A method for testing the integrity of a porous filter medium having an upstream side and a downstream side, said porous filter medium being incorporated into a housing, said housing comprising an interior separated by the porous filter medium into an upstream portion at the upstream side of the filter medium and a downstream portion at the downstream side of the filter medium, said housing further comprising an inlet in fluid connection with the upstream portion and an outlet in fluid connection with the downstream portion of the interior, said outlet being connected to a filtrate conduit comprising a closable end remote from said outlet, and said porous filter medium having a predefined bubble point; said method comprising: - providing the porous filter medium in a state wherein the pores of the filter medium are wetted and filled with a liquid; - providing the downstream portion of the interior of the housing and the filtrate conduit filled with a liquid so as to displace gas in the downstream portion of the interior and the filtrate conduit; - closing the remote closable end of the filtrate conduit; - draining the upstream portion of the interior and filling same with a testing gas at least to such an extent that essentially the whole upstream side of the porous filter medium is exposed to the testing gas while retaining the liquid in the downstream portion of the interior; - providing a gas-filled testing volume fluidly connected to the downstream portion of the interior via said filtrate conduit filled with liquid; - providing a means for measuring the pressure in the downstream portion of the interior; - creating a pressure of the testing gas in the upstream portion of the interior corresponding to a predetermined testing differential pressure and maintaining it at this pressure level, said predetermined testing differential pressure being lower than the predefined bubble point of the porous filter medium; - determining the pressure of the gas within the testing volume by the means for measuring the pressure after a predetermined time after the predetermined testing differential pressure has been established; said testing volume being selected such that, when a filter medium is tested having a bubble point corresponding to the predefined bubble point, a pressure increase of the gas within the testing volume of about 100 mbar or more is obtained within a period of 10 minutes; and - comparing said determined pressure of the gas within the testing volume to a pressure obtained within the predetermined time when testing a non-defective filter medium, wherein in case said determined pressure of the gas within the testing volume is higher by a predefined amount than a pressure obtained when testing a non-defective filter medium, said tested filter medium is qualified as defective. - providing the porous filter medium in a state wherein the pores of the filter medium are wetted and filled with a liquid; - providing the downstream portion of the interior of the housing and the filtrate conduit filled with a liquid so as to displace gas in the downstream portion of the interior and the filtrate conduit; - closing the remote closable end of the filtrate conduit; - draining the upstream portion of the interior and filling same with a testing gas at least to such an extent that essentially the whole upstream side of the porous filter medium is exposed to the testing gas while retaining the liquid in the downstream portion of the interior; - providing a gas-filled testing volume fluidly connected to the downstream portion of the interior via said filtrate conduit filled with liquid; - providing a means for measuring the pressure in the downstream portion of the interior; - creating a pressure of the testing gas in the upstream portion of the interior corresponding to a predetermined testing differential pressure and maintaining it at this pressure level, said predetermined testing differential pressure being lower than the predefined bubble point of the porous filter medium; - determining the pressure of the gas within the testing volume by the means for measuring the pressure after a predetermined time after the predetermined testing differential pressure has been established; said testing volume being selected such that, when a filter medium is tested having a bubble point corresponding to the predefined bubble point, a pressure increase of the gas within the testing volume of about 100 mbar or more is obtained within a period of 10 minutes; and - comparing said determined pressure of the gas within the testing volume to a pressure obtained within the predetermined time when testing a non-defective filter medium, wherein in case said determined pressure of the gas within the testing volume is higher by a predefined amount than a pressure obtained when testing a non-defective filter medium, said tested filter medium is qualified as defective.