CSF Analysis

Cerebrospinal fluid (CSF) analysis may be used to help diagnose a wide variety of diseases and conditions affecting the central nervous system (CNS). They may be divided into four main categories:

• Infectious diseases such as meningitis and encephalitis testing is used to determine if the cause is bacterial, tuberculous, fungal or viral, and to distinguish it from other conditions; may also be used to detect infections of or near the spinal cord or to investigate a fever of unknown origin.
• Bleeding (haemorrhaging) within the brain or skull
• Diseases that cause inflammation or other immune responses such as antibodies—these may include autoimmune disorders such as Guillain-Barré syndrome or sarcoidosis or diseases that cause the destruction of myelin such as multiple sclerosis
• Tumours located within the CNS - either primary tumours or secondary metastatic tumours

CSF analysis may be requested when a doctor suspects that a patient has a condition or disease involving their CNS. A patient’s medical history may prompt the request for CSF analysis. It may be requested when a patient has suffered trauma to the brain or spinal cord, has been diagnosed with cancer that may have spread (metastatic) or has signs or symptoms suggestive of a condition involving their CNS. 

The signs and symptoms of CNS conditions can vary widely and many overlap with a variety of diseases and disorders. They may have sudden onset, suggesting an acute condition such as CNS bleeding or infection or may be slow to develop, indicating a chronic disease such as cancer or multiple sclerosis.

Depending on a patient’s history, doctors may request CSF analysis when some combination of the following signs and symptoms appear:

• changes in mental status and consciousness
• confusion, hallucinations or seizures
• muscle weakness or lethargy, fatigue
• nausea (feeling sick)
• flu-like symptoms that intensify over a few hours to a few days
• fever or rash
• sudden, severe or persistent headache or a stiff neck
• sensitivity to light
• numbness or tremor
• dizziness
• difficulties with speech
• difficulty walking, lack of coordination
• mood swings, depression
• infants may be irritable, cry when they are held, have body stiffness, refuse food, and have bulging fontanels (the soft spots on the top of the head)

CSF usually contains a small amount of protein and glucose and may have a few white blood cells (WBCs).  

Any condition that disrupts the normal pressure or flow of CSF or the protective ability of the blood/brain barrier can result in abnormal results of CSF testing. For detailed explanations of what various test results may mean, see the sections below on:

• CSF physical characteristics
• CSF chemical tests
• CSF microscopic examination
• CSF infectious disease tests

Multiple tubes of CSF are often collected during a lumbar puncture to ensure the quality of samples for testing.

Meningitis due to infective causes is a medical emergency. Your doctor must rapidly distinguish between this and other causes. Because prompt treatment is crucial, your doctor may start you on a broad-spectrum antibiotic before the diagnosis has been definitely determined. 

To help diagnose your illness your doctor may want to know what recent illnesses and vaccinations you may have had, what symptoms you are experiencing, whether you have been in contact with any ill people, and what places you have recently travelled to.

The lumbar puncture is a special, but relatively routine, procedure. It is usually performed while you are lying on your side in a curled up fetal position, but may sometimes be performed in a sitting position. It is crucial that you remain still during the procedure. Once you are in the correct position, your back is cleaned with an antiseptic and a local anaesthetic is injected under the skin. When the area has become numb, a special needle is inserted through the skin, between two vertebrae, and into your spinal canal. It is gently advanced until it enters the subarachnoid space (located between the arachnoid and pia mater layers of the meninges) and cerebrospinal fluid (CSF) begins to flow. You may be asked to straighten out your legs at this point and relax your muscles. It is important not to move unless you are instructed to do so. An "opening" or initial pressure reading of the CSF is obtained. The doctor then collects a small amount of CSF in multiple sterile vials. A "closing" pressure is obtained, the needle is withdrawn, and a sterile dressing and pressure are applied to the puncture site. You will then be asked to lie quietly in a flat position, without lifting your head, for 30 minutes or more hours to avoid a potential post-test spinal headache.

The lumbar puncture procedure usually takes less than half an hour, sometimes much faster. For most patients it is barely felt and at worst moderately uncomfortable. The most common sensation is a feeling of pressure when the needle is introduced. Let your doctor know if you experience a headache or any abnormal sensations, such as pain, numbness, or tingling in your legs, or pain at the puncture site.

