In the detection of the presence of a biomarker or the like in a sample of a flowable substance, e.g. a powder or a liquid, usually a body fluid, such as blood, urine, or saliva, for example, a disposable sample receiver (3) is used, which has a receiving chamber (301) that is dimensioned to receive a predetermined volume and is surrounded by a depression (303) receiving any excess volume for which there is no room in the receiving chamber (301). The receiving chamber (301) has a bottom outlet (302) closed by a removable strip (33), e.g. a plastic strip or foil. Upon pulling away the strip (33) from the bottom outlet, the sample in the receiving chamber is emptied into a flow path (32) leading to at least one detection compartment (321) permitting direct visual inspection. Preferably, disposable sample receiver (3) is used in a detector assembly (1) including an electronic camera (23), a CPU (26) and a display (22). Hereby, the volume of the sample to be analyzed will always be the same, and by controlling the exact point of time when the sample is passed on into the flow path (32), a high degree of repeatability and accuracy is achieved, and thereby also a fail-safe system.