Fine-needle aspirate cytology (FNA) in cats and dogs

What it shows

FNA is the most frequently used way to collect cells for cytology. A fine needle, with or without a syringe for suction, enters the tissue; the cells are spread on a slide, stained and examined. It is used for lumps on and under the skin, enlarged lymph nodes, internal organs such as the liver, spleen, lungs or kidneys, and fluids.

Cytology looks at individual cells, not at how tissue is built. That is its strength and its limit: it often gives a working answer the same day, but the arrangement of cells, the grade of a tumour and whether it has spread into nearby tissue are judged by histology — the examination of a tissue piece from a biopsy.

Higher than reference — what it may be a sign of

• Inflammation — named by the main cell type: neutrophilic (suppurative), macrophagic, mixed or pyogranulomatous, eosinophilic, lymphoplasmacytic. Degenerate neutrophils or bacteria inside cells raise the question of infection, and the vet may ask for a culture.
• Cyst or benign mass — for example, a keratin-filled skin cyst, which is always benign; fat cells from the centre of a soft mass fit a lipoma.
• Hyperplasia — enlargement of normal tissue, which can look much like a benign tumour on cytology.
• Neoplasia — cells that are not expected at that site. The cytologist names the group: epithelial (carcinoma or adenoma), mesenchymal (sarcoma or a benign “-oma”), round cell (lymphoma, mast cell tumour, histiocytic and plasma cell tumours) or endocrine. Signs of malignancy, such as variation in cell and nucleus size, support but do not prove cancer.

Lower than reference

“No cytologic abnormalities” means the cells look like normal tissue for the site. It is reassuring but does not rule everything out: the needle may have missed the most telling cells. “Non-diagnostic” means the slide could not be read — too few cells, too much blood, ruptured cells, or surrounding fat sampled instead of the lump. Some tissues simply release few cells, especially some mesenchymal tumours. In both cases the vet decides whether to repeat the aspirate or take a biopsy.

Look at together with

• Histology (biopsy) — the diagnostic gold standard; it is routinely advised to confirm cytology and is often needed to tell a benign tumour from a malignant one. A tumour grade comes from histology.
• Flow cytometry and PARR — when an aspirate of a lymph node suggests lymphoma, these tests type the cells and check clonality.
• c-KIT mutation test — for mast cell tumours in dogs.
• Culture — when the aspirate shows neutrophilic inflammation and infection is possible.
• Impression smear cytology — cells taken from a surface rather than with a needle.

Units and conversion

There are no units. The report is a description of the cells — how many, which types, what they look like — and a conclusion such as “neutrophilic inflammation”, “consistent with lipoma” or “round cell neoplasia, likely mast cell tumour”, often with a comment on what to do to confirm it. In the app the result is stored as text, as written on the form.

Reference range

The expected result is on your lab’s form — there is no number to compare. The conclusion is read together with where the sample came from, how the lump looks and how fast it changes. Any growth that is surgically removed is worth sending for histology, even after a cytology result.

Cats and dogs

Sources

1. VCA Animal Hospitals — Cytology: Fine Needle Aspiration (FNA)
2. VCA Animal Hospitals — Cytology, Biopsy, and Histopathology
3. eClinPath (Cornell University College of Veterinary Medicine) — Cytologic patterns
4. eClinPath (Cornell University College of Veterinary Medicine) — Cytology: sample collection
5. Merck Veterinary Manual — Cytology (Diagnostic Procedures for the Private Practice Laboratory)
6. VCA Animal Hospitals — Adipose (Lipoma) Tumors
7. Merck Veterinary Manual (Cat Owners) — Tumors of the Skin in Cats