A sampling device is provided for concentrating a liquid specimen sample, including a filtration assembly, which includes a tubular container and a plunger. The plunger includes a plunger head and a plunger rod that is shaped so as to define an internal plunger space having a plunger-space proximal opening through a proximal end of the plunger rod. The sampling device is configured such that a filter is removable from the tubular container via the plunger-space proximal opening while the plunger head is within the tubular container. Other embodiments are also described.