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May/June 2014 Today's Veterinary Practice 19 LymPh Node CyToLogy | tvpjournal.com 3. The gesture above will guide the node between the thumb and forefinger. While each practitioner will develop their own feel for lymph node palpation, this technique may help isolate the deeper and more elusive lymph nodes. Normal sized lymph nodes can sometimes be difficult to pal- pate and properly aspirate, especially in overweight or heavily muscled (eg, Staffordshire terrier, some Labrador retriever) dogs. To increase the chance of success, before isolating the node itself: 1. Use a reference point as described above. 2. Make a mental note of normal structures that are palpated near the node. Fine-Needle Aspiration 1. Use a needle without a syringe attached; any gauge is accept- able, but my preference is to use a 22-gauge needle to avoid discomfort. 2. Once a node is trapped between thumb and forefinger, intro- duce the needle. 3. Redirect the needle by moving it in and out through the node several times, until—when looking into the needle hub—a tiny bleb is apparent within the needle's inner circumference; this avoids unnecessary hemodilution. Slide Preparation The following technique provides high-quality diagnostic slides for needle aspiration cytology: 1. Attach an air-filled syringe to the needle and expel only ½ drop from the needle onto each of 2 to 3 slides, which keeps each slide's sample the right consistency, avoiding prepara- tions that are too thick (Figure 1A). 2. Gently lay a clean slide crosswise on the droplet, allowing it to break the surface tension (Figure 1B). 3. While holding the slide on both ends with the free hand, gen- tly pull the spreading slide across the aspi- rate slide, which allows good smearing of the droplet (Figure 1C and 1D). This tech- nique avoids applying too much pressure on the sample and traumatizing the cells. Note: If you stain a slide in-house using rapid fixation stains in preparation for labo- ratory evaluation, always send at least one unstained slide, which allows the clinical pathologist to apply his or her own stain. CYTOLOgIC EVALUATION Lymphocyte Size Lymph nodes are predominantly comprised of small, mature lymphocytes (80%–90%). Lymphoid cells typically have high nuclear to cytoplasmic ratios. Size is important when determining wheth- er the lymphoid population is of concern. Cell size is typically compared to a red blood cell (RBC), but neutrophils are less likely to fold and pile up, and are slightly larger than RBCs. Therefore, if possible, the nucleus should be compared to a RBC, and the whole cell to a neutrophil, if any are present (Table 1, page 20). Figure 1. Optimal lymph node aspirate slides are not too thick, and the smear has a smooth, oval appearance. Findings That Complicate Diagnosis Reactive Lymph Nodes Reactive lymph nodes are characterized by increased numbers of plasma cells, notable due to their deeply basophilic cytoplasm and perinu- clear clear zone. • Increased numbers of lymphoblasts are often seen in reactive lymph nodes, but they should not total more than 50% of the lymphoid pop- ulation. • Increased numbers of neutrophils are also seen, and other inflammatory cells, such as eosinophils may also be present. Inflammatory Cells When inflammatory cells are present, they can sometimes obscure metastatic cells or make cytologic changes difficult to interpret. The pres- ence of mast cells in a lymph node near a mast cell tumor can be especially challenging. While some mast cells may represent cytokine signaling and chemotaxis attracting normal mast cells, a high proportion as well as clustering of mast cells may represent true metastatic disease. Infectious Organisms other findings that do not belong in lymph nodes include infectious organisms, which may be seen against a background of reactive lymphocytes and inflammatory cells. Fungal and bacterial causes of lymphadenopathy can stimulate increased num- bers of macrophages and neutrophils. TVP_2014-0506_Lymph Node Cytology.indd 19 5/24/2014 10:39:00 AM