Abstract: An adapter for allowing a cap assembly to fit securely on two or more medical device ports with different sizes, shapes, and/or dimensions. The cap assembly has a housing with an interior configuration corresponding with an exterior configuration of a first port and surrounding port region of a medical device. An adapter having an exterior configuration corresponding with the housing interior configuration, and an interior configuration corresponding with the second port and port region may be fitted within the housing. Alternatively, an adapter having a first configuration may be provided in the housing to configure the housing interior to correspond with the exterior configuration of the first port. The adapter may be shiftable into a second configuration with an interior corresponding with the exterior configuration of the second port. The first and second configurations of the housing interiors adapt the housing to fit securely on the corresponding port.