Apresentado no Congresso EUROMEDLAB, Innsbruck Austria 2009

1. A PROPOSED APPROACH TO ANALYTICAL METHOD
LONG-TERM STABILITY ASSESSMENT
BERLITZ, F.
Weinmann Laboratório - Porto Alegre, Brazil
Poster Code: W-B022
fberlitz@weinmann.com.br
ABSTRACT RESULTS
IMPRECISION ASSESSMENT
Background: Although efficient to evaluate the current assay performance, traditional QC approaches
often fail to assess the long-term method stability. The aim of this study was to propose an alternative
approach to assess the long-term performance of analytical methods.
Methods: We used the QC data for OCD Vitros FS 5,1 glucose assay. To access the method
imprecision, we used internal QC data, in a period of 4 months. These data were plotted in control
charts, with limits defined using the first 20 results and specification of 6.9%. To assess changes in
results using different reagents lots and calibrations, was generated an EWMA control chart. To
access the long-term method inaccuracy, we used PT data from CAP program. To evaluate the long-
term inaccuracy, a control chart was generated using SDI obtained from PT samples. The control
chart were analyzed using traditional control limits and rules. Aiming to access the long-term
systematic error (SE), the glucose results obtained in the laboratory were compared to the peer group
means, using the least squares regression. The obtained regression model was used to estimate the
SE at different medical decision levels.
Results: Long-term performance of studied assay was considered stable in most of the time, except
when using the 3rd reagent lot (Internal QC) and for the 3rd sample of the first PT kit and for the first
sample of second PT kit (Proficiency Testing).
Conclusions: The evaluated method met the quality specifications for their clinical use. The proposed
approach for the long-term evaluation of laboratory tests was useful and effective to evaluate the
method performance and the long-term assay stability.
Internal QC data showed that the long-term performance of the studied assay was stable in most of the time, except when using the 3rd reagent (“out of control” event presented
INTRODUCTION in both charts). The EWMA chart, using control limits adjusted for each reagent lot proved to be more efficient to evaluate changes in long-term method performance.
Quality Control strategies in clinical laboratory aim to track the analytical method performance,
INACCURACY ASSESSMENT
evaluating their performance against required quality performance characteristics. Basically, 2
performance characteristics are evaluated: imprecision and inaccuracy. The imprecision is evaluated
through internal QC daily routine, while the inaccuracy is commonly accessed by Proficiency Testing
(PT).
Although efficient to evaluate the current assay performance, traditional QC approaches often fail to
assess the long-term method stability. The evaluation of long-term performance is important to assess
the method stability under different conditions of analysis, as different reagents lots or different
technicians performing the test, for example. This long-term stability allows the consistency between
different laboratory results for the same patient obtained at different times, preserving the appropriate
clinical significance of these results for the medical decision.
OBJECTIVES Proficiency Testing (CAP) data showed that the long-term inaccuracy of the studied assay was stable in most of the time, except for the 3rd sample of the first PT kit and for the
first sample of the second PT kit (“out of control” event presented in SDI control chart). The systematic error (inaccuracy), obtained using the regression model from PT
samples, was less than 1% in all glucose medical decision points.
The aim of this study was to propose an alternative QC approach to assess the long-term performance
of analytical methods in clinical laboratory.
OVERALL PERFORMANCE ASSESSMENT
METHODS
To validate this new proposed approach we used the QC data for Glucose assay performed in OCD Vitros FS
A Medical Decision Chart for OCD Vitros FS 5,1 was generated using
5,1 equipment. To access the method imprecision, we used daily internal QC data, obtained using the control imprecision and inaccuracy data obtained from Internal QC and PT
material (Level I) provided by the equipment supplier, assayed between September and December 2008 (n = program, respectively. The long-term overall method performance was
greater than 4-sigma.
119). These data were plotted in control charts (Levey-Jennings), with control limits defined using the first 20
control results and specification limits (plotted in the same graph) of 6.9% (Ricos, 1999). To assess possible
changes in the assay results using different reagents lots and calibrations, was generated an EWMA time-
weighted control chart (weight: 0.5), with non-fixed data of mean and SD, different control limits for each
reagent lot used and specification limits considering the mean for all reagent lots. To access the long-term
method inaccuracy, we used PT data (CAP, Kit C, 2007 to 2008). To evaluate the long-term method
inaccuracy, a control chart was generated using SDI (SD Index) obtained from PT samples. The control chart
were analyzed using traditional control limits and control rules. Aiming to access the long-term systematic error CONCLUSIONS
(SE) of the method, the glucose results obtained in the laboratory to PT samples were compared to the peer
group means, using the least squares regression approach. The obtained regression model was used to During the studied period, the evaluated method met the quality specifications for their clinical use. The
estimate the SE at different medical decision levels to Glucose. Additionally, the imprecision and inaccuracy proposed approach for the long-term evaluation of laboratory tests, using various statistical tools, was useful
and effective to evaluate the method performance and the long-term assay stability, while ensuring the validity
and consistency between different results for the same patient at different times and clinical conditions.
were used to assess the overall method performance, using a “Medical Decision Chart”.