Fine needle aspiration. Small Animal Diagnostic and Treatment Techniques

You can use needle sizes from 25 gauge up to 20 gauge for performing a fine needle aspiration. A 12 - 20 mL syringe will generate adequate suction for aspiration of lymph nodes and most mass lesions

Most masses are heterogeneous with regard to cell types. To obtain a sample for cytologic evaluation that is representative of the mass as a whole, several areas within the mass should be aspirated. The needle is inserted into the mass, (a) negative pressure is created within the syringe by pulling back the plunger to 5-10 ml (b) . Maintaining this negative pressure, redirect the needle in several directions ( 3-5) within the mass lesion, without removing the tip of the needle from the mass. Release the syringe plunger (c) and remove the needle from the mass (d) .

Aspiration of lymph nodes is no more invasive than venipuncture. Skin preparation is similar to that for venipuncture; simply wet the hair coat with alcohol to better define the structure being aspirated. The degree of skin preparation prior to aspiration of other masses depends upon the suspected contents of the mass and your plans for use of the aspirated sample. If you plan to culture the sample, then the skin surface should be clipped of hair and cleaned with an antiseptic solution such as Betadine. If the mass is fluctuant, suggesting it is fluid filled, aseptic skin preparation is indicated to prevent contamination of the fluid in the mass, if not already septic.

If you are right handed, immobilize the mass with your left hand and hold the syringe in your right hand. Sample serveral areas of the mass as described on the previous screen. You only need to aspirate a few drops of fluid from the mass. This small amount of fluid will remain in the needle during aspiration; you will not see any sample enter the syringe. If blood is aspirated, release the syringe plunger and stop aspirating.

Large amounts of blood will dilute the cells obtained from the mass and may render the cytolology, nondiagnostic. If the mass is fluctuant, suggesting it is fluid filled, aspirate several ml of the fluid and put this sample in a tube containing an anticoagulant such as EDTA. This sample can be centrifuged to concentrate the cells in the sample.

After the needle is removed from the mass, the syringe is disconnected from the needle and is filled with air. Make sure the needle is removed from the syringe before aspirating air into the syringe to avoid aspirating the sample out of the needle.

The air in the syringe is used to blow the sample out of the needle onto one or more glass slides. Many glass slides have a frosted edge (a). This edge is rough textured and identifies the top of the slide. Place the drop of sample close to the frosted edge of the slide, making sure the rough side is up. You can expect to obtain enough sample from one aspiration to make 1-3 slides. The aspiration procedure can be repeated several times from the same mass.

A second slide is used as a spreader by placing it at a right angle to the slide on which the sample has been placed. Gently pull the top slide across the bottom slide to thinly smear the sample. Do not press down with the top slide; let the weight of the top slide distribute the sample. Excessive pressure applied during this process may rupture cells, rendering the cytology nondiagnostic. Both the top and bottom slides will contain cells and can be submitted for cytologic evaluation.

Another technique for fine needle aspiration is called the staple technique. A 25 to 20 gauge needle is used without an attached syringe. The needle is inserted and removed rapidly several times in succession from the mass, attempting to sample several regions of the mass. The sample is expelled onto a glass slide by attaching an air filled syringe. The sample is spread across the glass slide as previously described.