Integration is where purity is decided

The index published its first uncertainty budget for a purity figure in 2026-Q1, and the exercise reordered the priorities.

The budget

Combined standard uncertainty for a well-resolved main peak comes out at 0.24 pp, giving an expanded uncertainty of roughly 0.48 pp at 95 % confidence. With an unresolved shoulder it roughly doubles to 1.0 pp.

Three conclusions

1. Differences under 0.3 pp are not findings. They are inside the expanded uncertainty of either measurement. The index does not treat them as findings and readers should not either.
2. The empirical and theoretical numbers agree. Measured inter-lab limits of agreement run ±0.44 to ±0.52 pp for the high-volume laboratories, close to the calculated expanded uncertainty. That agreement is reassuring about both.
3. Rule AX-R05's threshold is justified. Its 1.5 pp trigger is roughly three times the expanded uncertainty of a well-behaved measurement, which is why the index is willing to call a divergence above it a finding.

Rule AX-R12

The largest controllable term in the budget is the integration boundary on an unresolved shoulder, so the index added a rule for it: peak asymmetry above 1.50, or resolution below 1.20 to the nearest named impurity. Either condition makes the reported purity a range rather than a number, and the record is marked integration-limited.

Tirzepatide accounts for a disproportionate share of instances, because its des-Aib2 impurity elutes 0.35 min early and frequently appears as an unresolved leading shoulder. Retatrutide and the other multi-Aib analogues follow for the same reason.