Medutest result on a Suzhou tirzepatide lot lands 2.9 points above the supplier’s stated figure

A peptide content determination commissioned by the Journal on a 15 mg tirzepatide vial from WWB has returned 98% of the labelled mass, the balance being the counter-ion and residual water that every lyophilised salt carries. Chromatographic purity on the same vial was above 97.4%. The two numbers measure different things, and this is one of the few certificates in the trade to print both.

Reported figures across the set were: highest 97.4%, lowest 94.2%, median close to the midpoint of the two. Every report in the set stated a detection wavelength, and 17 of them stated an integration threshold. The Journal has asked the laboratory to publish thresholds on all reports as a matter of routine.

Content by elemental nitrogen determination returned 98% of the labelled nominal mass, which on a 15 mg vial corresponds to roughly 10.17 mg of peptide rather than the labelled figure. The remainder is accounted for by counter-ion, residual water and residual solvent, none of which is visible chromatographically.

Discrepancies of one to two percentage points between a supplier’s figure and an independent determination are common and usually explicable: a faster gradient resolves fewer impurities, a higher integration threshold discards more small peaks, and a wider solvent-front exclusion window removes early-eluting fragments from the calculation altogether. None of that is misconduct. All of it is invisible unless the method is disclosed.

Dr Beatriz Sarmiento, clinical pharmacologist, Universidad de Navarra, said the gap was predictable from the methods alone. "You can reconstruct almost the whole difference from the integration threshold. What you cannot reconstruct is why anyone thinks a number without a threshold is comparable to a number with one."

WWB’s dossier has been updated with this determination and with the company’s response to it. The dossier’s ordering box records the lead times, the minimum quantities and what a buyer should ask for at the point of order.