Assessment of the perchloric acid hexokinase and plasma glucose oxidase methods using glucose error grids

Introduction: Isotope dilution mass spectrometry (IDMS) is a definitive method for glucose measurement and perchloric acid treated specimens analyzed with hexokinase (PCA- hexokinase) is a reference method. The performance of clinical laboratory plasma glucose methods are commonly assessed by evaluating their traceability to either the IDMS or PCA-hexokinase results. Glucose error grids have been used to determine the clinical accuracy of point of care glucose meters. The objective of the current study was to describe the variation amongst the PCA hexokinase, plasma glucose oxidase and the IDMS methods using the insulin dosing error (IDEA) grid, Parkes and Surveillance error grids.
Materials and Methods: Residual lithium heparin venous whole blood specimens (n= 156) from hospitalized adult patients were treated with perchloric acid prior to analysis with the IDMS and the Roche Cobas® hexokinase methods. The remaining specimen was centrifuged and the plasma analyzed using the Beckman DxC® glucose oxidase method. The variation of the PCA-hexokinase and plasma glucose oxidase methods relative to the IDMS was assessed using the Parkes, Surveillance Error grid (SEG) and IDEA error grids. For the IDEA grid, the effect of 0.5 mg/dL differences between methods on the size of insulin dose category error was determined by simulation using the protocol for critically ill patients (Karon et al, 2010). Patient data was plotted on the error grid to indicate the extent that observed glucose results are expected to affect changes of insulin dose. Results: Parkes error grid analysis for PCA- hexokinase results revealed 86.5% zone A and 13.5% zone B, and for plasma glucose oxidase results were 99.4% zone A and 0.6% zone B. With SEG 98.7% of the PCA-hexokinase results were within the assessable range, 78.6% indicated no clinical risk, 19.5% demonstrated slight (low risk) and 1.9% showed slight (high risk). Plasma glucose oxidase results within the SEG required range demonstrated 94.7% had no clinical risk and 5.3% had slight (low risk). IDEA grid analysis of the PCA-hexokinase results indicated 85.9% within ±1 insulin dose category, 98.7% within ±2 categories. Plasma glucose oxidase results with the IDEA grid showed 96.8% within ±1 insulin dose category, 98.1% within ±2 categories.
Conclusions: Error grid analyses demonstrated that an automated lab glucose method (plasma glucose oxidase) and a glucose reference method (PCA-hexokinase) displayed imprecision and inaccuracy relative to a definitive glucose method. Error grid analyses of candidate glucose methods relative to automated lab glucose or glucose reference methods should not exclusively attribute analytic error to the candidate method.