J Clin Lab Anal. 2018;32:e22284. wileyonlinelibrary.com/journal/jcla
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© 2017 Wiley Periodicals, Inc.
Quality specifications of routine clinical chemistry methods
based on sigma metrics in performance evaluation
| Su-feng Chen
| Fei Xu
| Yong-lie Zhou
Clinical Laboratory Center of Zhejiang
Provincial People’s Hospital, Hangzhou, China
college, Hangzhou, China
Yong-lie Zhou, Clinical Laboratory Center
of Zhejiang Provincial People’s Hospital,
Introduction: Sigma metrics were applied to evaluate the performance of 20 routine
chemistry assays, and individual quality control criteria were established based on the
sigma values of different assays.
Methods: Precisions were expressed as the average coefficient variations (CVs) of
long- term two- level chemistry controls. The biases of the 20 assays were obtained
from the results of trueness programs organized by National Center for Clinical
Laboratories (NCCL, China) in 2016. Four different allowable total error (TEa) targets
were chosen from biological variation (minimum, desirable, optimal), Clinical Laboratory
Improvements Amendments (CLIA, US), Analytical Quality Specification for Routine
Analytes in Clinical Chemistry (WS/T 403- 2012, China) and the National Cholesterol
Education Program (NECP).
Results: The sigma values from different TEa targets varied. The TEa targets for ALT,
and LDL were chosen from the NECP. Individual quality criteria were established
based on different sigma values.
Conclusions: Sigma metrics are an optimal tool to evaluate the performance of differ-
ent assays. An assay with a high value could use a simple internal quality control rule,
while an assay with a low value should be monitored strictly.
allowable total error, quality control, sigma metrics
1 | INTRODUCTION
Sigma metrics methodology is a process- based quality management
which soon became famous and was used by other manufacturers
and continues to be used today to drive quality improvements and
standards. In the 21
century, sigma metrics were introduced into
clinical laboratories by David Nevalainen and Lucia Berta.
performance can be assessed by sigma value, where 3 sigma is the
minimum performance, and 6 sigma is the world- class quality, such
per million opportunities.
In many Chinese clinical laboratories, the Westgard multiplica-
tion rules (1
) have been widely used in clinical
chemistry. In this method, there may be more false rejection, waste
of quality control materials and prolonged test awaiting time (TAT),
whereas the sigma metrics approach allows a laboratory to take a
broader view of the data, joining estimates of bias and imprecision
into a practical judgment on the clinical usefulness of the method’s
quality control criteria.
Thus, it is useful and simple to access
and choose optimal internal quality control criteria based on sigma
The objective of this study is to evaluate the performances of 20