How to transport pathology samples legally (UN3373 explained)

A plain-English guide to UN3373 / P650 packaging rules for pathology samples in the UK — what the labels mean, who is responsible and how to book a compliant courier.

By CourierFly Operations Team, Control room & operations · Published 24 September 2026 · Updated 19 October 2026 · 2 min read

UN3373 is the transport designation for "Biological substance, Category B" — the classification that covers most routine pathology and clinical specimen movements in the UK. If you send patient samples between clinics, labs, or hospitals, this rule applies to you. Here is a plain-English guide.

What UN3373 actually is

UN3373 is a UN transport number under the ADR (European road transport of dangerous goods) framework. It designates biological substances that can cause disease in humans or animals but are not in the highest-risk group. In practice, that means most human blood, tissue, urine and swab samples destined for a diagnostic lab.

The P650 packaging rule

UN3373 must be transported in triple packaging that complies with packing instruction P650. That means:

1. A leak-proof primary receptacle — the sample container itself.
2. A leak-proof secondary packaging — a sealed bag with enough absorbent material to soak up the entire liquid content if the primary leaks.
3. A rigid outer packaging — labelled with the UN3373 diamond mark and the words "Biological substance, Category B".

Rigid boxes such as the Versapak medical bag or a certified fibreboard courier box meet the rigid-outer requirement. The outer must survive a 1.2 m drop test and display the UN3373 mark clearly on one face.

Who is legally responsible

Under GB ADR, the consignor (the person sending the sample) is responsible for correct packaging and labelling, and for training staff who handle the pack. The carrier is responsible for ensuring the vehicle and driver comply with the road-transport rules — including any spill response equipment required.

What the driver needs

For UN3373 carried under the "small load" exemption in ADR 1.1.3.6 (the vast majority of pathology courier work), no ADR driver training certificate is required. The driver does need:

• Company training on UN3373 handling and spill response.
• A spill kit appropriate for the volume carried.
• Fire extinguisher on board (standard van kit).
• Chain-of-custody paperwork, if the sending lab requires it.

How to book a compliant courier

When you book a courier for pathology samples, confirm three things: (1) their drivers are trained on UN3373 / P650, (2) they carry spill kits, and (3) they can provide signed chain-of-custody documentation on request. CourierFly's medical courier service ships everything above as standard, with DBS-checked drivers on rota 24/7. Our control room on 0330 341 3180 can dispatch a UN3373-trained driver inside 30 minutes across the Birmingham catchment.