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Lumbar puncture

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What is a lumbar puncture or spinal tap?


Lumbar puncture is a procedure where a needle is inserted in the lower back to take out a small sample of cerebrospinal fluid(CSF). Sometimes, it might be also done to remove a small amount of CSF to reduce pressure within the head or to inject a medicine into the spine.

In a simple sense, lumbar puncture does for the nervous system, what a blood test does for the body taking out a small sample of fluid for evaluating any abnormality.

How does lumbar puncture or spinal tap works?

CSF is the crystal clear fluid that surrounds and cushions the brain and the spinal cord protecting it from injury. It also circulates nutrients and removes waste products from the brain. It maintains constant intracranial pressure. It thus provides an indirect assessment of brain status.

As the brain and the spinal cord remain in intimate contact with the CSF, evaluation of CSF helps in the diagnosis of many brain and spine diseases. It is therefore also used to instill medicines, anesthesia, or chemotherapy drugs directly to the brain and spinal cord bypassing other parts of the body.

Why is it done? What is the use of Lumbar puncture?

To collect CSF to test for certain brain and spine conditions: such as

Meningitis (inflammation of the outer layers covering the brain)

Encephalitis (inflammation of the brain)

– Myelitis (inflammation of the spinal cord)

– Autoimmune inflammatory conditions such as multiple sclerosis

– Subarachnoid hemorrhage (bleeding along the layers of the brain and spinal cord)

– Tumors (lymphoma and other cancer)

To measure CSF pressure to diagnose hydrocephalus, where there is increased intra-cranial pressure.

To take out small fluid to relieve increased intracranial pressure in certain cases of hydrocephalus.

To control CSF pressure during brain surgery to keep the brain relaxed.

To administer medicines directly into CSF so that it reaches the brain and spine directly such as chemotherapy agents, antibiotics, and others.

To instill anesthetic agent into spine to achieve anesthesia in the lower body.

To put contrast medium/dye (myelography) or radioactive substances (cisternography) into the spine to visualize it with images.

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Where is lumbar puncture done and who performs the test?

This test is performed by a doctor in his clinic or in the hospital. For assessment of the brain conditions, it is generally done by a neurologist or neurosurgeon who is a specialist in brain and spinal cord diseases.

How is Lumbar puncture done? What happens during Lumbar puncture?

The lumbar puncture takes about 30 to 45 minutes. An intravenous line might be placed in the arm to give fluids, sedatives, or any other medicine. General anesthesia is not required and the procedure is done under sedatives to make the person feel relaxed. The following steps are taken sequentially:

1. Positioning of the patient:

Generally, the person would be asked to lie on one side with knees drawn to the chest and chin drawn inwards. Sometimes a person might be asked to sit and lean forward. These positions increase the distance between the bones of the spine to allow clear access of the needle into the spinal canal without hitting the bone.

2. Preparation of the site of insertion:

After patients positioning the following things are done:

Cleaning of lower back skin with antiseptic to prevent getting infection into the spine and to the brain.

Injecting the local anesthetic agent into the skin to avoid pain during the procedure. This may cause little stinging for a brief period.

3. Insertion of the needle into the spinal canal:

After anesthesia, a special hollow needle is inserted between bones of the spine, usually between the 3rd and 4th lumbar vertebrae. This is the preferred site, as it avoids the needle from damaging the spinal cord which usually ends by 2nd lumbar vertebrae in most of the people.

During the insertion of the needle, the person doesn’t feel much pain. However, he might feel some pressure or a stinging sensation just before the needle pierced through the outer protective layer called dura which surrounds the spinal cord.
For detecting any condition the doctor usually collects between 5 to 20 ml of CSF.

It is important for a person to lie still during the procedure and let the doctor know if he is feeling pain.

If the lumbar puncture is done for other reasons following things are done after inserting the needle into the spinal canal accordingly:

• Measure CSF pressure:

To measure the pressure the inserted needle would be attached to a meter and the person would be asked to straighten his legs.

• Remove excess CSF to reduce pressure:

In cases of increased intracranial pressure or hydrocephalus, a catheter could be inserted to continuously drain out CSF and relieve increased pressure on the brain.

• Instill medicine or anesthesia:

In some cases, an antibiotic, chemotherapy drug, or anesthesia is injected after getting access into the spinal canal.

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What happens after the procedure?

Immediately after the procedure, the doctor would apply mild pressure to the puncture site and stick a bandage on it. The person would generally be asked to rest in the bed for about an hour. The person is directed not to perform any strenuous physical activity for at least 24 hours. For the remaining day, it is considered good to drink plenty of fluids.

In case of mild headache or back pain, the person could take non-prescription painkillers such as acetaminophen. If there’s a leakage of fluid or oozing of blood from the puncture site the person must immediately consult a doctor.

What is the preparation required for the test?

Several or a few days prior to lumbar puncture, the doctor might ask for the following things:

• Imaging test: such as CT scan or MRI, to confirm if the lumbar puncture would be needed and to check for its safety.

• Drug history: about intake of any blood-thinning medicine such as warfarin. Such patients might be asked to avoid these medicines to prevent bleeding during the test.

On the day of the test, the following things might be done for the preparation:

• Take regular breakfast before the test

• Take medicines as discussed with the doctor

• Change clothes and wear a hospital gown

• Sign the consent for the test, where the person authorizes the doctor to perform such a test, after understanding the associated risks, benefits, and its need.

