Formalin-fixed samples must be shipped in a leak-proof primary container inside a leak-proof secondary container. The secondary container must be rigid and contain sufficient absorbent material (paper towels, cotton/cotton balls, cellulose wadding) capable of absorbing the entire liquid content. If shipping by air, the primary or secondary container must be rated for a 95 pKa pressure differential.

In contrast, when shipping clinical specimens that are known to contain or are reasonably expected to contain pathogens, the shipment must adhere to the regulations governing Biological Substances, Category B. All Category B infectious substances require triple packaging for shipment:

1. Leak-proof Primary Container (Figure 23.1). The specimen must be placed in a leak-proof primary container with a positive closure (e.g. screw-on cap). The volume of the primary container cannot exceed 500 ml (16.9 oz) for a liquid diagnostic specimen or hold more than 500 g (1.1 lb) for a solid diagnostic specimen. When shipped by air, the primary or secondary container must be able to withstand a pressure differential of not less than 95 kPa (14 psi) in the range of −40 to 55 °C (−40 to 130 °F). For solid specimens, the primary receptacle must be sift-proof (i.e. the solid material cannot leak out of the packaging).