2007, Number 1
Rev Mex Patol Clin Med Lab 2007; 54 (1)
SIX SIGMA: determination of analytical goals based upon biological variability and technological evolution
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ABSTRACTBackground: SIX SIGMA appeared in the industry in 1979 in order to obtain quality improvement manufacture processes to a level of 3.4 defects per million units (DPMU). It involves all the system from agreed goals establishment with customer’s requirements, to statistical data measurement, team work, and continuous quality improvement. Objective: To briefly review and document SIX SIGMA principles and tools in order to evaluate its applicability on the clinical laboratory, including the importance of comparing the impact of SIX SIGMA level with pre-established analytical goals according to the analytical coefficient of variation of Tonks and Aspen that have been used in clinical laboratories for decades as precision indicators. Material and methods: This is a revision of the basic concepts and the fundamental methods of SIX SIGMA, including definitions, formulas and practical exercises and examples on the indicators of analytical variability based on pre-established reference limits and ranks of biological variability for the analytical control of quality according to Tonks, Aspen and SIX SIGMA criteria, emphasizing the applicability of the relative coefficient of variation CVR as practical tool for analytical goal management on any laboratory test. Results: The criterion of Tonks is equivalent to25.0% of the normal rank representing a biological standard deviation. The Aspen criterion is equivalent to 50% of Tonks level representing 12.5% of the normal rank. In order to reach SIX SIGMA level it is necessary to improve Tonks level precision in six times, which implies to reduce the analytical variability to 4.2% of biological variability. In order to evaluate the relation that exists between biological variability (BV) and the analytical variability (AV) it is recommended to calculate quotient AV / BV which will give a result of ‹ 1.0 according to Tonks, ‹ 0.50 according to Aspen and ‹0.17 when SIX SIGMA level is reached. Discussion: The establishment of attainable and challenging analytical goals is the first step in any quality control system. The relative coefficient of variation calculation allows laboratories a reliable and easy approach on analytical goal establishment for any analyte with the only condition of having suitable reference limits for the attended population and the calculation of the analytical coefficient of variation for the specific test. The advance in quality improvement depends on the establishment of the actual level in which the laboratory is at the present moment. In order to improve quality it is necessary to improve good laboratory practices and to elevate the automation level of the laboratory. The achievement of analytical goals will be different depending on the level in which they are applied in such a way that the results that are obtained in the Internal Program of Control of Quality should be better than those than are reached on External Evaluation Quality Schemes, because for statistical reasons the confidence intervals vary inversely depending on the number of variables that take part in the process.