Apparatus is provided including a tube and a plunger sized and shaped to be inserted into the tube. A distal end of the plunger is shaped to define one or more first passageways therethrough, and the plunger is shaped to define one or more compartments in fluid communication with the first passageways. A filter is coupled to the distal end of the plunger. The plunger is shaped to define a second passageway that passes through the plunger from a proximal end of the plunger to the distal end of the plunger, and is positioned to facilitate testing of the filter. The apparatus is configured such that while the plunger is advanced within the tube while fluid is within the tube, the fluid in the tube is pushed through the filter, through the first passageways, and into the one or more compartments. Other embodiments are also described.