The lumbar puncture is performed low in the back, well below the end of the spinal cord – usually between lumbar (L) vertebrae L4 and L5. There are spinal nerves in the location sampled, but they have room to move away from the needle. There is the potential for the needle to contact a small vein on the way in. This can cause a “traumatic tap,” which just means that a small amount of blood may leak into one or more of the samples collected. While this is not ideal, it is not uncommon. The evaluation of your results will take this into account.

Blood from the lumbar puncture may contaminate the first portion of CSF sample that is collected. However, there are usually three or more separate tubes used to collect CSF samples during one spinal tap procedure. The last tube that is collected during a lumbar puncture is least likely to have blood cells present due to the procedure and is usually the sample tested for the presence of blood in the CSF. Likewise, the last sample collected is used for infectious disease testing since it will not be contaminated with microorganisms from inserting the needle through the skin.

The appearance of the sample of CSF is usually compared to a sample of water.  

• Pressure of the CSF can be measured when opening (starting) and closing (finishing) the collection.
  • Increased CSF pressure may be seen with a variety of conditions that increase pressure within the brain or skull and/or that obstruct the flow of CSF, such as tumours, infection, abnormal accumulation of CSF within the brain (hydrocephalus), or bleeding.
  • Decreased pressure may be due to dehydration, shock, or leakage of CSF through an opening (another LP site or sinus fracture).
• Colour of the fluid — normal is clear and colourless. Changes in the colour of the CSF are not diagnostic but may point to additional substances in the fluid. Yellow, orange, or pink CSF is said to be xanthochromic. It may indicate the breakdown of blood cells due to bleeding into the CSF or the presence of bilirubin. Green CSF may also sometimes be seen with bilirubin or infection.
• Turbidity — Cloudy or turbid CSF may indicate the presence of white or red blood cells, microorganisms, or an increase in protein levels.
• Viscosity — Normal CSF will have the same consistency as water. CSF that is “thicker” may be seen in patients with certain types of cancers or meningitis.

A few routine tests are usually performed on CSF samples.

• CSF glucose – normal is about 2/3 the concentration of blood glucose. Glucose levels may decrease when cells that are not normally present use up (metabolise) the glucose. These may include bacteria or cells present due to inflammation (WBCs) or shed by tumours.
• CSF protein – only a small amount is normally present in CSF because proteins are large molecules and do not cross the blood/brain barrier easily. Decreases in CSF protein are not generally considered significant. Increases in protein are most commonly seen with:
  • Meningitis and brain abcess
  • Brain or spinal cord tumours
  • Multiple Sclerosis
  • Guillain-Barré Syndrome
  • Syphilis

If any of the initial tests are abnormal or if the doctor has reason to suspect a specific condition, then additional testing may be requested. This may include one or more of the following:

• CSF protein electrophoresis — separates different types of protein. Oligoclonal bands may be seen with multiple sclerosis and Lyme disease.

• CSF IgG (Immunoglobulin G) — increased in some conditions, such as multiple sclerosis, herpes encephalitis, connective tissue diseases.

• CSF lactic acid — often used to distinguish between viral and bacterial meningitis. The level will usually be increased with bacterial and fungal meningitis while it will remain normal or only slightly elevated with viral meningitis.

• CSF lactate dehydrogenase (LD) — used to differentiate between bacterial and viral meningitis; may also be elevated with leukaemia or stroke.

• CSF glutamine — may be increased with liver disease: hepatic encephalopathy and Reye syndrome

• CSF amino acid analysis – may be requested in the investigation of several inherited metabolic diseases, particularly those presenting with unexplained seizures

• CSF bilirubin – seen following subarachnoid haemorrhage (SAH). Requested when SAH is suspected but evidence of a bleed has not been demonstrated by a computed tomography (CT) scan

• CSF Amyloid – neurodegenerative markers may be requested to aid the diagnosis of Alzheimer disease (AD).

