Retest date and expiry date on a certificate of analysis

A certificate of analysis for a peptide batch carries several dates, and two of them cause most of the confusion we see: the retest date and the expiry date. They sound alike. They are often read as one and the same. They answer different questions, and the difference decides what we do with a vial once the date has passed.

The dates on a certificate

Before the two difficult ones, the easy ones. A certificate normally states a date of manufacture, which is when the batch was produced, and a date of analysis or of release, which is when the reported measurements were made or signed off. These two fix the data in time. Every figure on the page describes the batch as it was on the date of analysis.

The remaining date looks forward. It says for how long the manufacturer is prepared to stand behind those figures, under the stated storage conditions. That is where the two terms part ways.

What a retest date says

A retest date is the date after which the material should be examined again before use. It is not a statement that the material has degraded. It is a statement that the manufacturer’s evidence runs out at that point. Up to that date, the supplier has stability data, or reasoned experience, suggesting that a batch stored as instructed still meets its specification. After it, nobody has checked.

The consequence is in the name. Material past its retest date can be tested again. If it still meets the specification, it can be assigned a new date and used. The original certificate stays valid as a record of the batch at release, and the new measurement is added to it.

What an expiry date says

An expiry date is stricter. It marks the end of the period in which the material is expected to stay within specification, and after it the material is not meant to be used, retested or otherwise. Expiry dates are the rule for finished products with a defined shelf life. For bulk substances and reference materials, retest dates are more common.

For research peptides, practice varies. We have seen certificates with a retest date, certificates with an expiry date, certificates that give only “store at −20 °C” and a date of manufacture, and certificates with no forward-looking date at all. We do not think there is one convention, and we would be wary of anyone who claims there is.

The condition attached to both

Neither date means anything without the storage conditions printed next to it. A retest date of two years at −20 °C, protected from light and moisture, describes a sealed vial kept exactly so. It says nothing about a vial that spent a summer week in a parcel depot or a month on a fridge door.

This is the point at which the certificate and our own records have to meet. The supplier vouches for the batch up to the moment it leaves their hands. From receipt onwards, the storage history is ours to document. If we cannot show how a vial has been kept, the date on the certificate no longer applies to it in any strict sense, however far in the future it lies.

How we read the date in practice

On receipt, we follow the same four steps for every batch.

1. We find the forward-looking date and note which kind it is, in the supplier’s own wording.
2. We copy it into the batch record together with the storage conditions it is tied to.
3. We check that our storage location actually provides those conditions.
4. We set a reminder one month before the date.

When the reminder arrives, the decision depends on the kind of date and on how much material is left. For a retest date with several vials remaining, we run one vial through HPLC and compare the chromatogram with the one on the certificate: main peak at the expected retention time, purity within the specification, no new peaks of any size. If the comparison holds, we record the result, set a new internal date and carry on. If it does not, the batch is withdrawn.

With one vial left, retesting would consume what we are trying to qualify, so the vial is simply retired. For an expiry date, there is nothing to decide.

When no date is given

The awkward case. A certificate with a date of manufacture and nothing else leaves the judgement to us. We ask the supplier first, in writing, whether a retest period exists for that product; quite often it does and was merely left off the document. If the answer is no, we set a conservative internal retest interval counted from the date of manufacture, not from the date of receipt, and we write down that the interval is ours and not the supplier’s.

We would rather admit that the figure is a convention than pretend it is knowledge. Lyophilised peptides differ a good deal in how well they keep, depending on sequence, and a single interval cannot be right for all of them.

For readers who want the document as a whole, PEPTIDÁE has published an article on the certificate of analysis for research peptides, in Romanian. And when we look at any catalogue entry, for example a BPC-157 product page from the same catalogue, the questions we take to the batch documents are the ones above: which date, of which kind, under which conditions.

One line in the record

In our batch table the two terms have separate columns, and only one of them is ever filled for a given batch. It looks pedantic. It prevents the single most common mistake, which is someone reading “retest” as “expiry” and discarding usable material, or reading “expiry” as “retest” and keeping what should have gone.