The most sophisticated laboratory result is worthless if the defence can argue the sample might have been swapped, altered or contaminated on its way there. That is what the chain of custody exists to prevent: a documented, unbroken record of every person who handled an exhibit, when, and why — from the scene to the courtroom.
What it actually is
Chain of custody is a chronological paper (and digital) trail. For each item it records:
- who collected it, and when and where,
- a unique exhibit number and description,
- every transfer of the item between people or storage, signed and dated,
- where it was stored, and under what conditions.
Why it matters
Its single job is to prove continuity and integrity: that the item analysed in the lab is the same item recovered at the scene, unaltered. Without that, a court cannot be sure what the result actually relates to. A solid chain lets powerful evidence stand; a broken one can see it excluded or its weight sharply reduced.
How integrity is maintained
Good practice is practical and consistent:
- tamper-evident, sealed packaging, signed across the seal,
- labelling at the moment of collection, not later,
- secure, access-controlled storage with logs,
- minimal handling, and a record every time the seal is broken for examination.
How it gets challenged
Defence scrutiny focuses on the gaps:
- an item unaccounted for during a period of time,
- a broken or missing seal,
- inconsistent or incomplete paperwork,
- storage that could have degraded or contaminated the sample.
None of these necessarily means anything did happen to the evidence — but they introduce doubt, and doubt is often enough.
The bottom line
Chain of custody is the least glamorous part of forensic work and one of the most decisive. The discipline that protects a result is built long before the science begins — at the scene, with a label, a signature and a sealed bag.