Why oldest first
Reagents expire. If newer stock is used while older stock sits behind it, the old stock expires unused and the laboratory pays twice. First-in, first-out also keeps lot-to-lot variation predictable for your quality control.
What a batch record should hold
At minimum: the item, the batch or lot number, the expiry date, the quantity received, the date received and the supplier. With those, every result can be traced to the batch that produced it.
Traceability when something goes wrong
When a supplier issues a recall, you need to know within minutes which tests used the affected batch. That is only possible if consumption is recorded against batches, not just against items.
Expiry as a process, not a surprise
Expiry alerts should reach the right person early enough to use, transfer or dispose of stock, and disposal should itself be recorded.
Common questions
FIFO uses the batch received first; FEFO uses the batch that expires first. For reagents the two usually agree, and expiry should win whenever they differ.
Anything that can be recalled or that expires benefits from it. Many laboratories track reagents and controls at batch level and bulk consumables at item level.