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Cytology - Fine Needle Aspiration (FNA)

What is cytology?

Cytology is the microscopic examination of cells that have been collected from body tissues. By examining the appearance of these cells and looking for inflammation or infection, it is often possible to diagnose specific diseases or determine the nature of a pet's illness (see article 'Cytology - General' for an introduction to Cytology).


When is cytology by Fine Needle Aspiration performed?

"Fine needle aspiration is typically used to diagnose 'lumps and bumps' found on the body."

Fine needle aspiration (FNA), also called fine needle biopsy, is the most frequently used technique in cytology and is typically used to sample 'lumps and bumps' found on the body. However, it is also used to evaluate:

  • internal organs such as the liver, lung, lymph node, or kidney
  • body fluids such as urine or joint fluid
  • abnormal accumulations of fluid (called effusions) usually found in the chest and abdomen, and sometimes around the heart.


How are cells collected with Fine Needle Aspiration?cytology_-_fna_samples-1

The FNA technique is very simple: a sterile fine gauge needle is attached to an empty syringe, the needle is introduced into the tissue and the plunger of the syringe is pulled back while the needle is held in the tissue. This creates suction, which aspirates or pulls cells or fluid from the site into the syringe.

Cytology specimens are sometimes collected during an ultrasound examination. These samples are called "ultrasound guidedfine needle aspirates since the ultrasound image is used to locate the site to be sampled and helps the veterinarian position the needle correctly before aspriating the sample.


What happens to the collected cells?

When solid tissue is sampled, a small amount of material accumulates in the hub of the needle. This material is immediately and gently expelled onto a clean glass slide, spread in a thin layer, and rapidly dried by waving the slide in the air or by placing it in front of a fan or portable hair dryer. This is called making an "air-dried smear". cytology_-_fna_samples-2The slide is then stained with special dyes and examined under the microscope.

When a fluid sample is collected, air-dried smears are often prepared directly from the material in the syringe, and the remaining fluid is placed in transport tubes or containers. The smears and the containers are then sent to the laboratory for further analysis. This typically includes measurement of the cellularity and protein content of the fluid, as well as preparation of additional slides. If the sample is very thin and watery, sometimes the sample is concentrated before the slides are made, which provides more cells to look at. The slides are then stained with special dyes, and examined.

Is any special preparation required before collecting the sample?

For routine sampling of lumps and bumps on the body surface, there is usually no special preparation required, although a simple disinfectant like alcohol may be applied to the skin prior to sample collection. However, when samples are collected from internal organs or need to be tested for bacteria, sterile surgical technique must be used during collection and handling of the sample. This involves shaving the fur, cleaning and disinfecting the skin, and wearing surgical gloves etc., just as would be done in preparation for surgery.

Is cytology by FNA always diagnostic?

"Cytology by fine needle aspiration is not always diagnostic..."

Cytology by FNA is not always diagnostic, but in those cases where the results do not provide a definitive diagnosis, they usually contribute valuable information that ultimately leads to a final diagnosis.


What is the next diagnostic step after cytology?

The next diagnostic step after cytology is histology. Histology is the microscopic examination of solid tissue collected surgically from the pet. Histology focuses on tissue architecture and looks at how cells and tissues are organized, and determines if there is any disruption in the normal pattern.

"If your pet has a growth surgically removed, always request that the tissue be sent away for histological examination."

In most cases, histology will provide a definitive diagnosis, and is generally considered the diagnostic "gold standard". Histology may be needed to determine if a tumor is benign or malignant, and is routinely recommended to confirm the cytological findings. If your pet has a growth surgically removed, always request that the tissue be sent away for histological examination.

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