A lumbar puncture, also called a spinal tap, tests for infections, bleeding, cancer, and inflammatory diseases of the brain and spinal cord. It does this by collecting a small sample of cerebrospinal fluid (CSF), the clear liquid that surrounds and cushions your central nervous system. Doctors analyze the fluid’s appearance, protein levels, sugar content, cell counts, and pressure to detect problems that blood tests and imaging often miss.
Bacterial vs. Viral Meningitis
Meningitis is one of the most common reasons for an urgent lumbar puncture. The fluid sample can distinguish between bacterial and viral meningitis, a distinction that changes treatment dramatically. In bacterial meningitis, the CSF shows a surge of white blood cells (specifically a type called neutrophils), elevated protein, and glucose that drops below 50% of your blood sugar level. Viral meningitis looks different: the white blood cells are mostly lymphocytes, protein is mildly elevated, and glucose stays normal.
This difference matters because bacterial meningitis is a medical emergency requiring immediate antibiotics, while viral meningitis typically resolves on its own. A lumbar puncture can also detect fungal meningitis and infections like encephalitis (inflammation of the brain itself) and syphilis that has spread to the nervous system.
Subarachnoid Hemorrhage
When someone has the worst headache of their life and a CT scan comes back normal, a lumbar puncture is often the next step. CT scans are excellent at catching brain bleeds early on, but their accuracy drops as hours pass. The spinal fluid is checked for a yellowish discoloration called xanthochromia, caused by the breakdown of red blood cells into bilirubin. If bilirubin is present in the fluid above a specific threshold, it confirms bleeding around the brain even when imaging looks clean.
Multiple Sclerosis
A lumbar puncture plays a supporting role in diagnosing multiple sclerosis (MS). The lab looks for proteins called oligoclonal bands, which are immunoglobulins produced when the immune system is actively attacking the central nervous system. The key finding is two or more of these bands appearing in the spinal fluid but not in the blood. Their presence points toward MS, though it doesn’t confirm it on its own. Other inflammatory conditions of the nervous system can also produce oligoclonal bands, so doctors interpret the result alongside MRI findings and clinical symptoms.
Guillain-Barré Syndrome
Guillain-Barré syndrome causes rapidly worsening weakness as the immune system attacks the nerves outside the brain and spinal cord. The CSF in Guillain-Barré shows a characteristic pattern: protein levels are high, but the cell count stays normal. This combination, called albuminocytologic dissociation, is one of the diagnostic criteria for the condition. It helps separate Guillain-Barré from infections, which would cause both protein and cell counts to rise together.
Cancer of the Brain and Spinal Cord
Certain cancers can spread to the fluid surrounding the brain and spinal cord, a condition known as leptomeningeal disease. A lumbar puncture allows doctors to examine the fluid under a microscope for cancer cells. This is particularly relevant for leukemia and lymphoma. In acute lymphoblastic leukemia, for example, fewer than 10% of newly diagnosed adults have cancer cells in their spinal fluid at diagnosis, but checking is routine because finding them changes the treatment plan. The fluid is also tested at follow-up appointments and at relapse to monitor whether cancer has reached the central nervous system.
Measuring Intracranial Pressure
Beyond analyzing the fluid itself, a lumbar puncture measures the pressure inside your skull. Normal CSF opening pressure falls between 10 and 25 cm of water. Pressure above 25 cm (and in some cases up to 28 cm, depending on body size) suggests intracranial hypertension, a condition where too much pressure builds around the brain. This measurement is central to diagnosing idiopathic intracranial hypertension, a condition that causes severe headaches and vision changes, particularly in younger women. Abnormally low pressure, below 10 cm, can indicate a CSF leak.
Alzheimer’s Disease and Dementia
Lumbar punctures are increasingly used in the evaluation of cognitive decline. The spinal fluid can be tested for specific proteins associated with Alzheimer’s disease, helping distinguish it from other forms of dementia. This is especially useful early in the disease course, when imaging may not yet show clear changes, or when the clinical picture is ambiguous.
What the Lab Actually Measures
A standard CSF analysis looks at several values. In a healthy adult, the fluid contains no more than 5 white blood cells per cubic millimeter. Protein ranges from 18 to 58 mg per deciliter. Glucose runs at roughly two thirds of your blood sugar level measured in the preceding two to four hours, which is why a blood draw is typically done at the same time as the puncture. Deviations from these ranges point the diagnosis in different directions: high white cells suggest infection or inflammation, high protein with normal cells suggests nerve damage, low glucose suggests bacterial or fungal infection consuming it.
The fluid’s appearance also tells a story. Normal CSF is crystal clear. Cloudy fluid suggests infection. Pink or red fluid indicates bleeding, though this can also happen if a small blood vessel is nicked during the procedure (a “traumatic tap”). The lab can usually distinguish the two.
When a Lumbar Puncture Cannot Be Done Safely
There are situations where performing a lumbar puncture is dangerous. The most serious is when pressure inside the skull is unevenly distributed, because removing fluid from below can cause brain tissue to shift downward. This is why most patients get a CT scan before a lumbar puncture. Signs on imaging that make the procedure unsafe include a visible shift of the brain’s midline structures or compression of the fluid spaces at the base of the brain. Infection at the skin where the needle would be inserted is the other absolute reason to hold off.
What Recovery Looks Like
The most well-known side effect is a post-procedure headache, sometimes called a spinal headache. It happens because the puncture site continues to leak small amounts of fluid after the needle is removed, temporarily lowering pressure around the brain. The headache typically worsens when you sit or stand and improves when you lie flat. Incidence varies widely, from under 2% to as high as 40%, depending on needle type, needle size, and individual risk factors.
Modern practice reduces the risk considerably. Clinical guidelines recommend using a pencil-point (non-cutting) needle, which parts the tissue fibers rather than slicing through them, resulting in a smaller hole that seals faster. If a cutting needle is used, a narrower gauge and specific orientation of the needle bevel also lower the odds. Bed rest after the procedure, despite being widely recommended in the past, has not been shown to prevent these headaches. Most spinal headaches resolve within a few days on their own, though persistent cases can be treated with a procedure that patches the leak.