the present invention is a container assembly that includes an inner tube formed from a plastic that is substantially inert to bodily fluids and an outer tube that is formed from a different plastic . collectively , the container assembly is useful for providing an effective barrier against gas and water permeability in the assembly and for extending the shelf - life of the container assembly , especially when used for blood collection . the inner container is spaced from the outer container at most locations . however , the inner container includes an enlarged top configured to engage the outer container . the enlarged top has a roughened outer surface to permit an escape of air from the space between the containers .