An apparatus and method are provided for treating wound with negative pressure. The apparatus includes a wound cover, a first pump for providing said negative pressure at a chosen pressure level to the wound, a canister, a first conduit between the wound cover and the canister, first means for measuring the pressure within the canister, and a second conduit which connects the canister with the first pump. A circulating pump is arranged to intermittently transport a gas volume from the canister to the wound region via a third conduit and back to the canister via the first conduit in order to press out possible exudates from the first conduit to the canister. A second means analyzes the pressure curve over time of said pressure in the canister and a third means indicates by alarm when said pressure curve differs from a normal cyclical form.