Abstract
In clinical laboratories, the performance of a process as Sigma Metric (SM) is calculated by the equations derived by Westgard. In the present study, we found that the Westgard equations do not reflect the real performance of the process and that the SM calculated using these equations is lower than the real SM. We measured the substance concentration of ten analytes (glucose, urea, creatinine, cholesterol, calcium, magnesium, phosphorus, LDH, sodium, and potassium) in serum and calculated the SM for each using the Westgard equations and z transformations. The SM values for the same measurand using the Westgard equations based on either the absolute or the relative (percentage) results were not equal to each other, and those related to calcium and sodium were even lower than 0. The SM obtained from the z transformation was higher than that from the Westgard equations, and none were lower than 0. We concluded that the equations suggested by Westgard to calculate the SM do not cover all of the data produced by the process and do not reflect reality. From our research, the z transformation was the optimum method of calculating the actual SM for the process.
This is a preview of subscription content, log in via an institution to check access.
Similar content being viewed by others
References
Klee GG, Westgard JO (2012) In: Burtis CA, Ashwood ER, Bruns DE (eds) Tietz textbook of clinical chemistry and molecular diagnostics, 5th edn. Elsevier Saunders, St. Louis
Schoenmakers CHH, Naus AJM, Vermeer HJ, van Loon D, Steen G (2011) Practical application of Sigma Metrics QC procedures in clinical chemistry. Clin Chem Lab Med 49:1837–1843
Harry SR (2000) Six sigma: the breakthrough management strategy revolutionizing the world’s top corporation. Currency, New York
Westgard JO, Westgard SA (2006) The quality of laboratory testing today. An assessment of σ metrics for analytic quality using performance data from proficiency testing surveys and the CLIA criteria for acceptable performance. Am J ClinPathol 125:343–354
Fraser CG (2001) Biological variation: from principles to practice. AACC Press, Washington
Dawson B, Trapp RG (2004) Basic and clinical biostatistics, 4th edn. Lange Medical Books/McGraw-Hill, New York
Author information
Authors and Affiliations
Corresponding author
Additional information
Papers published in this section do not necessarily reflect the opinion of the Editors, the Editorial Board and the Publisher.
A critical and constructive debate in the Discussion Forum or a Letter to the Editor is strongly encouraged!
Rights and permissions
About this article
Cite this article
Coskun, A., Serteser, M., Kilercik, M. et al. A new approach to calculating the Sigma Metric in clinical laboratories. Accred Qual Assur 20, 147–152 (2015). https://doi.org/10.1007/s00769-015-1113-8
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s00769-015-1113-8