Disclosed is a convenent sample preparation method for a medium suspected of containing contaminants, the method comprising a) passing a known volume of said medium through a filter from an influent side to an effluent side thereby concentrating the contaminants on the influent side of the filter, b) contacting the influent side of the filter with a liquid vehicle containing at least one substrate that through interaction with the contaminants each produces a detectable moiety, c) and allowing the substrate to interact with the contaminants on the influent side of the filter for a period of time, which is sufficient to allow the detectable moiety to be detected in the liquid vehicle. The method may further comprise a detection step, where the amount of detectable is determined in the liquid vehicle, preferably after the liquid vehicle has been separated from the contaminant, e.g. by passing the liquid vehicle through the filter and performing a measurement on the contaminant free liquid vehicle. Also disclosed is a kit for exercising the inventive method.