A method for controlling the flow of liquid through a flexible tube in a blood collection set is provided. The set includes a venipuncture needle (12), a tube (14) for receiving blood and which provides branches to a blood collection container (18) and to a sampling pouch (20), a first flow control clamp (22) and an irreversibly closable flow control clamp (30) on the two branches. The method allows blood to flow from the tube (14) to the sampling pouch (20), irreversibly closing the flow control clamp (30) on the branch to the sampling pouch (20) whereby that flow-path is irreversibly closed and thereafter allowing blood to flow from the tube (14) to the collection (18) container via the other branch. The irreversibly closable flow control clamp (30) includes a flexible body (34) apertures (40, 44) and having two legs (36, 38) moveable between a spaced apart and a closed position, a tube contacting member (48) for clamping a tube passing through the apertures when the legs are in the closed position.