• CSF antibody tests – specific antibodies to may be requested as part of investigations into neuropathy

Normal CSF has no or very few cells present and appears clear. If the CSF sample appears clear, a small drop of undiluted CSF is examined under a microscope, and cells are counted manually. However, if the CSF is very cloudy or bloody, which can indicate the presence of many cells, the specimen may be run on an automated cell counter.

If the number of cells present is increased (for example, more than 5/mm³ ), the laboratory will most likely perform a cell differential . This may not be done if there are fewer than 5/mm³ cells present. To perform a differential, labs will often use a special centrifuge (cytocentrifuge) to concentrate the cells at the bottom of a test tube. A sample of the concentrated cells is placed on a slide, treated with special stain, and an evaluation of the different kinds of WBCs present is performed. Sometimes if there are too many cells present in the centrifuged sample, an accurate differential may be difficult to perform. In those cases, the specimen may be diluted, cytocentrifuged, and then stained.

If cancer is suspected or has been previously diagnosed, the sample is usually cytocentrifuged regardless of the number of cells counted, and a differential is performed.

• CSF total cell counts
  • Red blood cell (RBC) count. Normally no red blood cells are present in the CSF. The presence of red blood cells may indicate bleeding into the CSF or may indicate a “traumatic tap” - blood that leaked into the CSF sample during collection.
  • White blood cell (WBC) count. Normally less than 5 cells are present per cubic mm of adult CSF. A significant increase in white blood cells in the CSF is seen with infection or inflammation of the CNS.
• CSF WBC differential
  • an increase in neutrophils with a bacterial infection
  • an increase in lymphocytes with a viral or Mycobacterial (TB) infection
  • sometimes an increase in eosinophils with a parasitic infection
  • abnormal and increased numbers of WBCs may be seen with leukaemia that is present in the CNS
  • abnormal cells may be present with cancerous tumours
  • immune disorders of the CSF, such as multiple sclerosis, may also cause a slight increase in lymphocytes.
  • an increase in the different types of WBCs with a variety of other conditions, including brain abscess, following seizures or bleeding within the brain or skull, metastatic tumour, Guillain-Barré syndrome, and inflammatory disorders such as sarcoidosis.

• CSF cytology – a cytocentrifuged sample is treated with a special stain and examined under a microscope for abnormal cells. This is often done when a CNS tumour or metastatic cancer is suspected. The presence of certain abnormal cells, such as tumour cells or immature blood cells, can indicate what type of cancer is involved.

In addition to chemistry tests, such as protein and glucose, other routine tests may be performed to look for microorganisms if meningitis or encephalitis is suspected.

• CSF gram stain for direct observation of microorganisms under a microscope. There should be no microorganisms in CSF fluid. If bacteria or fungi are present on a CSF gram stain, then the patient has bacterial or fungal meningitis or encephalitis.

• CSF culture and sensitivity is used to detect any microorganisms, which will grow in the culture. If bacteria are present, they can be tested in the laboratory to predict the best choices for antimicrobial therapy for the patient and prophylaxis of close contacts, if needed. If there are no microorganisms present, it does not rule out an infection; they may be present in small numbers or unable to grow in culture due to prior antibiotic therapy.

If any of the initial tests are abnormal or if the doctor strongly suspects a CNS infection, then additional testing may be ordered. This may include one or more of the following:

• CSF molecular testing – detection of viral, bacterial or fungal genetic material (DNA, RNA) by PCR testing in adjunct to culture and sensitivities. Examples include herpes virus, enteroviruses, tuberculosis (TB) or cryptococcus. A positive PCR test indicates an infectious cause of meningitis or encephalitis.

• CSF Cryptococcal antigen – to detect a specific fungal infection

• Other CSF antigen tests – depending on which organism(s) are suspected

• Specific CSF antibody tests – depending on which organism(s) are suspected

Other CSF tests for infectious diseases that are less commonly requested include:

• CSF AFB smear and culture may be positive with tuberculosis and with other mycobacteria

• CSF syphilis testing, using the Venereal Disease Research Laboratory (VDRL) nontreponemal test, Treponema pallidum particle agglutination assay (TPPA) or rapid plasma reagin (RPR) testing. Results may be positive with neurosyphilis (involvement of the brain by syphilis); a negative does not rule out brain involvement.