Are there any side effects or pitfalls of Lumbar puncture?

Lumbar puncture is usually a safe procedure with only some people developing any serious side effects.

• Post-procedure headache: it is found that about one third (32%) patients develop headaches after the lumbar puncture procedure, also known as spinal headache. This is most likely occurs due to leakage of the CSF from the hole that develops when the needle pierces the covering of the spinal cord. This CFS leakage in turn leads to a fall in the CSF volume producing headache. These headaches can be intense and generally lasts from a few hours to a few days.

Spinal headache is more common in young adults between 18–30‐year age group. There are some methods that can be used to reduce the incidence of spinal headache.

• Back pain or leg pain: a person may feel pain in the lower back near the puncture site which some time may radiate to legs.

• Accidental puncture or injury of the spinal cord may rarely occur with the needle.

• Bleeding may occur at the site of the needle puncture rarely into the spinal canal.

• Brain herniation may occur after the lumbar puncture due to a sudden drop in the CSF pressure especially in cases of brain tumor which causes significant pressure within the skull.

What does the lumbar puncture show? How are the results read? How the normal and abnormal results look like on Lumbar puncture?

The physical appearance and thorough lab investigations for chemical composition of the CSF and presence of abnormal micro-organisms or antibodies can help to differentiate between normal and abnormal CSF and to diagnose specific diseases.

The doctor can inform about any gross changes in the physical appearance or changes in the CSF pressure immediately after the procedure. The results of subsequent laboratory assessment of CSF may take up to 48 hours with some results available within few hours.

Physical appearance:

The normal CSF is clear and colorless in appearance. Abnormal CSF may appear different in color such as:

• Pinkish which may indicate presence of blood suggesting a diagnosis of subarachnoid hemorrhage (SAH). The discoloration of CSF is seen in more than 90% of people within 12 hours of SAH.

• Cloudy or yellowish colored CSF may indicate an infection( possibly meningitis), high bilirubin, or blood which after breaking down can give rise to yellowish color.

• Greenish in color which may indicate the presence of bilirubin or an infection (pus in CSF).


ComponentsNormal CSF(Abnormal CSF) Increased values(Abnormal CSF) Decreased values
CSF pressure 

60 to 200 mm H2O (above 8 years age) &up to 250 mm H2O in obese patients.

More than 250 mm H2O- intracranial hypertensionLess than 60 mm H2O-Intracranial hypotension, seen in injury causing CSF leak.
Protein18 to 58 mg per dLInfections (meningitis, brain abscess etc), Intracranial bleeding,

Multiple sclerosis,Guillain Barré syndrome,

Tumors&Inflammatory conditions

Blockage in CSF flow

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Can be seen in rapid CSF production
Glucose45 to 80mg/dL or about 2/3rd of blood glucose levelsUsually, the CSF glucose is not above 300 mg/dL- if above, then sign of high blood sugarInfection- bacterial, or fungal tubercular meningitis and others.

Low blood sugar

WBC (White Blood Cells )0 to 5 cells/mm3 (all mononuclear WBCs )Infections (meningitis, brain abscess etc),


Multiple sclerosis

RBC0-10 RBCssuggests Bleeding into CSF or

Trauma due to lumbar puncture

Gamma globulin7 to 12 % Inflammatory conditions such as

Multiple sclerosis

Guillain Barré syndrome Neurosyphilis

Different labs have different parameters and the range may slightly vary accordingly.

Presence of other abnormalities in CSF could also suggest specific conditions as follows:

• Microorganisms: The presence of micro-organisms such as bacteria, viruses, fungi and others in the CSF suggest infection.

1. Staining and microscopic evaluation:

These micro-organisms are detected under the microscope after staining the CSF sample with specific dyes:

• Gram staining is done if bacteria is suspected as the cause of meningitis.

• Acid fast bacillus (AFB) staining is done if tuberculosis is suspected as cause of meningitis.

• India ink staining is done for fungus (Cryptococcus).

Gram stain could make diagnose in 60-80% of untreated cases of bacterial meningitis and only in 40-60% of partially treated cases. Acid fast staining could help in diagnosing only 37% cases on initial test. The sensitivity can be improved to 87% if four smears are made. Sensitivity could also be increased by testing the sediment of CSF.

2. Culture:

Culture is the gold standard test for detecting the causative microbes in cases of meningitis. Here the sample of CSF is put on specific nutrient medium for micro-organisms and allowed to grow into size over a period of time. At least 15 mL of CSF is to be taken out for culture.

• Sensitivity: culture is able to detect the microbe in 56% cases on the first sample. The detection rate can be improved to 83% if four separate samples are cultured.

• Timing: The CSF culture should be done before starting antibiotic treatment as it can decrease the sensitivity of test.

• Duration: The test takes about 6 weeks time to show results.

3. Polymerase chain reaction (PCR):

PCR is the most sensitive rapid test for the detection of microbes in CSF. Its main advantage over culture is, it is much faster, requires lesser amount of CSF, and is more sensitive in certain circumstances. PCR is especially useful in detecting viral meningitis with sensitivity of about 95 to 100% for several viruses. The sensitivity of PCR for tuberculous meningitis is 54 to 100 percent and could replace AFB staining and culture as the test of choice. Although PCR is costlier, it can be potentially cheaper by reducing the need for overall diagnostic testing and intervention.

• Cancer cells: The presence of tumor cells or immature blood cells in CSF can indicate certain types of cancer.

• Antibodies

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