What Is a Fine Needle Aspirate?

A fine needle aspirate, usually shortened to FNA, is a way of collecting a small sample of cells from a lump so someone can look at them under a microscope. It's the first test almost any veterinarian will reach for when they find a mass on your dog.

The procedure itself is unremarkable. A thin needle, usually the same gauge used for vaccinations, goes into the lump. Cells are drawn or teased out, squirted onto a glass slide, spread thin, stained, and examined. Most dogs need no sedation. The whole thing takes a couple of minutes and feels to your dog like an injection.

What it gives you is a look at individual cells rather than intact tissue. For some things that's plenty. An FNA can confirm a lipoma, a cyst, or simple inflammation, and it reliably identifies the group of cancers called round cell tumors, which includes lymphoma and mast cell tumors. For other masses it narrows the category without naming the tumor, and a biopsy follows.

Two things are worth knowing before you get the result. Cytology can't grade a tumor or tell you whether surgery removed all of it. And a result saying "no cancer found" is considerably less reliable than one saying cancer is present.

Key points

• An FNA takes minutes, usually needs no sedation, and results typically come back within 0 to 2 days.
• It gives a definitive answer for lipomas, cysts, inflammation, and round cell tumors like lymphoma and mast cell tumors.
• It can't grade a tumor, assess how deeply it has invaded, or check surgical margins. Those need a biopsy.
• When cytology says a mass is cancer, it's almost always right. One study put the positive predictive value at 99.4%.
• When cytology says a mass isn't cancer, it's wrong roughly a third of the time. Negative predictive value in that same study was 68.7%.
• Around one sample in six comes back with too few cells to interpret. That's not a negative result, it's no result.
• Fatty and fibrous masses are the ones most likely to yield poor samples.
• Complications are rare. Brief soreness and a small bruise are the usual extent of it.

Detailed explanation

What actually happens

Your dog is held still, often just by a nurse with a hand on the shoulder. The area may get a quick wipe. A needle goes into the mass, sometimes attached to a syringe so cells can be drawn back, sometimes not, since simply moving the needle within the tissue collects cells by capillary action. Your veterinarian may take two or three samples from slightly different spots.

The contents get expelled onto slides, spread out, dried, and stained. Then either your vet looks at them there and then, or they go to a laboratory.

Most dogs tolerate this without any sedation. Dogs who are anxious, or masses in awkward places like the face, mouth, or paw, may warrant light sedation, which is a conversation worth having if your dog finds the vet stressful.

Why it's the first step

Because nothing else answers the question. The 2026 AAHA Oncology Guidelines are explicit that the type and likely behaviour of a mass cannot be determined by feeling it or by imaging. A mass that feels like a fatty lump can be something else. Mast cell tumors in particular imitate almost anything.

So when an examination or a scan turns up a mass, sampling it is the logical next move. And the widely used rule of thumb for owners and general practitioners is that a skin mass the size of a pea or larger, or one that's been there a month, should be aspirated rather than watched.

The reason to act rather than monitor comes down to surgery. If a mass turns out to be malignant, the first operation is the best chance of removing it completely. Waiting lets tumors grow, which makes clean removal harder, particularly on legs, the head and the neck where there isn't much spare tissue.

What an FNA can tell you

For some masses it's genuinely definitive. Cytology often gives a firm answer for benign things like dermal cysts, lipomas and inflammation, and for the round cell tumors, a group that includes lymphoma, mast cell tumors and plasma cell tumors. Mast cells are particularly distinctive under a microscope, which is why mast cell tumors are frequently diagnosed on a simple aspirate.

For other masses, cytology sorts cells into broad families. It can usually say whether cells look epithelial or mesenchymal in origin, and often whether they look benign or malignant. That's enough to plan the next step even when it doesn't name the tumor.

An enlarged lymph node is another common target. This matters more than people expect, because node size is an unreliable guide to whether cancer has spread. Nodes can be enlarged from infection or inflammation, and normal-sized nodes can contain cancer cells. Sampling settles it.

