To circumvent the problem of potential non commutability of calibrators in the reference measurement system, the C-STFT sees it as a conditio sine qua non to establish traceability by use of panels of commutable serum samples at the level of the "working calibrator" (14), (15). This implies that the panels, certified by a RMP or surrogate, are used as working calibrator of the manufacturers' master procedures. In addition, the C-STFT uses the "Step-Up" design (16). It essentially comprises a sequence of method comparisons with selected sets of commutable samples (17). The design allows to decide after each step whether the step-up to the next phase can be made, or is premature. It also lets investigate the intrinsic quality of participating assays and document assay consistency over time.
Progress in laboratory testing of thyroid disease