Storage life versus expiry dating

Two dates can apply to the same material, and they come from different places. One is printed on the container by whoever made it. The other is assigned by the laboratory that uses it. Confusing the two is a common finding when an assessor walks through a stockroom, because the printed date is easy to see and the other is often not written down at all.

What an expiry date means

An expiry date marks the end of the period during which a material is expected to stay within its specification, provided it has been stored as the label states. It comes from stability data: the maker has stored samples under defined conditions, tested them at intervals, and set the date at a point the data support. ICH Q1A(R2) describes how such studies are designed for pharmaceutical substances and products, and similar principles are applied to reagents and reference materials. ISO 17034, the standard for reference material producers, expects producers to assess stability and to tell users the period over which the certified values remain valid.

Two conditions are built into every expiry date. The first is that the container is unopened. The second is that storage has matched the label. Opening the container, or storing it outside the stated conditions, takes the material out of the situation the date was based on.

Retest dates are different again

Some materials carry a retest date instead of an expiry date. ICH Q1A(R2) uses this idea for drug substances: a retest period during which the material is expected to stay within specification, after which it should be tested again before it is used. A retest date is a checkpoint rather than an end. A material that passes the retest can be used, and the result is recorded. An expiry date is the end: after it, the material is not used.

What a storage life means

A storage life, sometimes called an in-use period or opened life, is the laboratory's own limit for a material after something has changed: a container has been opened, a powder has been dissolved, a stock solution has been diluted, or a portion has been moved to a new container. The maker's date no longer describes that material in that state, so the laboratory has to assign a new one.

The new date can come from three places. The maker may publish a stability statement for the opened or prepared material. The laboratory may generate its own data, by testing a prepared solution at intervals until it drifts. Or, without data, the laboratory's procedure may set a conservative default for each type of preparation. What matters is that the source is written down, so the date can be defended when someone asks.

Rules that apply to both

A few rules hold whichever kind of date is in play:

• A prepared material cannot outlast what it was made from. A solution expires on the earliest of its own assigned date and the dates of every component used to make it.
• A date is valid only under the stated storage condition. A temperature excursion can end it early, and the excursion record should say so.
• Dates are not extended without evidence. Relabeling a material with a later date needs data, a documented decision and the name of the person who made it.
• The procedure should say how a date given only as a month and year is read, so that everyone reads it the same way.

What the label has to carry

Because the storage life is the laboratory's own decision, making it visible is the laboratory's job. A prepared solution or opened container should be labeled with its identity, its concentration where relevant, the date it was prepared or opened, the person responsible, the storage condition, and the date after which it is not used. The same details belong in the preparation record, which should also link the preparation to the lots it was made from.

An assessor checking this will often pick a bottle from the shelf and ask three questions: when was it opened, what is its limit, and where does that limit come from. If the label answers the first two and the procedure or stability file answers the third, the material is in order. A container with a maker's date and nothing else, sitting open on a shelf, answers none of them.