Traditional cytology preparations, such as blood smears, aspirates of masses, and fluid evaluation, can all be performed on postmortem samples. However, the workhorse of postmortem cytology is the touch imprint. While touch imprints of lesions are not a substitute for histology, these preparations allow for rapid evaluation of cellular morphology and often are helpful in identifying infectious agents and distinguishing between inflammatory and neoplastic conditions. Neoplastic cells can often be further classified as round, epithelial, or mesenchymal. The number of cells that adhere to the slide on a touch imprint depends on the tissue and the disease process. In a fresh cadaver, round cell tumors and inflammatory cells readily exfoliate, while touch imprints of sarcomas and fibrotic tissue are usually paucicellular. The degree of exfoliation alone may be a clue to the nature of the disease process; however, in a postmortem sample, autolysis can also affect the propensity of cells to exfoliate.

The first rule of making a touch imprint is to use a clean blade and make a fresh cut into the area of interest. If a blade has previously been used to cut other tissues, cells and debris on the blade of the knife or scalpel will contaminate the imprint. Cut a small block (0.5–1 cm) out of the tissue of interest, cutting the surface to be used for the imprint first. Hold the tissue block with forceps and blot the surface with a clean paper towel (Figure 19.1a). Touch the tissue gently to the slide multiple times, moving to a clean area of the slide for each imprint (Figure 19.1b).

Formalin, and even formalin fumes, can negatively affect cytological preparations, and contamination of the tissues or smears should be avoided.

When collecting bone marrow for histology, it is easy to also make a smear of the marrow. Hematogenous cells undergo rapid autolysis, so the smear should be made as soon as possible. A moistened, clean, natural-bristle paint brush can be used to make a bone marrow smear. Alternatively, the proximal femur, which has been opened to obtain the histology sample of bone marrow, can be removed and the cortex chipped away with bone forceps until the marrow is exposed. The femur can then be used like a pencil to make a serpiginous smear on a glass slide (Figure 19.2). Forceps can also be used to hold a piece of bone marrow to make a smear.