What it can't tell you

Cytology looks at loose cells, not tissue architecture, so several things are simply out of reach:

• Grade. How aggressive a tumor looks under the microscope comes from a biopsy, with rare exceptions.
• Invasion. How deeply a mass has grown into surrounding tissue needs intact tissue to assess.
• Margins. Whether surgery removed all of it can only be answered by examining the removed piece.

This is why a mass is often aspirated first and biopsied later. The two tests answer different questions.

How accurate is it, really

This is the part worth understanding properly, because the accuracy isn't symmetrical.

In a study of 292 skin and subcutaneous masses from dogs and cats, cytology agreed with the eventual biopsy result in 90.9% of interpretable cases. Sensitivity for detecting cancer was 89.3% and specificity was 97.9%.

The two numbers that matter most for owners are these. Positive predictive value was 99.4%, meaning that when cytology said a mass was cancer, it essentially always was. Negative predictive value was 68.7%, meaning that when cytology said a mass wasn't cancer, it was wrong about a third of the time.

A separate study of 300 canine masses found much the same pattern: nearly all cytologic cancer diagnoses were confirmed, while about 36% of the "not cancer" calls turned out to be false negatives.

So the practical rule is this. If the aspirate says cancer, believe it. If the aspirate says no cancer but the lump is growing, changing, or bothering your dog, don't consider the matter closed. Recheck it, resample it, or biopsy it.

When the sample comes back non-diagnostic

Roughly one sample in six yields too few cells to interpret. In the studies above, retrieval rates were 83.2% and 82.7%.

This is not a negative result. It means nobody could see enough to form a view. Some masses simply don't give up their cells easily, and fatty and fibrous tumors are the usual culprits, which is a little unfortunate given that those are also the masses owners are most likely to be told to keep an eye on.

If your dog's aspirate comes back non-diagnostic, the options are usually to repeat it, sometimes with ultrasound guidance for better targeting, or to move to a biopsy. What isn't appropriate is treating it as reassurance.

Who reads the slide

This makes more difference than most owners realise, and it's a fair thing to ask about.

Many practices examine slides in-house, which is fast and cheaper. AAHA guidance suggests this is reasonable when the clinician is confident in the diagnosis, such as a clear-cut mast cell tumor, when cost is a real constraint, or when enough information has already been gathered to plan a biopsy anyway.

Sending slides to a clinical pathologist is recommended when there's uncertainty about the cell type or whether a mass is malignant, when no further diagnostics are planned before starting treatment, and when a more detailed characterisation would change the treatment plan.

If your dog's result was ambiguous and read in-house, asking whether a pathologist should review it is a reasonable question.

Aspirating things you can't feel

FNAs aren't limited to lumps under the skin. With ultrasound guidance, samples can be taken from the liver, spleen, kidneys, lymph nodes deep in the abdomen, and masses in the chest. This is how internal disease often gets diagnosed without surgery.

Reliability varies by site. Lung aspirates in one study matched the eventual biopsy diagnosis in 82% of cases with no complications recorded. Liver aspirates produce a usable sample about 88% of the time, though agreement with biopsy findings is more variable, and liver cytology is better at spotting some conditions than others.

Some organs are approached cautiously or avoided. Splenic masses are the clearest example, since they're often highly vascular and bleeding is a genuine risk. Your veterinarian may prefer imaging and surgical removal over aspirating a spleen.

Risks

Very low, which is the main argument for doing it early rather than waiting.

Expect a small amount of soreness, possibly a little bruising, occasionally a small firm swelling at the site that settles within a day or two. Bleeding is uncommon and usually trivial. Infection is rare.

The specific one worth mentioning is mast cell tumors. Handling these masses can trigger the cells to release histamine, which causes local swelling and redness, and occasionally more widespread effects. Your veterinarian will handle a suspected mast cell tumor gently for this reason, and may give an antihistamine beforehand. This is a manageable risk rather than a reason to avoid sampling.

Owners occasionally ask whether putting a needle into a tumor can spread it. In practice this is not considered a meaningful risk for the great majority of masses, and the information gained substantially outweighs it.

