Cutaneous cytology: A quick review of an indispensable test



Wayne Rosenkrantz, DVM, DACVD
Cutaneous cytology immediately and inexpensively reveals whether infectious, inflammatory, or neoplastic processes are affecting the skin, and it can help you determine which diagnostic path to continue on. For example, finding atypical cells will generally lead you to perform a skin biopsy for histologic evaluation.

To use this invaluable tool, you'll need a basic knowledge of sample collection techniques and keratinocyte morphology. And you'll need to know how to identify infectious organisms, inflammatory cells, and artifacts. Cytologic evaluation is most useful in identifying inflammatory or infectious processes. Interpreting samples obtained from neoplastic lesions requires specialized training, but tumors such as those involving mast cells can readily be detected. An easy way to expand your cytologic evaluation skills in practice is to obtain duplicate samples—one to read in-house and one to submit to a clinical pathologist—for comparative interpretation.

THE RIGHT TOOLS

The most important piece of equipment is a good-quality, well-maintained binocular microscope with a halogen light source and multiple objectives (4X, 10X, 40X, and oil immersion 100X). Different types of immersion oil are available, but I recommend type B for its versatility and high quality.

For simplicity, use a commercial modified Wright's stain such as Diff-Quik (Dade Behring), but keep in mind that it gives less nuclear detail than a vital stain such as new methylene blue. In my practice, we rarely perform Gram stains because of the time commitment and because differentiating gram-negative vs. gram-positive organisms usually does not change our immediate clinical decisions. Many other special stains are available through your clinical pathology laboratory. Keep the stain clean—change staining tubs weekly, and use separate containers for contaminated samples such as anal gland or fecal cytology.

Heat fixation can be accomplished with a blow dryer, Bunsen burner, lighter, or match. Heat fixation prevents sample loss from the slide during the staining process, especially samples that are waxy, greasy, or exudative. Hold the sample over a flame for only a few seconds to avoid overheating and damaging the sample.

SAMPLING TECHNIQUES

Direct impression

The easiest and often the most effective way to obtain a sample for cutaneous cytologic examination is a direct impression with a glass slide. Either press the glass slide against the site you want to sample, or place it at a slight angle to the skin surface and then scrape the skin surface while dragging the slide onto the abraded surface you have created. This latter technique may increase the amount of material you collect, but it also can damage some of the inflammatory cells, creating proteinaceous nuclear strands.

Transparent acetate tape

Use transparent acetate tape to collect material from dry lesions that will not readily stick to a glass slide or to reach areas that are challenging to sample with a glass slide. With this technique, do not perform heat fixation, and omit the Diff-Quik fixation (clear) solution step because the tape will not lie flat on the slide. Put one drop of the third solution (the purple stain) or a drop of new methylene blue onto a glass slide, and then apply the unstained tape preparation directly over the stain. You can examine these samples with all of the objectives (including oil immersion).

Fine-needle aspiration

Use syringes and needles to aspirate material from nodules, tumors, plaques, abscesses, or other lesions in which a deep (nonsurface) sample is required. Attach a 20- or 22-ga needle to a 6- or 12-ml syringe. Insert the needle into the lesion, hold the needle stable, and pull the plunger back fully several times. Keep the plunger neutral (apply no negative pressure) when you withdraw the needle and syringe. Unless the sample is fluid, most of the sample you want will be in the needle lumen. Next, remove the needle from the syringe, pull the plunger back, and reattach the needle to the syringe. Then rapidly push the plunger to expel the sample onto a slide, and smear the sample before fixation and staining.