Beyond total expanded uncertainty: Budget composition governs grey zone and reference change value in anti-TPO antibody testing over assay generations.

ISO/TS 20914:2019 yields total expanded uncertainty (U%) without decomposing its components. Grey zone requires the full budget, including systematic uncertainty; RCVincrease 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 (usys) per Nordtest TR 537. uRw, EQA-derived usys, and ucal 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 usys-exclusive U%. Upward logarithmic reference change value (RCVincrease) (Z = 1.645) from uRw and CVI. aTPOII lower combined uRw at Lv1 (8.27% vs 10.30%) but higher at Lv2 (6.86% vs 4.82%). RMSdev increased (3.78% → 7.04%), and uref increased (0.74% → 2.99%), making usys 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 usys narrowed grey zone widths by 5.5% and 24.5% (normalized: 53.2% and 51.2%); RCVincrease: 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 RCVincrease 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 usys where material commutability can be established.