What is cytology?
Cytology is the microscopic examination of cells that have been collected from the body. By examining the appearance of these cells, including their number, size, shape, color, internal characteristics, and how they fit together with their neighbors, it is often possible to make a diagnosis of a specific disease process.
Image via Wikimedia Commons / Joel Mills (CC BY-SA 3.0.)
When is cytology performed?
Cytology is most often used to diagnose the nature of 'lumps and bumps' found on the surface of the body. However, cytology can also be used to evaluate:
- internal organs, such as the liver, lungs, lymph nodes, and kidneys
- body fluids, such as urine or joint fluid
- abnormal accumulations of fluids (called effusions), especially in the chest and abdomen
- various surfaces of the body, both external and internal (e.g., mouth, eyes, breathing passages, or vagina)
What information can cytology provide?
The most important thing cytology can tell us is whether a problem is caused by inflammation or by neoplasia (abnormal growth of tissue). If there is inflammation, cytology can often identify the underlying cause, such as a bacterial infection, embedded foreign body, or allergies. If the sample appears to be neoplastic, cytology can usually determine which type of tissue is involved and whether the neoplasm is malignant (cancerous) or benign.
How does this information help my veterinarian manage a problem?
If the problem is inflammatory in nature, your veterinarian can take steps to look for a specific cause (bacteria, parasite, fungus, allergies, foreign bodies, etc.) in order to provide the appropriate treatment.
If cytology indicates neoplasia, the appearance of the cells can often indicate the type of tumor and predict how it will behave. This is important because with malignant tumors, further testing (e.g., chest X-rays) may be required to determine whether it has spread. This is helpful for treatment planning, including whether surgical removal is warranted and how quickly it should be scheduled, and whether other treatment modalities, such as chemotherapy or radiation therapy, may be helpful. When the nature of the tumor is known ahead of time, the surgeon is better able to plan how to remove the tumor and predict what may happen afterwards, answering questions such as:
- Will it return?
- Will it spread to other parts of the body?
- Is it gone for good?
How are cells collected?
There are several different methods for collecting cells from tissues, depending on where the problem is and what type of tissue is involved. The most common technique is called fine needle aspiration or fine needle biopsy. This simple technique involves using a sterile, fine gauge needle attached to an empty syringe. The needle is inserted into the middle of the tissue in question and the plunger of the syringe pulled back to create suction, withdrawing (or aspirating) cells. Cells can be aspirated from solid tissues, such as organs or tumors, and fluid-filled tissues, such as joints or cysts (see handout “Fine Needle Aspiration”).
There are several other techniques used to collect cells. A flaky, bald patch of skin might be sampled by doing a skin scraping to rub off the top layers of cells so they can be examined. An ulcerated or oozing skin sore might be sampled by doing an impression smear, which involves pressing a clean glass slide firmly on the area to collect surface cells and debris for evaluation. Discharges from the nostril, eye, ear, prepuce or vagina might be collected using a cotton-tipped swab, and then spread on a slide where it can be examined. A less commonly used technique is called a lavage or flush, where sterile fluid is washed over an area, such as the nasal passage or trachea (airway), in order to dislodge the surface cells. The flushed fluid, containing surface cells, is then collected for examination. See handout “Cytology - Collecting Cells from Surfaces” for further information.
What are the advantages of cytology?
Cytology is a simple, quick, relatively painless, and non-invasive method of gathering information about medical and surgical conditions. Minimal equipment is required, and sampling can often be performed without sedation or anesthesia. With a relatively small investment of time and resources, cytology can often provide a definitive diagnosis. Howver if a diagnosis is not made from cytology, it often is still helpful to categorize the condition, which then helps your veterinarian decide what further steps are needed to reach a definitive diagnosis.
What are the limitations of cytology?
The main limitation of cytology is that the cells collected may not tell the whole story about what is happening in the tissue. This may happen if samples are very small, or if the most diagnostic (or representative) cells are not present in the sample. In addition, some tumors do not play by the rules: sometimes the cells may look benign, but the tumor is malignant, or the cells look malignant, but the tumor is benign.
What is the next diagnostic step after cytology?
The next diagnostic step after cytology is histology. Histology is the microscopic examination of a sample (small piece) of tissue that has been collected surgically. Histology not only focuses on the individual cells, but on how they are structurally related to one another (the tissue architecture). It provides information on how the cells are organized and how related tissues (e.g., layers of skin) interact with each other, looking for any disruptions in the normal patterns.
"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 as such, it is generally considered the diagnostic 'gold standard'. It is routinely recommended to confirm cytological findings (e.g., of a fine needle aspiration) and is often needed to determine if a tumor is benign or malignant. If your pet has a growth surgically removed, always request that the tissue be sent away for histological examination.