Filtering procedure designed to isolate and/or quantify a substance to be examined from a sample, wherein said procedure comprises the following steps: - Introduction of a first quantity of the sample, via a sample inlet, into a filter module which has a housing with a first opening forming the sample inlet and a second opening covered by a filter forming a sample outlet, wherein the filter is designed to retain a first part of the sample and to allow a second part of the sample to pass through, wherein the substance to be examined is contained in the first part of the sample or in the second part of the sample; - Routing of the sample from the sample inlet through the filter and the sample outlet by applying a differential pressure between a side of the filter facing towards the sample inlet and a side of the filter facing away from the sample inlet, wherein the second part of the sample leaves the filter module as filtrate through the sample outlet; - While the sample is conducted from the sample inlet through the filter to the sample outlet, A series of consecutive measured values of a measured variable representing the differential pressure and/or a volume flow through the filter is recorded; and - The measured variables are compared with a predefined reference value of the measured variable; characterized in that the filtering procedure further comprises the following step: - If the measured values of the measured variable that are recorded consecutively are equal to the reference value over a predefined time and/or are within a predefined value range around the reference value, the filtering procedure is stopped. - Introduction of a first quantity of the sample, via a sample inlet, into a filter module which has a housing with a first opening forming the sample inlet and a second opening covered by a filter forming a sample outlet, wherein the filter is designed to retain a first part of the sample and to allow a second part of the sample to pass through, wherein the substance to be examined is contained in the first part of the sample or in the second part of the sample; - Routing of the sample from the sample inlet through the filter and the sample outlet by applying a differential pressure between a side of the filter facing towards the sample inlet and a side of the filter facing away from the sample inlet, wherein the second part of the sample leaves the filter module as filtrate through the sample outlet; - While the sample is conducted from the sample inlet through the filter to the sample outlet, A series of consecutive measured values of a measured variable representing the differential pressure and/or a volume flow through the filter is recorded; and - The measured variables are compared with a predefined reference value of the measured variable; - The measured variables are compared with a predefined reference value of the measured variable; characterized in that the filtering procedure further comprises the following step: - If the measured values of the measured variable that are recorded consecutively are equal to the reference value over a predefined time and/or are within a predefined value range around the reference value, the filtering procedure is stopped. - If the measured values of the measured variable that are recorded consecutively are equal to the reference value over a predefined time and/or are within a predefined value range around the reference value, the filtering procedure is stopped.