Embodiments of negative pressure wound therapy apparatuses and methods for using such apparatuses are disclosed. In some embodiments, a negative pressure wound therapy apparatus includes a controller configured to determine a level of exudate in a canister (or a dressing) based at least in part on one or more characteristics of pressure signals generated by a negative pressure source and monitored by a pressure sensor. One such characteristic of the pressure signals can be amplitude, which may increase as a level of exudate in the canister (or dressing) increases. The canister (or dressing) can include a filter configured to become occluded in order to prevent overflow of the canister (or dressing). The controller can be additionally configured to detect and indicate a canister (or dressing) pre-full condition before the filter becomes occluded. More efficient and reliable operation of the negative pressure wound therapy apparatus can thereby be attained.