Beyond the basics: Sigma scores in laboratory medicine with variable total allowable errors (TEa).

Area of Science:

• Clinical Chemistry
• Laboratory Quality Management
• Medical Diagnostics

Background:

• Clinical laboratories rely on accurate results for diagnosis and treatment monitoring.
• Internal quality control (IQC) and external quality assurance schemes (EQAS) evaluate laboratory performance.
• Sigma metrics, calculated using bias, CV%, and Total Error Allowable (TEa), are key error indicators.

Purpose of the Study:

• To determine sigma scores for 20 routine chemistry parameters using six different TEa sources.
• To assess the impact of TEa source variability on sigma metric evaluation.
• To develop an internal quality control plan for underperforming parameters.

Main Methods:

• Analyzed 20 routine chemistry parameters over 12 months using IQC (CV) and EQAS (bias).
• Calculated sigma scores using the formula: Sigma = (TEa - Bias%) / CV%.
• Compared sigma values derived from six distinct TEa sources (CLIA'88, CLIA'24, BDV, RCPA, RiliBak, EMC/Spain).

Main Results:

• Sigma values varied significantly across different TEa sources.
• CLIA'88, RiliBak showed more liberal TEa goals, while RCPA and Biological Variation were stricter.
• Identified error sources for low sigma scores, including reagent aging and instrument maintenance, and implemented a quality control plan.

Conclusions:

• The choice of TEa source substantially influences sigma metric evaluation.
• Harmonization and standardization of sigma metrics are necessary for reliable laboratory performance assessment.
• Establishing global consensus on TEa goals requires international collaboration among experts.