The calibrator value assignment protocol of the Abbott enzymatic creatinine assay is inadequate for ensuring suitable quality of serum measurements.

The measurement of creatinine in serum is a key indicator of glomerular function of the kidney and its pivotal role in this evaluation has been further increased by recommendations issued by nephrology societies for using equations to estimate the glomerular filtration rate (GFR) [1]. In 2006 the reference system for the measurement of serum creatinine, using suitable higher-order reference materials and reference methods, was completed and internationally promoted [2]. Particularly, the U.S. National Institute for Standard and Technology (NIST) developed the standard referencematerial (SRM) 967 (“Creatinine in frozen human serum”), in which there are two concentration levels with values assigned by isotope dilution-mass spectrometry (IDMS) andwith proven commutability with native serum samples for most of the available commercial creatinine methods [3]. In vitro diagnostic (IVD) manufacturers (re)calibrated their creatinine measurement systems to the internationally agreed reference system to implement the SI traceability of blood creatinine results, with the consequence that for clinical laboratories only standardized assays should remain available. IVD manufacturers may employ different metrological chains and spend substantially different amounts of the total budget of measurement uncertainty just by selecting one or the other of the available traceability chains [4]. This is a central issue as the selection and application of a reference measurement system for calibration of routine methods should be associated with the fulfilment of measurement uncertainty goals based on medical relevance, so that results are suitable for patient management [5]. Using the information on the biological variation of the analyte [www.westgard.com/biodatabase1.htm], to be acceptable, the degree of expanded uncertainty (U) of creatinine measurements for clinical laboratory using unbiased assays on patient samples should stay within approximately ±6.0% or ±9.0% (desirable or minimum quality level, respectively, for imprecision multiplied by a coverage factor of 2), when both the accumulated uncertainty of the corresponding traceability chain and the uncertainty due to the random effects of measurement are included. In our laboratory the serum creatinine measurement is performed with the enzymatic (creatinine hydrolysis to creatine by creatininase) assay (cod. 8L24) on the Abbott Diagnostics Architect c16000 platform. Accordingwith the IVDEUDirective 98/79/EC [6], the analytical system is CEmarked and the calibrator (Multigent Clin ChemCalibrator, cod. 6K30) (CAL) is traceable to theNIST SRM967,with aUof 0.059mg/dL (1.48%) at