A pre-assembled medical fluid flow system and method of making same are disclosed. The system includes a first sterilized fluid flow system component, a second sterilized fluid flow system component and a flow path for providing fluid communication therebetween. The flow path includes an isolated portion. An openable closure isolates the isolated portion from the first sterilized fluid system component, and a microorganism filter is disposed between and isolates the isolated portion of the flow path and the second sterilized fluid flow system component. Therefore, fluid from the isolated portion of the flow path must flow through the microorganism filter to reach the second sterilized fluid flow system component.