QUARTERLY BEAT
JULY 2026
Numerous prior veterinary and human studies have found conflicting literature favoring both larger and smaller gauge needle sizes or finding equivocal results.
Figure 2. Linear probe (left), microconvex probe (right). Photo courtesy of Tom Thompson, MSU-CVM photographer.
The decision to hold the needle with the dominant or nondominant hand is inconsequential and should be based on the operator’s preference. Most individuals insert the needle along the probe marker side of the transducer, but the opposite side may be needed due to anatomic considerations. An appropriate window is one where the needle may be advanced the shortest distance possible, ideally without entering another body cavity or organ. Any imaging plane may be used to achieve this goal and surrounding tissue should be evaluated to know what is adjacent to the planned needle path. The focal zone should be placed at the lesion of interest. Needle visualization is easy and rapid with most modern ultrasound machines. In cases where visualization is challenging, fanning/rocking the transducer and/or turning off compound imaging usually helps. Slow advancing or retracting the needle may be performed next but done so cautiously to avoid tissue damage or vessel laceration. Although discussed in some texts, aspirating a small volume of saline or air can interfere with visualization of the target or dilute the diagnostic sample.
Figure 3. Prepare your disinfectant and equipment needed for ultrasound- guided FNA. Photo courtesy of Tom Thompson, MSU-CVM photographer.
The results of three large, well designed, recent veterinary studies (Arai 2019, Llanos 2023, and Launay 2023) concluded that there is not a significant difference in diagnostic quality between larger gauge (22G or 23G) and smaller gauge (25 or 27G) needles. Arai et al. prospectively looked at results of 50 masses from cutaneous, subcutaneous, and intracavitary locations and found 25G needles resulted in less blood contamination but increased cell trauma. Similarly, Llanos et al. prospectively evaluated 282 splenic aspirates from 94 dogs and 348 aspirates from 116 dogs and also did not find significant differences in diagnostic quality. Curiously, more blood contamination was noted with hepatic aspirates when using a 22G needle as compared to a 25G needle, but no differences were seen with the spleen. Finally, Launay et al. also found no difference in the final cytology diagnosis between 23G, 25G, and 27G needles when obtaining splenic aspirates but did observe that 23G needles were of significantly higher cellularity. It is possible that diagnostic yield could vary among other organs or even disease etiology, but more studies would be needed to investigate this concern. For example, results of a preliminary investigation (Blacklock 2018) on five canine malignant tumors suggested that an aspiration technique (i.e., pulling back on the syringe during sampling, discussed further below) with a 25G needle and a 10 mL syringe yielded more cells for melanoma, lymphoma, and anal gland adenocarcinoma as compared to an advance/withdraw technique, a needle only technique, using 21G or 23G needles, or while aspirating with 2 or 5 mL syringes. However, this study had several limitations. The tumors were aspirated outside the patient after surgical removal and appeared to be cutaneous/ subcutaneous, the sample size was low, and the tumors were aspirated
EQUIPMENT
By definition, a “fine needle aspirate” is the use of a needle less than 20-guage (G), with anything larger considered a needle core biopsy (Liffman 2017). A 21G - 25G, 1.0 – 1.5-inch needle is most commonly used. Longer needles (e.g., 2.5 – 8 inch) with a stylet may be used for deeper lesions but are technically more challenging to place into the lesion accurately due
to the increased travel distance. Needle size has been extensively studied in both human and veterinary medicine. In theory, a larger gauge needle is easier to see with ultrasound, less likely to bend, can collect more cells, and is less likely to damage
cells; however, it could cause more blood contamination in the sample (Liffman 2017). All of the above would be opposite for a smaller gauge needle.
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