Transferability of clinical laboratory data within a health care region

Analytical data for S-Creatinine and S-Urate are presented from seventeen laboratories in the Swedish Uppsala-Orebro regional quality assessment program. The bias and imprecision as well as the instability of the measurement procedures in the participating laboratories were estimated over three 14-week periods. Bias was estimated by a linear least squares fit of the difference between measured and assigned values vs. assigned values, and expressed in absolute and relative terms. Instability of the measurement procedures was estimated by comparing slope and intercept of regression lines of measured vs. assigned values from three fourteen week periods. According to our experiences we recommend regression analysis to describe the performance of the analytical methods of a laboratory over time. The results show that most laboratories fell within the limits of +/- 15% bias for S-Creatinine above 100 mumol l-1 and +/- 17% for S-Urate at concentrations above 250 mumol l-1. Various steps to reduce the inter-laboratory variability are suggested, including numerical correction of individual laboratory results using correction functions. In a few laboratories, instability was too high to allow for numerical corrections of analytical results.