UN3373 is the dangerous-goods classification for Biological Substance, Category B: clinical and diagnostic samples that may contain an infectious substance below the highest-risk threshold. To travel by road they must be packed to packing instruction P650, triple packaging with absorbent material and a rigid outer box marked with the UN3373 diamond. This guide sets out what that means in practice.
UN3373 packaging is the triple-packaging system set out in packing instruction P650 of the ADR (the European agreement for dangerous goods by road, applied in the UK by the Carriage of Dangerous Goods Regulations) and, for air, the IATA Dangerous Goods Regulations. The three layers are:
The complete package must be capable of passing a 1.2 metre drop test without leakage from the primary receptacle, which is why laboratories buy pre-tested UN3373 kits rather than improvising with a jiffy bag.
A Category B biological substance is an infectious substance that does not meet the criteria for Category A. Category A (UN2814, infectious substance affecting humans; UN2900, affecting animals) is material capable of causing permanent disability or life-threatening or fatal disease in otherwise healthy humans or animals when exposure occurs. Everything infectious below that line is Category B, UN3373.
In practice, most routine clinical samples sent to pathology are Category B: blood, urine, swabs, tissue and cultures from patients who are, or may be, infected with something other than the listed high-consequence pathogens. The sender’s judgement, using the ADR or IATA list of Category A examples, decides the classification. The UK Health Security Agency and the Health and Safety Executive publish guidance for laboratories and clinics on making that decision.
There is a third group: exempt specimens, where a professional judgement finds minimal likelihood of pathogens. These travel in leak-proof triple packaging marked “Exempt human specimen” or “Exempt animal specimen” and are not dangerous goods at all.
Beyond the three layers, the UN3373 packaging requirements that a courier checks at the door are the marks on the outer box:
For road transport under the ADR there is no dangerous-goods transport document required for UN3373 and no quantity limit per package. For air, IATA limits liquids to 1 litre per primary receptacle and 4 litres per package, and solids to 4 kg per package, and the airline’s requirements apply.
How to transport blood samples by courier, step by step, for a Category B consignment:
Keep the time between sampling and testing as short as the test requires; a dedicated vehicle driven direct is the usual way to meet a same-day cut-off. See our biological sample courier service.
Dry ice (solid carbon dioxide, UN1845) is itself a dangerous good because it sublimates into gas that can build pressure and displace oxygen. When it is used to keep a UN3373 package cold, the rules are: the dry ice goes outside the secondary packaging, the outer packaging must allow the gas to escape (never a sealed plastic drum), and the package carries the UN1845 mark, the words “DRY ICE” or “CARBON DIOXIDE, SOLID”, and the net weight in kilograms.
Gel packs and validated cool boxes avoid the dangerous-goods marking altogether and are the usual choice for samples that need 2 to 8°C for a few hours. First Choice Transportation vehicles are not refrigerated, so the cold chain is the packaging’s job, which is the normal arrangement for Category B samples.
Regulations are revised regularly; the sender is responsible for packing to the edition in force on the day of shipment. This guide was last reviewed in October 2026.
UN3373 is the United Nations number for Biological Substance, Category B: human or animal material that contains or may contain an infectious substance that is not in the highest-risk form. It is a dangerous-goods classification, so the package must be packed to packing instruction P650 and marked with the UN3373 diamond before a courier accepts it.
Category A (UN2814 for humans, UN2900 for animals) is an infectious substance capable of causing permanent disability or life-threatening disease in healthy people, and needs packing instruction P620 and a trained dangerous-goods carrier. Category B (UN3373) is everything infectious that does not meet that threshold, packed to P650. Most clinical samples are Category B.
A sample where a professional judgement, based on the patient’s history and local conditions, finds minimal likelihood that pathogens are present: for example routine blood tests from a healthy person. It travels in leak-proof triple packaging marked “Exempt human specimen” (or “Exempt animal specimen”), without the UN3373 mark, and is not classed as dangerous goods.
The sender. The laboratory, clinic or researcher sending the sample decides whether it is Category A, Category B or exempt, packs it to the matching packing instruction and applies the marks. The courier checks that the outer package is correctly marked and undamaged before accepting it, and refuses it if it is not.
For road transport under the ADR, P650 sets no maximum quantity per package, although the primary receptacle must be leak-proof and the packaging drop-tested. For air transport under the IATA regulations the limits are 1 litre per primary receptacle and 4 litres per package for liquids, and 4 kg for solids, so check the mode before packing.
Yes. Category B samples and exempt specimens are carried in a dedicated vehicle at ambient temperature, collected within 30 to 45 minutes across London and within the hour UK-wide, with the outer package and paperwork checked at collection and a signed, photographed handover at the laboratory. Category A substances are not carried. Call 020 8202 2008.
First Choice Transportation Ltd is a registered UK company (No. 15276847) based in Southgate, North London, founded by Allam, who has 15 years’ experience in courier and private hire work, with 30 drivers across the UK.
The driver checks the UN3373 marks and the paperwork before accepting the package.
The sample is the only consignment in the vehicle, kept upright and secured, driven direct to the laboratory.
30 to 45 minutes across London and the Home Counties; 45 minutes to an hour elsewhere, 24 hours a day.
Signature and photo on handover, texted to you as proof of delivery.
