A method and device is provided for collecting filterable material to perform assays on a filtration apparatus by applying negative-pressure to the inlet chamber of the filtration apparatus which is independent of negative or positive pressure applied to its fluid path. The negative-pressure applied to the inlet chamber serves to actively aspirate fluid and promote flow into the inlet chamber via the fluid pathway. The negative-pressure applied to the inlet chamber facilitates the use of either negative or positive-pressure based fluid-filtration approaches. Use of the invention improves the functionality and scope of applications that can be performed with enclosed filtration apparatuses.