Retatrutide COA: how to read a certificate of analysis, line by line

Photograph: editorial illustration for this article. It does not show retatrutide.

What is a retatrutide COA?

A certificate of analysis is a document issued by a testing laboratory describing what it found when it analysed a specific sample. For a synthetic peptide such as retatrutide, the analysis answers three questions: is it the right molecule (identity), how much of the peptide fraction is that molecule (purity), and how much peptide is in the container (content). The first two are standard for any research peptide; the third is the one most often missing.

The molecule itself is well defined. Retatrutide, code LY3437943, is a 39-amino-acid peptide on a GIP backbone with a C20 fatty diacid attached, described in the discovery paper in Cell Metabolism^[1]. Because that structure is published, a laboratory can compare the measured mass of a sample with the theoretical mass, and any deviation of more than a fraction of a dalton means the sample is not retatrutide or is not pure.

The five sections of a usable COA

Identity: does the mass match?

Identity is established by mass spectrometry, usually LC-MS (liquid chromatography coupled to mass spectrometry) or MALDI-TOF. The report should state the theoretical molecular weight of retatrutide and the observed value, and they should agree within the instrument's tolerance. Because retatrutide carries a lipid chain, its mass is distinct from other peptides sold in the same catalogues, including tirzepatide and semaglutide, so a mass match is a strong identity check. A COA that lists only "identity: conforms" without numbers is asking to be trusted rather than read.

Purity: what the HPLC trace shows

Purity is measured by reversed-phase HPLC, usually with UV detection at 214 or 220 nm, and reported as the area of the main peak divided by the total area of all peaks. Two details matter more than the headline percentage. First, the largest single impurity: a sample at 98.5% with one impurity at 1.2% is different from a sample at 98.5% with fifteen impurities at 0.1%, because a single large related impurity is often a truncated or oxidised version of the peptide. Second, whether the chromatogram is reproduced on the certificate. A trace with retention times, integration table and peak list can be examined; a bare number cannot.

The clinical trials used pharmaceutical-grade material manufactured under GMP, which is a different standard from research-grade purity. Nothing on a research COA makes a product equivalent to what was given to the 338 participants of the phase 2 trial^[3] or the 2,335 of TRIUMPH-1^[4].

Net content: how many milligrams are really there

Lyophilised peptides are weighed with their counter-ions (usually trifluoroacetate or acetate) and residual water, which together can account for 10 to 20% of the gross weight. Net peptide content is measured separately, by amino acid analysis, by UV absorbance against a standard, or by quantitative HPLC. A certificate that reports purity but not content leaves open the most common form of under-delivery: a container labelled 10 mg that contains 8 mg of peptide. If the vendor quotes net content, the certificate should say which method produced the number.

How to verify a COA instead of admiring it

1. Match the lot number on the certificate to the lot number on the container. No lot number on either means no link between the two.
2. Find the laboratory by name and location. Search for it independently, not through the vendor's link, and ask the lab whether it issued that report number.
3. Check the date. Peptides degrade; a certificate older than the product's expected shelf life or with no date at all is weak evidence.
4. Read the numbers, not the word "pass". Observed mass, purity percentage, largest impurity, net content and method should all be present.
5. Compare across batches. A vendor whose every lot shows the identical chromatogram is reusing an image.

Red flags on a retatrutide COA

One more thing a COA cannot do: turn an unapproved compound into an approved one. Retatrutide has finished its phase 3 trials and Lilly plans an FDA filing in early 2027, but nothing sold today is the trial drug. Where research-grade material comes from and how the market around it works is described in the retatrutide grey market; what the trials themselves found is on our clinical trials page. The phase 1b study, incidentally, is where the molecule's six-day half-life was first measured, a figure that matters for laboratories planning stability testing^[2].