CSF cytology (cells of cerebrospinal fluid) in cats and dogs

What it shows

Cytology looks at single cells rather than a piece of tissue. CSF holds very few cells, so the lab concentrates them onto a slide, for example with a cytocentrifuge, and a pathologist reads the stained smear.

Normal CSF contains small lymphocytes and monocytes in varying proportions; rare neutrophils can also be a normal finding. Some cells are not part of normal CSF: eosinophils, plasma cells, large foamy activated macrophages and reactive lymphocytes. Their presence points to inflammation, but on its own it does not say what caused it. Red blood cells are not normal either; most often they come from the needle touching a small vessel, less often from real bleeding.

Higher than reference — what it may be a sign of

There is no “higher” here; instead the report names which cells dominate. Typical patterns:

• Neutrophils — bacterial infection, where neutrophils are often degenerate and may contain bacteria, and in dogs steroid-responsive meningitis-arteritis, where they look intact. In cats, feline infectious peritonitis (FIP) can also give many neutrophils or a mixed picture.
• Mainly lymphocytes — viral infections usually give a mild to moderate mononuclear, mostly lymphocytic, increase.
• Eosinophils — parasitic and fungal infections; less often eosinophilic inflammation without an infectious cause.
• Granulomatous inflammation — mostly mononuclear cells or a mixed picture; an infection with fungi or protozoa and inflammation without infection can look alike under the microscope.
• Organisms — fungi and occasionally protozoa can be seen in CSF, and bacteria are sometimes identified on stained smears. For fungi and protozoa blood tests are usually still needed to confirm the infection.
• Tumour cells — seen only occasionally. The exception is lymphoma: malignant lymphocytes have been reported in the CSF of dogs and cats with lymphoma of the brain or spinal cord.

Any of these patterns is a clue for the vet, not a diagnosis.

Lower than reference

“No abnormal cells seen” is reassuring but rules little out. The CSF of animals with neurological disease is not always abnormal: tumour cells are infrequently shed into the fluid, in rabies the fluid stays normal, and a sample with very few cells may not be enough to judge. Only occasionally does CSF cytology give a specific answer — mainly when organisms or tumour cells are seen — so the result is never read on its own.

Look at together with

• CSF cell count and protein — cytology and the numbers come from the same sample and are read together.
• Bacterial culture — when neutrophils look degenerate or bacteria are suspected; in dogs culture of CSF is less often positive than in large animals.
• Infectious tests — for example tests for Cryptococcus and Toxoplasma antibodies, or antibodies to canine distemper in CSF, which help tell active distemper infection from vaccination.
• Neurological examination and MRI — CSF findings support the most likely diagnosis together with breed, age and signs.

Units and conversion

There are no units: CSF cytology is a written description. Labs list the cell types they see, sometimes with the share of each type in per cent, note whether cells look reactive or degenerate, mention organisms or atypical cells and end with a short conclusion. In the app the result is stored as text, as written on the form.

Reference range

The reference range is on your lab’s form or in the pathologist’s comment. There is no number to compare: the expected picture is a few small lymphocytes and monocytes, no eosinophils and no organisms. Because cells deteriorate quickly, the sample is processed within 30 to 60 minutes of collection, which is why it is often sent to a lab that can handle it at once.

Cats and dogs

Sources

1. VCA Animal Hospitals — Cerebrospinal Fluid Collection and Examination
2. Vernau W, Vernau KA, Bailey CS. Cerebrospinal Fluid. In: Clinical Biochemistry of Domestic Animals, 6th ed. Elsevier, 2008 (PMC7150067)
3. Merck Veterinary Manual — Meningitis, Encephalitis, and Encephalomyelitis in Animals
4. Merck Veterinary Manual — Neoplasia of the Nervous System in Small Animals