CSF analysis (cerebrospinal fluid) in cats and dogs

What it shows

Healthy CSF is clear and colourless, with very few cells — mostly small lymphocytes and monocytes — and far less protein than blood. Red blood cells are not part of normal CSF; when they appear, the needle has most often touched a small vessel during collection.

The form usually shows two numbers: the nucleated cell count per microlitre (an increase is called pleocytosis) and total protein, which rises when the barrier between blood and nervous tissue becomes leaky or when protein is made inside it. A pathologist also reads a smear, and which cells dominate — neutrophils, lymphocytes and monocytes, or eosinophils — often says more than the count itself.

Higher than reference — what it may be a sign of

• Many neutrophils: bacterial infection (protein often above 100 mg/dL; neutrophils often damaged, bacteria sometimes seen) or, in dogs, steroid-responsive meningitis-arteritis, where neutrophils look intact and the culture is negative.
• Mainly lymphocytes and monocytes, more protein: viral infections such as canine distemper, and meningoencephalitis of unknown origin in dogs, where more than half of the cells being mononuclear supports the suspicion; the final answer comes from tissue examination.
• Eosinophils: parasitic and fungal infections; rarely eosinophilic inflammation without infection.
• Feline infectious peritonitis (FIP) in cats: many cells, neutrophils or a mix, and markedly increased protein.
• Protein up, cell count normal: in a study of cats with brain tumours, most had a mild protein rise and a normal total white cell count, though the share of cell types was often changed.
• Blood from the collection itself is common and is taken into account when the numbers are read.

Lower than reference

A very low count or protein has no special meaning — for cells the reference starts at zero. What matters more is the other way round: normal CSF does not rule out disease. In animals with neurological disease the fluid is not always abnormal; disease near the membranes around the brain changes CSF more than a deep lesion inside brain tissue. In rabies, for example, CSF stays normal.

Look at together with

• Neurological examination and MRI — CSF supports the most likely diagnosis together with breed, age and signs; on its own it is rarely specific.
• Protein electrophoresis — when CSF protein is markedly increased, shows which kind of protein is up.
• Bacterial culture — when bacteria are seen or cells are poorly preserved.
• Antibody tests for Toxoplasma or Ehrlichia; in cats high IgM antibodies to Toxoplasma in serum or CSF support toxoplasmosis.

Units and conversion

CSF protein is reported in mg/dL or in g/L: 1 g/L equals 100 mg/dL, so 0.3 g/L is 30 mg/dL. Mind the scale: a CSF protein of 30 means milligrams, while blood protein is counted in grams. The cell count is given in cells/µL; many forms add the red cell count and the percentage of each cell type. In the app each row keeps the unit printed on the form.

Reference range

The reference range is on your lab’s form. It depends on the species and on where the fluid was taken: in dogs, fluid from the lower back had fewer cells but more protein than fluid from the cistern at the back of the head (in one group of healthy dogs, protein 3–23 mg/dL in the cistern and 18–44 mg/dL in the lumbar region). For cisternal fluid, a textbook chapter regards 0–2 cells/µL as normal in dogs, 3 as questionable and 4 as clearly abnormal; in cats 3 cells/µL or more is abnormal. Cells break down quickly, so the sample is processed within 30–60 minutes, often where a laboratory can handle it at once.

Cats and dogs

Sources

1. VCA Animal Hospitals. Cerebrospinal Fluid Collection and Examination (Know Your Pet).
2. Vernau W, Vernau KA, Bailey CS. Cerebrospinal Fluid. In: Clinical Biochemistry of Domestic Animals, 6th ed. Elsevier, 2008 (PMC7150067).
3. Meningitis, Encephalitis, and Encephalomyelitis in Animals. Merck Veterinary Manual, professional version.
4. Inflammatory and Infectious Diseases of the Spinal Column and Cord in Animals. Merck Veterinary Manual, professional version.
5. Rand JS, Parent J, Percy D, Jacobs R. Clinical, cerebrospinal fluid, and histological data from thirty-four cats with primary noninflammatory disease of the central nervous system. Can Vet J. 1994;35(3):174–181.