Beyond total expanded uncertainty: Budget composition governs grey zone and reference change value in anti-TPO...

ISO/TS 20914:2019 yields total expanded uncertainty (U%) without decomposing its components. Grey zone requires the full budget, including systematic uncertainty; RCV_increase requires imprecision alone, presupposing knowledge of budget composition. The MU budget of anti-TPO antibody testing across four Atellica IM modules was quantified over two assay generations (aTPOI and aTPOII) using an extended approach that supplements ISO/TS 20914:2019 with an EQA-derived systematic uncertainty component (u_sys) per Nordtest TR 537. u_Rw, EQA-derived u_sys, and u_cal were combined in quadrature; within-IQC-lot CVs pooled by root-mean-square (2 independent cycles; aTPOI: 15 samples, aTPOII 8 samples; 8 months IQC). Grey zones derived using the overlap-based approach (C1+U%/100 and C1-U%/100,k = 2) and u_sys-exclusive U%. Upward logarithmic reference change value (RCV_increase) (Z = 1.645) from u_Rw and CV_I. aTPOII lower combined u_Rw at Lv1 (8.27% vs 10.30%) but higher at Lv2 (6.86% vs 4.82%). RMS_dev increased (3.78% → 7.04%), and u_ref increased (0.74% → 2.99%), making u_sys the co-dominant component in aTPOII (Lv1: 40.2%; Lv2: 43.4%). U% was comparable at Lv1 (24.96% vs 24.11%) but diverged at Lv2 (13.23% vs 23.21%). Excluding u_sys narrowed grey zone widths by 5.5% and 24.5% (normalized: 53.2% and 51.2%); RCV_increase: 37.9-42.7% and 33.4-38.5%. Budget composition differed between generations despite comparable U% at Lv1. Knowledge of composition is a prerequisite for correct grey zone and RCV_increase derivation. Nordtest TR 537 supplementation addresses this gap. Uncertainty budget decomposition into systematic and random components is a practical step during assay generation transitions; EQA-based peer-group deviation estimation may further characterize u_sys where material commutability can be established.