Lumbar puncture is used to do the following:
Evaluate intracranial pressure and cerebrospinal fluid (CSF) composition (see table Cerebrospinal Fluid Abnormalities in Various Disorders Cerebrospinal Fluid Abnormalities in Various Disorders Lumbar puncture is used to do the following: Evaluate intracranial pressure and cerebrospinal fluid (CSF) composition (see table Cerebrospinal Fluid Abnormalities in Various Disorders) Therapeutically... read more )
Therapeutically reduce intracranial pressure (eg, idiopathic intracranial hypertension Idiopathic Intracranial Hypertension Idiopathic intracranial hypertension causes increased intracranial pressure without a mass lesion or hydrocephalus, probably by obstructing venous drainage; cerebrospinal fluid composition is... read more )
Administer intrathecal drugs or a radiopaque contrast agent for myelography Myelography X-rays taken after a radiopaque agent is injected via an intra-arterial catheter show individual cerebral arteries and venous structures of the brain. With digital data processing (digital subtraction... read more
Relative contraindications include
Infection at the puncture site
Bleeding diathesis
Increased intracranial pressure due to an intracranial mass lesion, obstructed CSF outflow (eg, due to aqueductal stenosis or Chiari I malformation), or spinal cord CSF blockage (eg, due to tumor cord compression)
If papilledema or focal neurologic deficits are present, CT or MRI should be done before lumbar puncture to rule out presence of a mass that could precipitate transtentorial or cerebellar herniation Brain Herniation Brain herniation occurs when increased intracranial pressure causes the abnormal protrusion of brain tissue through openings in rigid intracranial barriers (eg, tentorial notch). Because the... read more 
.
Lumbar puncture procedure
For the procedure, the patient is typically in the left lateral decubitus position. A cooperative patient is asked to hug the knees and curl up as tightly as possible. Assistants may have to hold patients who cannot maintain this position, or the spine may be flexed better by having patients, particularly obese patients, sit on the side of the bed and lean over a bedside tray table.
An area 20 cm in diameter is washed with iodine, then wiped with alcohol to remove the iodine and prevent its introduction into the subarachnoid space. A lumbar puncture needle with stylet is inserted into the L3-to-L4 or L4-to-L5 interspace (the L4 spinous process is typically on a line between the posterior-superior iliac crests); the needle is aimed rostrally toward the patient’s umbilicus and always kept parallel to the floor. Entrance into the subarachnoid space is often accompanied by a discernible pop; the stylet is withdrawn to allow CSF to flow out.
Opening pressure is measured with a manometer; 4 tubes are each filled with about 2 to 10 mL of CSF for testing. The puncture site is then covered with a sterile adhesive strip.
A post–lumbar puncture headache Post–Lumbar Puncture and Other Low–Pressure Headaches Low-pressure headaches result from reduction in cerebrospinal fluid (CSF) volume and pressure due to lumbar puncture or spontaneous or traumatic CSF leaks. (See also Approach to the Patient... read more occurs in about 10% of patients.
Lumbar puncture
This lumbar puncture is done with the patient in the lateral decubitus position and the lumbar puncture needle inserted at the L3-L4 interspace.  |
CSF color
Normal CSF is clear and colorless; ≥ 300 cells/microL produces cloudiness or turbidity.
Bloody fluid may indicate a traumatic puncture (pushing the needle in too far, into the venous plexus along the anterior spinal canal) or subarachnoid hemorrhage Subarachnoid Hemorrhage (SAH) Subarachnoid hemorrhage is sudden bleeding into the subarachnoid space. The most common cause of spontaneous bleeding is a ruptured aneurysm. Symptoms include sudden, severe headache, usually... read more 
. A traumatic puncture is distinguished by
Gradual clearing of the CSF between the 1st and 4th tubes (confirmed by decreasing the red blood cell [RBC] count)
Absence of xanthochromia (yellowish CSF due to lysed RBCs) in a centrifuged sample
Fresh, uncrenated RBCs
With intrinsic subarachnoid hemorrhage, the CSF remains uniformly bloody throughout collection; xanthochromia is often present if several hours have passed after ictus; and RBCs are usually older and crenated. Faintly yellow fluid may also be due to senile chromogens, severe jaundice, or increased protein (> 100 mg/dL).
CSF cell count and glucose and protein levels
Cell count and differential and glucose and protein levels aid in the diagnosis of many neurologic disorders (see table Cerebrospinal Fluid Abnormalities in Various Disorders Cerebrospinal Fluid Abnormalities in Various Disorders Lumbar puncture is used to do the following: Evaluate intracranial pressure and cerebrospinal fluid (CSF) composition (see table Cerebrospinal Fluid Abnormalities in Various Disorders) Therapeutically... read more ).
Normally, CSF:blood glucose ratio is about 0.6, and except in severe hypoglycemia, CSF glucose is typically > 50 mg/dL (> 2.78 mmol/L).
Increased CSF protein (> 50 mg/dL) is a sensitive but nonspecific index of disease; protein increases to > 500 mg/dL in purulent meningitis, advanced TB meningitis, complete block by spinal cord tumor, or a bloody puncture. Special examinations for globulin (normally < 15%), oligoclonal banding, and myelin basic protein aid in diagnosis of a demyelinating disorder.
CSF staining, testing, and culture
If infection is suspected, the centrifuged CSF sediment is stained for the following:
Larger amounts of fluid (10 mL) improve the chances of detecting the pathogen, particularly acid-fast bacilli and certain fungi, in stains and cultures. In early meningococcal meningitis Meningococcal Diseases Meningococci (Neisseria meningitidis) are gram-negative cocci that cause meningitis and meningococcemia. Symptoms, usually severe, include headache, nausea, vomiting, photophobia, lethargy,... read more 
or severe leukopenia Overview of Leukopenias Leukopenia is a reduction in the circulating white blood cell (WBC) count to read more , CSF protein may be too low for bacterial adherence to the glass slide during Gram staining, producing a false-negative result. Mixing a drop of aseptic serum with CSF sediment prevents this problem. When hemorrhagic meningoencephalitis is suspected, a wet mount is used to search for amebas.
Latex particle agglutination and coagglutination tests may allow rapid bacterial identification, especially when stains and cultures are negative (eg, in partially treated meningitis). CSF should be cultured aerobically and anaerobically and for acid-fast bacilli and fungi.
Except for enteroviruses, viruses are seldom isolated from the CSF. Viral antibody panels are available.
Venereal Disease Research Laboratories (VDRL) testing and cryptococcal antigen testing are often routinely done. Polymerase chain reaction (PCR) tests for herpes simplex virus Herpes Simplex Virus (HSV) Infections Herpes simplex viruses (human herpesviruses types 1 and 2) commonly cause recurrent infection affecting the skin, mouth, lips, eyes, and genitals. Common severe infections include encephalitis... read more 
and other central nervous system (CNS) pathogens are increasingly available.
Specialized CSF tests can be done; they include tests for specific antibodies in various disorders, such as autoimmune encephalitis (see also Mayo Clinic: Encephalopathy Autoimmune Evaluation Algorithm: Spinal Fluid). The autoimmune encephalitides are antibody-mediated brain disorders that target specific neuronal antigens; symptoms include an altered level of consciousness, seizures, and cognitive and behavioral dysfunction.
