Pleocytosis is an abnormally high white blood cell count in cerebrospinal fluid (CSF), the clear liquid that surrounds your brain and spinal cord. In adults, normal CSF contains up to 5 white blood cells per microliter. Anything above that threshold is considered pleocytosis. It’s not a disease on its own but a lab finding that signals inflammation or infection somewhere in the central nervous system.
How White Blood Cells End Up in Spinal Fluid
Your brain and spinal cord are protected by the blood-brain barrier, a tightly sealed layer of cells lining the blood vessels of the central nervous system. Under normal conditions, very few immune cells cross this barrier. The small number that do make up the baseline count of roughly 70% lymphocytes and 30% monocytes found in healthy adult CSF.
When something triggers inflammation, whether an infection, an autoimmune process, or a tumor, the blood-brain barrier becomes more permeable. Immune cells latch onto the walls of tiny blood vessels using sticky surface proteins called adhesion molecules. Chemical signals then guide them through the vessel wall and into the surrounding brain or spinal cord tissue, and eventually into the CSF. The type of white blood cell that predominates in the fluid often points toward the underlying cause.
Types of Pleocytosis and What They Suggest
Lymphocytic Pleocytosis
A rise in lymphocytes is the most common pattern in viral infections of the central nervous system, such as herpes simplex encephalitis and enteroviral meningitis. It also appears in chronic conditions like tuberculosis and in autoimmune diseases. About 40% of people with multiple sclerosis have lymphocytic pleocytosis at the time of diagnosis, and in those patients it correlates with greater blood-brain barrier disruption and a heightened immune response. Cell counts in MS rarely exceed 50 per microliter; when they do, doctors typically suspect something else, particularly an infection.
Neutrophilic Pleocytosis
Neutrophils are the immune system’s rapid responders, and a neutrophil-dominant CSF profile classically points to bacterial meningitis. But the picture isn’t always that clean. In one study of viral central nervous system infections, enterovirus was responsible for 64% of cases that showed a neutrophil-predominant CSF, compared with only 33% of lymphocyte-predominant cases. Herpes viruses, by contrast, accounted for 46% of lymphocytic profiles but just 20% of neutrophilic ones. So while neutrophils raise concern for a bacterial cause, they don’t rule out a viral one, especially early in the course of illness when the immune response is still shifting.
Eosinophilic Pleocytosis
This rarer variant is defined by either 10 or more eosinophils per microliter or eosinophils making up at least 10% of the total CSF white cells. The most common trigger is a parasitic worm invading the central nervous system, particularly a roundworm called Angiostrongylus cantonensis (rat lungworm). Certain fungal infections can also cause it. In a study of patients with coccidioidal meningitis, a fungal disease common in the southwestern United States, 70% had eosinophilic pleocytosis. Drug reactions and some autoimmune conditions round out the list of possible causes.
How Pleocytosis Is Detected
The only way to measure CSF white blood cells is through a lumbar puncture, commonly called a spinal tap. A needle is inserted into the lower back to collect a sample of fluid. Guidelines recommend drawing at least 10 milliliters to avoid the need for a repeat procedure, and the sample should reach the lab within two hours so the cell count stays accurate. Cells in CSF break down quickly outside the body, so delays can lead to falsely low results.
One complication is the “traumatic tap,” where the needle nicks a small blood vessel and introduces blood into the sample. Since blood carries its own white cells, this can mimic pleocytosis. Clinicians use a standard correction formula, subtracting one white blood cell for every 1,000 red blood cells in the sample, to adjust for contamination. They may also use the “three tube test,” collecting fluid in sequential tubes to see if the red cell count drops (a sign of contamination rather than true bleeding).
Normal Ranges Vary by Age
The threshold of 5 white blood cells per microliter applies to adults and older children. Newborns are a different story: up to 20 white blood cells per microliter can be normal, and finding a few neutrophils in a newborn’s CSF isn’t unusual. This makes interpreting spinal tap results in infants trickier and is one reason neonatal meningitis workups require careful clinical judgment beyond just a cell count.
Pleocytosis as a Clue to Cancer
Elevated CSF white blood cells can also signal that cancer has spread to the membranes covering the brain and spinal cord, a condition called leptomeningeal disease. In a study of 116 patients with primary central nervous system lymphoma, 36% had pleocytosis at diagnosis. Pleocytosis proved to be a more reliable indicator of cancerous cells in the spinal fluid than elevated protein levels. Among patients whose CSF contained lymphoma cells, 86% had white blood cell counts above 5 per microliter, while only 14% had counts in the normal range. Protein elevation, by comparison, had far lower specificity (34% versus 74% for cell count), making it a less useful screening marker.
What the Cell Count Level Can Tell You
The degree of pleocytosis helps narrow the diagnosis. Mild elevations of 5 to 20 cells per microliter are common in conditions like MS or early viral infections. Moderate counts in the hundreds are typical of viral meningitis. Counts in the thousands, sometimes exceeding 10,000, often point to bacterial meningitis or a brain abscess. But no single number is definitive. Doctors interpret pleocytosis alongside other CSF findings like protein and glucose levels, the type of white cell that dominates, and the patient’s symptoms and imaging results. It’s one piece of a larger diagnostic puzzle, not a diagnosis in itself.