The reference values listed in Table S3 (see ESM) and Fig. 2 are based specifically on measured decadic total attenuance at 280 nm assuming a theoretical extinction coefficient (ε) of 1.42 (mL mg−1 cm−1) . The extinction coefficient was calculated according to the method reported by Pace et al. , and further corrected for glycan mass fraction via a correction factor of 0.977 . Uncertainty associated with the theoretical extinction coefficient has not been fully evaluated; therefore the reported value is not traceable to the SI unit of mass. A comprehensive uncertainty budget is reported in the ESM such that a future experimental determination of ε may be incorporated if deemed necessary via stakeholder feedback. The combined standard uncertainty provided in Table S3 (see ESM) is intended to represent, at the level of one standard deviation, the effect of the combined components of uncertainty including Type A measurement uncertainty and Type B components related to the analysis, consistent with the International Standards Organization/Joint Committee for Guides in Metrology (ISO/JCGM) Guide . Each of the three 14HB-D lots were measured to have a concentration to within ±2uc of one another (Fig. 2) which demonstrates that good inter-lot reproducibility was achieved. These data indicate the lifecycle management plan using bulk homogenization produced highly reproducible lots, and therefore future lots of RM 8671 are expected to be consistent with respect to concentration.Fig. 2Mean concentration determined for each lot of NISTmAb using UV-Vis spectrophotometry. Error bars represent ±2uc based on statistical treatment of data as described in ESM (n = 10 for RM 8671 lots, n = 2 for PS 8670)