sodium and Potassium Results: Patients Sera and Commercially Available Control Sera

Abstract

Sodium and potassium were measured in sets of 102 to 107 patients sera, and in 31 commercially available control sera. The results from four routine analytical methods/systems (indirect potentiometry: two; direct potentiometry and enzymatic assay: one each) were compared with those from a flame photometry-based reference method. In the assay of patient sera, substantial agreement was observed in some comparisons, clinically relevant bias in others. The inter-assay changes observed for the control sera differed significantly from those shown by the patients sera (i.e. commercial control sera were non-commutable) in about 12% of the comparisons, as a whole. Recalculation of serum sample results with a single control serum as calibrator lowered or increased the bias originally present according to whether the serum itself was commutable or not. Moreover, the inter-method variability in the assay of commercial control sera was lower with commutable sera, higher with non-commutable sera. With the exception of liquid sera stabilized with ethylene glycol, there was no evident link between any specific characteristic of the commercial control sera (matrix and physical state) and their degree of commutability. The routine measurements were performed with the following four methods/instruments/systems. Method 1 Indirect potentiometry, E2A instrument, from Beckman, reagents and calibrators from the same source. Method 2 Indirect potentiometry, Hitachi 717 analyzer, from Boehringer Mannheim, reagents and calibrators from the same source. Method 3 Direct potentiometry, Cobas Mira S plus analyzer, from Roche, reagents and calibrators from the same source.

Evaluation of Commutability of Control Materials

“…A recent study shows that the improvements, which have been quite notable during the last 20 years, concern more particularly precision, which is evaluated in clinical laboratories by the internal quality control (1). Several authors confirm this opinion (4)(5)(6)(7)(8) in various clinical chemistry applications such as the measurement of substrates and of the catalytic activity of enzymes, the measurement of electrolytes or of proteins determined by immunochemical methods. A much proposed explanation is that control materials are not always commutable and therefore are not always representative of the patients' specimens.…”

Commutability of Calibration and Control Materials for Serum Lipase

“…Both single-point and two-point mathematical recalibrations were used because intermethod comparison with patient sera revealed constant and proportional components of intermethod differences. The differences [(original y-axis value) Ϫ (x-axis value)] and [(recalculated y-axis value) Ϫ (x-axis value)] were then computed, and the distributions of such differences were compared to assess the effect of recalibration (12)(13)(14).…”

“…The ability of CCMs to show intermethod behavior comparable to that observed when measuring the same quantity in patient sera is referred to as "commutability" (8)(9)(10). Many commercially available CCMs lack such commutability for many analytes (10)(11)(12)(13)(14), including lipase (3 ). The lack of commutability may not be attributable to the declared characteristics of the CCMs, but rather to the interaction between the characteristics of the CCMs and the specificity of the analytical methods.…”