Stopper having a cavity for reagents and an assay method using said stopper

A closure device suitable for use in performing an assay, said device having its basic construction designed into a closure assembly suitable for closing a diagnostic test vessel, said closure device comprising a body part, which is suitable for tight mounting on a mouth of said vessel and is provided with a cylindrical bore, said body part including a lid suited for closing the body part bore end facing said diagnostic test vessel in an openable manner, and a plunger with a diameter compatible with the bore of the body part, said plunger being slidably mounted in the bore so as to permit its movement into a sealed position with respect to the body part thus effecting the formation of a sealed reagent storage chamber in the space remaining between the lid and the plunger and into an opening position effecting the opening of the lid, wherein an inner wall of the bore passing axially through said body part is provided with at least one groove, whose radial depth is so deep as not to be within the reach of the outer diameter of the plunger, said groove extending from the end of the bore into which the plunger is inserted, axially along the inner wall of the bore, over such a length of the bore wall as to maintain a gas flow communication between said reagent storage chamber and the end of the cylindrical bore when the plunger is mounted into a partially inserted position.