A measuring cell includes a cavity for receiving a test sample to be used in a particle detection apparatus. The measuring cell is configured as an optical waveguide for guiding a light beam. The waveguide has a core which has a refractive index nK, extends along a longitudinal axis of the waveguide, has a cross-sectional area AK of less than 80 μm2 in a cross section perpendicular to the longitudinal axis, and which is surrounded by a cladding having a smaller refractive index than nK. The cavity forms a channel. The channel extends along the longitudinal axis, is formed inside of or in contact with the core, and has at least one open end with an opening area AH of less than 0.2 μm2.