Pen retention apparatus

A pen retention device for releasably securing a pen to a tether is disclosed. The device includes the pen having an end and an annular interior surface defining a bore, a cap comprising an expandable retention interface disposed within the bore proximate the end, and an anvil securedly attached to the tether. The expandable retention interface includes a radially outwardly directed surface in contact with the annular interior surface. The expanding retention interface further includes a radially inwardly directed surface in operable communication with the anvil. The expanding retention interface is responsive to an application of force less than a release force by the tether via the anvil to expand and secure cap to the pen, and is responsive to an application of force greater than the release force to release the cap from the pen.