Cytology

Cytology is the study of cells under a microscope: the cell, the smallest unit the body is built from, placed under the lens to be seen up close. The whole craft begins with getting hold of them, and there are several routes. They can be collected from a fluid, from a washing (rinsing a surface and keeping the rinse along with whatever it carried off), from a brushing (running a fine brush over a surface so cells catch on the bristles), or from a fine needle aspiration (going in with a thin needle and drawing cells out).

It's like recognizing a fabric from a few threads left in your hand, without needing the whole garment. In cytology, what reaches the microscope is those loose cells, brought in by one of those routes, and the exam is done on them.