Re-test Period vs Shelf Life: What a COA Date Really Means
Share
A re-test date is not an expiry date. That sentence sounds like a technicality until you are standing in a warehouse with a drum that passed its re-test date last week, trying to decide whether to quarantine it or use it.
Here is what the distinction actually commits you to. A re-test period applies to an active substance. It is the period during which the material is expected to stay within specification when stored as stated, and once it is reached, a representative sample has to be re-examined before the material is used. The material has not been declared unusable. It has been declared unverified. A shelf life applies to a finished medicinal product, and it means the end of the line for that batch.
The distinction matters commercially because the two words get used interchangeably in supplier correspondence, and one of them is much cheaper for a buyer than the other.
Two dates, two different obligations
| Re-test period | Shelf life | |
|---|---|---|
| Applies to | Active substance, and intermediates | Finished medicinal product |
| What it means | The material remains within specification if stored as stated. Beyond the date, analysis is required before use. | The product may no longer be used. There is no route back in on the basis of retesting. |
| Can it be extended | Yes, for a specific batch, with supporting scientific data and long-term results for that batch. Extending the period for future batches needs its own data and may need a regulatory filing. | No, not without a variation. |
| Established by | Stability studies on the substance as made by that process | Stability studies on the product in its marketed container closure system |
Some companies assign an expiry date to an active substance instead of a re-test period. That is a legitimate choice rather than an error, and it is more conservative than it needs to be. It is also worth knowing which convention the supplier you are buying from has adopted, because it changes what you can do at the end of the period.
Where the numbers come from
Stability data for a new active substance is generated against a standard set of conditions, and the design is what allows a period to be proposed at all.
| Study | Storage condition | Minimum data at submission | Testing points |
|---|---|---|---|
| Long term | 25 °C ± 2 °C / 60% RH ± 5% RH, or 30 °C ± 2 °C / 65% RH ± 5% RH | 12 months | 0, 3, 6, 9, 12 months, then every 6 months to 24 months, then annually |
| Intermediate | 30 °C ± 2 °C / 65% RH ± 5% RH | 6 months | 0 and 6 months at minimum |
| Accelerated | 40 °C ± 2 °C / 75% RH ± 5% RH | 6 months | 0, 3, 6 months |
Three details in that table are worth pulling out.
The applicant chooses between the two long-term conditions. Both are acceptable under the guideline, and the choice signals the markets a supplier is prepared to support. Where a product is destined for hot and humid climates, the long-term condition is normally set at 30 °C / 75% RH, which comes from regional guidance rather than from Q1A(R2) itself. The ICH guideline as written addresses climatic zones I and II, meaning temperate and Mediterranean. If your market is a zone III or IV country, the stability package that supports a period derived under Q1A(R2) conditions may not be the package your registration needs.
There is no intermediate condition if you choose 30 °C / 65% RH as the long-term condition. The intermediate study exists purely as a bridge when the long-term condition is 25 °C / 60% RH and something goes wrong under accelerated conditions. Reading a stability summary without knowing which long-term condition was chosen makes the rest of the table uninterpretable.
Three primary batches is the usual minimum. For a substance supplied from a single process, that is straightforward. It is also the reason a change of manufacturing site is not a paperwork formality, which is a point we cover in what a second source involves.
What counts as significant change
For a drug substance the definition is blunt: significant change is failure to meet the specification. There is no percentage trigger and no judgement call. That is a useful asymmetry to remember, because the drug product side of the same guideline uses a set of threshold criteria, including a five per cent change in assay from the initial value and any degradation product exceeding its acceptance criterion.
The consequence is structural. If significant change happens under accelerated conditions while the long-term study is running at 25 °C / 60% RH, intermediate testing gets triggered, and the proposed re-test period has to be justified against the intermediate data as well. A stability package where everything held at accelerated conditions is the easy case. The interesting ones are the packages where something moved and the company can show you exactly how it reasoned from there.
What the date obliges you to do
European GMP guidance is explicit that only starting materials released by quality control and within their re-test period may be used. That is a hard gate, and it is why the re-test date on a certificate is not decorative.
