Diagnosing Lumps in Dogs: Fine Needle Aspirates (FNAs) and Biopsies

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This article was updated on November 25th, 2023

It is usually not possible to accurately diagnose lumps and bumps from their appearance alone, although a veterinarian will have their suspicions. This article explains the process used to confirm an accurate diagnosis, including Fine Needle Aspirates (FNAs) and Biopsies.

About Fine Needle Aspirates (FNAs) and Biopsies

Your veterinarian will likely advise sampling the lump and sending the cells to a lab for analysis. This helps confirm an accurate diagnosis and identify the best treatment plan. The cost of this will vary depending on the clinic and which sort of biopsy procedure is used.


Simple “fine needle aspirations” (FNAs) cost about $150-300, while biopsies done under anesthetic may cost $400-600.

What are Fine Needle Aspirates (FNAs)?

An FNA means a needle is poked into the lesion and some cells are sucked out for analysis. As this is done quickly and in a conscious dog, it’s relatively inexpensive. This is the test of choice when a Mast Cell Tumor is suspected but is also often used to diagnose lumps such as lipomas and cysts.

The downside of an FNA is we could “miss” the important cells and only suck up blood or fat, so we don’t always get a diagnosis.

What are biopsies?

A biopsy is a much larger sample taken and dogs are usually sedated or put under general anesthetic. They may need a suture or two to close the skin after. This sort of procedure requires an aseptic technique and at least two staff members. The dog must be monitored during and after, as they wake up.

As a much larger sample is taken, it is more likely that we will get a definitive diagnosis. The pathologist is often able to provide us with a lot more detail about what the mass is, and what grade it may be.

How does a vet decide between an FNA and a biopsy?

There are a few considerations:

  • Biopsies are often taken when we are concerned for a cancerous tumour such as a sarcoma or when the mass is very diffuse or spread out.
  • A biopsy is often the best way of confirming a lesion is cancerous and of determining what grade it is. We are able to get more information as there are a lot more cells to look at compared to an FNA, so this method is much more accurate.
  • Large growths are usually biopsied before removing them, so we can plan the lump removal surgery and determine if the patient might need extra therapy, such as chemotherapy. Rather than FNA or biopsy a small growth, it may simply be removed in full, and then sent to the lab for analysis.
  • Very vascular lesions might be FNA’ed rather than biopsied, to minimise bleeding.
  • It is not uncommon for an FNA to yield an inconclusive results, especially when the mass is not exfoliative; meaning not many cells are shed. It can be frustrating for an owner to pay $100-200 for an FNA, only to get no definitive answer from it. In these cases, we may have to do a biopsy.


  • Dr. Linda Simon, Veterinarian

    Dr Linda Simon (MVB MRCVS) has 10 years of experience as a veterinarian. She is a veterinary surgeon with a special interest in geriatric patient care, dermatology and endocrinology. She is a member of the British Royal College of Veterinary Surgeons. She graduated top of her class from UCD School of Veterinary Medicine in Dublin in 2013. Linda has also worked as a locum vet in a range of clinics, including 24 hour emergency clinics and busy charity clinics.

Disclaimer: This website's content is not a substitute for veterinary care. Always consult with your veterinarian for healthcare decisions. Read More.

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