Cost, timing, and what happens next

An FNA is one of the least expensive diagnostic tests in veterinary medicine, particularly compared to what follows if a mass is left to grow. In-house results can come back the same visit. Laboratory results usually take one to two days.

What happens next depends on the answer. A confirmed benign mass may need nothing more than monitoring. A confirmed cancer usually means staging tests to work out how far it has spread, followed by a treatment plan. An ambiguous or non-diagnostic result means repeating the aspirate or moving to a biopsy.

If the wait is difficult, and it usually is, ask when to expect the call and who will make it.

When to contact a veterinarian

Make an appointment and ask for an aspirate if you find:

• Any new lump the size of a pea or larger
• A lump that has changed in size, shape, firmness or appearance
• A lump that has been present for a month
• Firm swellings under the jaw, in front of the shoulders, or behind the knees
• A sore that hasn't healed within two weeks

Contact your veterinarian after an aspirate if your dog:

• Has swelling, redness, or heat at the site that is worsening rather than settling
• Seems painful at the site more than a day later
• Has widespread swelling, hives, or facial puffiness, which can follow sampling of a mast cell tumor
• Is bleeding from the site and it hasn't stopped

Go back and ask about resampling if:

• The result came back non-diagnostic
• The result said no cancer but the lump is still growing or changing
• You were told to monitor a lump that has since changed
• The result was ambiguous and read in-house rather than by a pathologist

Questions to ask your veterinarian

• Can this be aspirated today, during this visit?
• Will the slides be read here or sent to a clinical pathologist? Which would you recommend?
• What might this test tell us, and what will it not be able to tell us?
• If it comes back non-diagnostic, what's the next step and what does that cost?
• Does my dog need sedation for this?
• Should the nearby lymph nodes be sampled too, even if they feel normal?
• If this is a mast cell tumor, do we need to give anything before handling it?
• How long until we have results, and who will call me?
• If it comes back as cancer, what happens next?
• If it comes back benign but the lump keeps growing, when should I bring my dog back?

Sources

American Animal Hospital Association. (2026). 2026 AAHA oncology guidelines for dogs and cats: Section 3. Tumor diagnostics and staging. https://www.aaha.org/resources/2026-aaha-oncology-guidelines-for-dogs-and-cats/section-3-tumor-diagnostics-staging/

American College of Veterinary Surgeons. (n.d.). Mast cell tumors. https://www.acvs.org/small-animal/mast-cell-tumors/

Christensen, J., Johnson, K., Ettinger, S., Garrett, L., Gordon, I., Ireifej, S., Love, A., & Wisecup, M. (2026). AAHA oncology guidelines for dogs and cats. Journal of the American Animal Hospital Association, 62(1), 1-37. https://doi.org/10.5326/JAAHA-MS-7549

Ettinger, S. (n.d.). When to get a lump checked by a veterinarian. DogCancer.com. https://www.dogcancer.com/perspectives/diagnosis-and-medical-procedures/when-to-get-a-lump-checked-by-a-veterinarian/

Ghisleni, G., Roccabianca, P., Ceruti, R., et al. (2006). Correlation between fine-needle aspiration cytology and histopathology in the evaluation of cutaneous and subcutaneous masses from dogs and cats. Veterinary Clinical Pathology, 35(1), 24-30. https://doi.org/10.1111/j.1939-165X.2006.tb00084.x

Simeonov, R. (2010). The accuracy of fine-needle aspiration cytology in the diagnosis of canine skin and subcutaneous masses. Comparative Clinical Pathology. https://doi.org/10.1007/s00580-010-1075-5

Veterinary Radiology and Ultrasound. (2002). Correlation between fine-needle aspiration cytopathology and histopathology of the lung in dogs and cats. https://pubmed.ncbi.nlm.nih.gov/12118686/

The Veterinary Journal. (2019). A randomised controlled comparison of aspiration and non-aspiration fine-needle techniques for obtaining ultrasound-guided cytological samples from canine livers. https://www.sciencedirect.com/science/article/abs/pii/S1090023318306452