Two practical consequences follow for a buyer.
- Check the date against your production schedule, not against today. Material that arrives with eleven months left on a two-year period is a different proposition from material that arrives with three. The date that matters is the date on which you will consume it, plus whatever safety margin your own procedures require.
- Ask for the stability summary, not just the period. A supplier who states a re-test period without being able to describe the study behind it is passing on a number. The summary is usually a page: conditions, batch numbers, time points, and the parameters that were tested.
Why two suppliers quote different periods for the same molecule
Two suppliers can sell material that passes the same pharmacopoeial tests and state different re-test periods, and neither is lying. The period describes the stability of a substance as made by a particular process. A different route, a different purification, a different residual profile or a different particle size distribution can all shift it.
There is a trap in reading this as a quality ranking. A shorter period does not automatically mean worse material; it can mean a more conservative stability programme, an earlier stage of data accumulation, or a company that has not yet run the long-term study out far enough to propose anything longer. What a shorter period does reliably tell you is something about the stability package, and a stability package is a fair thing to ask about.
One more limitation is worth stating plainly, because buyers occasionally try to use a supplier's stability data for something it cannot support. Active substance stability data cannot substitute for the stability evidence a finished medicinal product requires. A stable substance tells you nothing about how it behaves once it is formulated with excipients in a particular container. If your registration needs product stability data, the substance package is an input rather than an answer.
Five questions worth putting to a supplier
- Does your certificate state a re-test period or an expiry date?
- Which long-term storage condition was the period derived from?
- How many batches supported it, and at what scale?
- What does the accelerated data show, and did anything trigger intermediate testing?
- What is your change-notification commitment if the period is revised downward?
The last question is the one that separates a partner from a vendor. A re-test period is a live number. It can be extended, and it can be shortened, and a supplier that has thought about the second case has thought about what happens to your stock if it does.
Frequently asked questions
Is a re-test date the same as an expiry date?
No. A re-test period applies to an active substance: the material is expected to remain within specification during that period, and a representative sample must be re-examined before use once the date is reached. A shelf life applies to a finished product and cannot be extended by retesting.
Can a re-test period be extended?
Yes, for a specific batch, where extension is supported by scientific knowledge, long-term stability information and testing of that batch stored under its labelled conditions. Extending the period for future batches requires its own supporting data and may require a regulatory filing.
What storage conditions are used in a stability study?
Under ICH Q1A(R2), long-term testing runs at 25 °C / 60% RH or 30 °C / 65% RH for at least 12 months, accelerated testing at 40 °C / 75% RH for 6 months, with an intermediate condition at 30 °C / 65% RH used when significant change occurs under accelerated conditions.
What is significant change for a drug substance?
Failure to meet the specification. Unlike the drug product definition, there is no percentage threshold for a drug substance.
Can expired material be retested and released?
For an active substance within a re-test period framework, material past its re-test date can be re-examined and released if it still conforms. Material past the shelf life of a finished product cannot be dealt with that way.
Why do suppliers state different re-test periods for the same substance?
Because the period reflects the stability of the substance as produced by a particular process. Different routes, purification steps or physical properties can produce different periods without either result being inaccurate.
The question that surfaces the stability package
Ask what the accelerated data showed. Everyone can quote a re-test period. The supplier who can tell you what happened at 40 °C, and what they did about it, is the one whose period you can actually rely on.
The primary texts behind this piece are ICH Q1A(R2), the ICH quality guidelines including Q1E on the evaluation of stability data, and ICH Q7 for the re-test date convention on active substances.
What happens to material once it arrives is covered in what to test when material arrives. What we ship against is on the ursodeoxycholic acid page, and specification questions for a specific quantity can be raised through a quotation request.
This article describes stability framework and date interpretation for quality, regulatory and procurement professionals. It states no therapeutic use, dose or indication for any substance, and no order quantity or lead time. No re-test period for any specific product is stated here; confirm against the current stability data for the material in question.