Six Sigma is a more efficient way to control quality, but the lack of TEa targets for many analytes and the sometimes inconsistent TEa targets from different sources are important variables for the interpretation and the application of Sigma metrics in a routine clinical laboratory. Sigma metrics are a valuable means of comparing the analytical quality of two or more analyzers to ensure the comparability of patient test results.

Although manufacturers are compelled by the European IVD Directive, 98/79/EC, to have traceability of the values assigned to their calibrators if suitable higher order reference materials and/or procedures are available, there is still no equivalence of results for many measurands determined in clinical laboratories. The adoption of assays with metrological traceable results will have a significant impact on laboratory medicine in that results will be equivalent across different laboratories and different analytical platforms. The IFCC WG on Allowable Errors for Traceable Results has been formed to define acceptable limits for metrological traceability chains for specific measurands in order to promote the equivalence of patient results. These limits are being developed based on biological variation for the specific measurands. Preliminary investigations have shown that for some measurands, it is possible for manufacturers to assign values to assay calibrators with a measurement uncertainty that allows the laboratory enough combined uncertainty for their routine measurements. However, for other measurands, e.g., plasma sodium, current assays are too imprecise to fulfil limits based on biological variation. Although an alternative approach based on probability theory is being investigated, the most desirable approach would be for industry to improve measurement methods so that they meet clinical requirements.

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A semiautomated fluorescence polarization immunoassay (FPIA) for total T3 performed using the TDx analyzer has been evaluated. Precision of the assay at a low level of T3 (< 1 .O ng/mL) is not good with CVs of about 17% observed. Precision at moderate to high levels (1.7-5.6 nglmL) is good (CVs = 11.3-4.1 O/O). Low end imprecision is attributable to a lack of sensitivity, with a reliable detection limit of only about 0.9 nglmL. An average recovery of added T3 of 96% was noted, and the assay is not affected by interference due to lipemia, bilirubinemia, or hemolysis. A methods cornparison study of the FPIA test and total T3 by RIA yielded a regression equation of Y = 1.08 x -.28, r = 0.95. The TDx FPIA total T3 test is a quick, semiautomated procedure, offering the advantage of a stable, stored calibration curve. It is useful for quantitating elevated levels of total T3 as found in hyperthyroid conditions, and its performance is comparable to or better than RIA procedures for this purpose. However, it cannot be recommended for the evaluation of patients with T3 values below 1.0 nglmL, and RIA tests are still preferable for this analytical range.