Evaluation of the impact of standardization process on the quality of seruma creatinine determination in Italian laboratories

Background: Creatinine determination in serum is the key indicator of kidney glomerular function. A reference measurement system for standardization of creatinine measurements is available and virtually all IVD manufacturers have recently aligned their creatinine assays to this system. The aim of this work was to verify if and how these standardization efforts have improved the state of the art of creatinine determination in Italy. Methods: An analysis of Prolarit EQAS results using control materials with target values assigned by a traceable method (enzymatic assay calibrated against the NIST SRM 967) was carried out. Results: Results obtained during 2006, 2010, and 2011 schemes by participating laboratories showed a general good alignment at creatinine concentrations ~2.00 mg/dL, with 2011 results – except for one method group – well inside the desirable bias (±4%). At higher concentrations, whereas the overall bias was small in 2010, for some groups using alkaline picrate (AP) methods it became significantly negative in 2011. The performance markedly worsens at creatinine physiologic concentrations, where a significant positive bias (up to ~20%) is still present for most of the AP-based analytical systems. Unexpectedly, with few exceptions, no evident improvement in individual assay bias was noted from pre- (2006) to poststandardization (2011) periods. The enzymatic method groups were the only always presenting an acceptable bias for all concentration levels, in addition to showing the lowest between-laboratory variability. The number of laboratories using enzymatic methods, however, still remains only 7% of the total. Conclusions: Our EQAS performance data indicate that most of the current “standardized” creatinine methods based on AP reaction do not perform well, mainly at the lower creatinine concentrations. This inaccuracy of creatinine measurements can adversely impact the estimation of glomerular filtration rate by equations and the evaluation of kidney function in pediatrics.