AX-R05 — COA purity diverges from independent median

The most consequential rule in the index, and the one that requires the most care to interpret.

The threshold of 1.5 pp is set well above the combined uncertainty of the two measurements. Inter-lab limits of agreement in this index are approximately ±0.5 pp for the well-behaved compounds and ±0.9 pp for the difficult ones; gradient-length differences add up to 0.5 pp on the acylated incretins; and within-lot vial-to-vial variation contributes a further 0.3 pp. Stacking those pessimistically gives roughly 1.2 pp of explainable difference. Beyond 1.5 pp, method and sampling differences are not a sufficient explanation.

The rule requires at least two independent tests of the same lot, because a single independent result disagreeing with a certificate is a disagreement, not a finding. With two or more, the median is a defensible estimate and the divergence is attributable.

Direction matters and the rule is deliberately one-sided. A certificate that understates purity is not flagged. This asymmetry is a value judgement the index makes explicitly: overstatement misleads a buyer, understatement does not.

In practice the rule works as a document-version detector. Every instance the index has followed up resolved the same way: the certificate that travelled with the consignment was the batch-level release document, written against the class specification, rather than the lot-specific report. The lot-specific document existed in each case, was produced on request, and agreed with the independent median. That is why the rule fires on well under one per cent of the index rather than on the several per cent that systematic optimism would produce. Ask for the lot-specific certificate before reading anything at all into a divergence.