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A systematic approach to cytology (Proceedings)

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Oct 01, 2008

Cytology is an extremely useful tool that can be employed in any practice. Samples can be acquired quickly and easily, using instruments that are readily available in any veterinary practice from the single veterinarian hospital to the largest referral centers. The strengths of cytology lie in the rapidity with which the samples can be obtained, processed and evaluated. In addition, it is an affordable and generally minimally invasive. Using a systematic, step-wise approach, any specimen can be evaluated and useful interpretations derived.

An exhaustive review of sample acquisition is well beyond the scope or purpose of this lecture. That being said, interpretation of the objective material on a slide can be markedly influenced by the source of the sample. I am a clinical pathologist that believes that the person that can offer the most accurate interpretation of a sample is the person who took it (or was present when it was obtained). Confidence in the source of a sample is important, whether reading the slide in house or sending the slides away for a second opinion, an accurate description of sample acquisition should be available, should the person interpreting the smears want it. The interpretation of a slide is a marriage between the objective material contained within the smear(s) and the additional material provided (i.e. history, physical examination, description of the lesion, etc.). Different people use more or less of the additional information; however, it should be available, should it be needed.

The interpretation of a smear begins when the needle is inserted into the lesion. The gross appearance, texture or appearance using diagnostic imaging can be used in conjunction with the findings on the slide. A gritty feel when the needle is inserted can be indicative of mineralization, a finding which should be corroborated in the smears. An initially clear serous effusion or cerebrospinal fluid that turns bloody should be noted to make interpretation possible. Once the sample is obtained, its gross appearance should be noted. A smear that appears to be composed of water droplets that does not dry upon standing is most likely composed of lipid. If this gross appearance is not noted prior to staining it may go undocumented, Depending on stain used, cell lipid may be removed or dissolved and thus not be present to identify. In addition to lipid, the smear should be evaluated for the overall cellularity. Comparing samples to a smear of pure blood can help get an idea of blood contamination vs. cells from tissue. Finally, gross examination of a smear can help direct you to areas of higher cellularity, which tend to be areas of higher yield.

After gross examination of the smear, it can then be stained for initial evaluation. The type of stain is up to the person evaluating the smears; however, a rapid polychromatic stain is generally employed. Automated stainers are available as well. As long as the stain used provides a consistent, contaminant-free product, you should be fine. Unstained smears should be set aside to perform additional "special" stains if required, either by you or the reference lab that the sample is sent to. Gram stain, New Methylene Blue, Acid Fast, PAS, GMS and immunocytochemical stains are all examples of stains that may be indicated based on the findings of the smears stained with a polychromatic stain.