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

A peptide content determination commissioned by the Journal on a 15 mg tirzepatide vial from WWB has returned 96% of the labelled mass. Chromatographic purity on the same vial was above 97.4%. The two numbers measure different things and only one of them was on the certificate.

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 96% 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.

Rowena Ashcombe, community pharmacist and diabetes educator, 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 date of the request for comment. The dossier’s "what we could not verify" box records the questions still outstanding.