Abstract: A planar device for absorbing substances, especially proteins, such as enzymes, from body fluids, especially from gingival crevicular fluid or lacrimal fluid, comprising a receptor element (10), a support element (20), characterized in that—said receptor element (10) is hydrophilic and has a pore size of from 0.22 ?m to 5 ?m, especially from 0.5 ?m to 3 ?m, and consists of a plastic material or mixture of plastic materials, especially of an inert plastic material or a mixture of inert plastic materials;—said support element (20) is hydrophobic and covers one surface of the receptor element (10) at least partially, and—wherein the device further comprises a discriminator element (40) which is situated at the opposing surface of the receptor element (10).