A microfluidic cartridge includes first and second sides and at least one flow channel and an inlet to the flow channel(s) for feeding a liquid sample, the flow channel(s) include a plurality of first optical detection sites. A detector assembly includes a slot for inserting the microfluidic cartridge and a first fixed light source with a beam path and an optical reader for reading out optical signals from at least one of the first optical detection site(s). When the microfluidic cartridge is inserted to a first predetermined position into the slot, one of the first optical detection sites of the microfluidic cartridge is positioned in the beam path of the first light source, and when the cartridge is inserted to a second predetermined position into the slot, another one of the first optical detection sites of the microfluidic cartridge is positioned in the beam path of the first light source.