Sigma Metrics in Thyroid Testing: Striving for Perfection- An Observational, Cross-sectional Study

Abstract

Introduction: Six Sigma is a potent tool for evaluating the quality of the analytical phase by combining bias, imprecision, and Total Allowable Error (TEa). Considering the variation in TEa values from different sources, analysis on the sigma scale needs to be carefully monitored. Aim: To assess the performance of Thyroid Stimulating Hormone (TSH) and free Thyroxine (fT4) on the Sigma scale. Materials and Methods: The present observational, cross- sectional study Immunoassay laboratory at the Department of Biochemistry, IPGMER and SSKM Hospital, Kolkata, India, from February 2021 to January 2022. The study involves 12 months of tri-level (L1, L2, L3) Internal Quality (IQ) control data and External Quality Assessment (EQAS) data. The bias percentage was obtained from EQAS, and the Coefficient of Variation (CV%) was obtained from IQ Control (IQC) data run on Advia Centaur CP (CLIA) each month. Sigma (σ) was calculated applying TEa from the desirable biological variation database. Sigma values of L1, L2, L3 of TSH and fT4 have been calculated using Microsoft spreadsheet software version 2010, applying the formula σ= (TEa-bias)/CV. Results: The CV% and bias% were found to be within an acceptable range, always less than the cut-off percentage of imprecision (I%) and inaccuracy (B%) for TSH and fT4 in the desirable specifications for imprecision and inaccuracy (updated 2014). However, sigma levels are near the satisfactory mark, found to be <5σ over 17 months for TSH and 23 months for fT4, considering all sigma values of L1, L2, and L3 IQ levels. Better sigma values (>5σ) in tri-level IQ for TSH with a higher numeric TEa% value (23.7%) were obtained in more months, whereas for fT4 with a lower TEa% value (8%), better sigma values (>5σ) were obtained in a lesser number of months. Conclusion: The present study establishes that Sigma values are affected by the numeric values of TEa% of a particular parameter taken from the source. Sigma values showed average performance despite satisfactory CV% and bias% for fT4 and TSH, creating chaos in the laboratory’s operational routine. The Sigma matrix is a good indicator, but it is difficult to maintain a good sigma value for parameters that have low TEa%. It becomes crucial to choose appropriate TEa to plan a quality control strategy for thyroid hormones.