In the second part of our series, Suneeta Kochhar and William Marshall guide you through the basics of interpreting CSF results

When to take a cerebrospinal fluid sample

After taking a history and doing a physical examination it is important to order appropriate investigations and interpret the results in order to formulate a diagnosis and make treatment decisions.

Cerebrospinal fluid (CSF) investigations are usually done when meningitis is suspected. But examining CSF may also be useful when diagnosing suspected subarachnoid haemorrhage, multiple sclerosis, and certain polyneuropathies, such as Guillain-Barré syndrome and chronic inflammatory demyelinating polyneuropathy.

CSF is produced by the choroid plexus and circulates in the subarachnoid space--a sample is usually obtained by lumbar puncture. A lumbar puncture is also done for intrathecal administration of drugs to treat pain and severe spasticity, measurement of CSF pressure when there is clinical evidence of rising intracranial pressure, and for the therapeutic removal of CSF in benign intracranial hypertension to prevent visual disturbances and secondary brain injury.

Obtaining a CSF sample

Contraindications

A lumbar puncture should not be done if intracranial pressure is raised, or if an intracranial lesion with mass effect is a possibility. This is because doing a lumbar puncture with raised intracranial pressure carries a risk of herniation of the cerebellar tonsils--also called coning. You may suspect this in a patient with a severe headache, nausea, focal neurology, or papilloedema.

Other relative contraindications to consider include clotting abnormalities, as there may be a risk of prolonged bleeding, and skin infection, which potentially would lead to contamination at the site of needle insertion. When considering whether or not to do a lumbar puncture, always ask someone more senior for help.

SIMON FRASER/RVI NEWCASTLE/SPL

Doctor performs lumbar puncture on patient

Procedure

The patient usually lies with their back on the edge of the bed in the left lateral position with their knees brought up to their chin. This opens up the gaps between the spinous processes. In adults, the L3-4 intervertebral space is marked. This level is used for lumbar puncture since the spinal cord ends at the L1-2 disc in adults. When marking this level remember that the plane through the iliac crests is at L4.

Using sterile technique, open a lumbar puncture pack. Inject the local anaesthetic lignocaine into the skin, and then insert a spinal needle aiming towards the umbilicus. You will feel resistance as you pass through the spinal ligaments and dura mater. Once you have passed through these structures, you will notice reduced resistance as the needle enters the subarachnoid space. After withdrawing the stylet, you can collect CSF and measure CSF pressure with a manometer.

About one third of patients develop a post-lumbar puncture headache within 24 hours. Reducing CSF leakage by using the smallest needle you can--usually 22G--and keeping the patient flat after the procedure helps to minimise the risk of developing a headache.

Note appearance of CSF

Collect approximately 2 ml of fluid in numbered bottles in a sequential manner. At this stage, it is important to note the appearance of the CSF. Is the sample xanthochromic (yellow), bloodstained, or turbid due to increased white blood cells? Then send the samples straight to the laboratory for biochemistry and microbiology tests.

Microbiology

Examination of CSF is critical to the diagnosis of meningitis, and the appearance of the CSF may be suggestive of a diagnosis. If you suspect meningitis, you should usually do a lumbar puncture immediately, provided that there are no signs of raised intracranial pressure, which may occur in meningococcal disease. However, the investigation should not delay treatment.

CDC/M S MITCHELL

Neisseria meningitidis group C in spinal fluid

Other tests in meningitis

The CSF pressure in meningitis is typically raised. Laboratory tests you may need to do include Gram staining, Ziehl-Nielsen stain for acid fast bacilli in tuberculosis, virology, microscopy, cell counts, and CSF culture. Initially the results may be normasl, but if igns and symptoms persist then you should consider repeating the lumbar puncture. Other tests of relevance include a full blood count, urea and electrolytes, blood culture, chest x ray, as well as, syphilis serology. For example, a chest x ray may show evidence of pulmonary tuberculosis, which may suggest a causative organism for a patient with meningitis.

Other tests you may request to diagnose the causative agent in meningitis include nucleic acid amplification by polymerase chain reaction for herpes simplex virus, and India ink staining, and more recently latex agglutination, for cryptococcal antigen in immunocompromised patients.

If investigations are not available, a diagnosis of meningitis may be made on clinical grounds. Classically a patient may complain of neck stiffness, fever, headache and photophobia, and a petechial rash may be present if there is septicaemia.

CDC/EDWIN P EWING,JNR

Acid-fast Mycobacterium tuberculosis

Characteristics of CSF in meningitis

Acute meningitis is usually due to bacterial or viral infection. If the infection is bacterial, for example with Neisseria meningitidis or Streptococcus pneumoniae, you should treat promptly with benzylpenicillin to prevent morbidity and mortality. Chronic meningitis is normally bacterial or fungal, for example infection with Mycobacterium tuberculosis or Cryptococcus neoformans.

In the case of bacterial meningitis, the CSF may be turbid consisting largely of polymorphonuclear neutrophils--this is why the cell count is helpful. Glucose concentration is reduced due to bacterial metabolism, and protein concentration is increased.

Mononuclear cells predominate in tuberculous meningitis, glucose is reduced, and protein is increased. In contrast, the CSF is normally clear in viral meningitis and protein may be slightly elevated, and mononuclear cells predominate.

Biochemistry

Glucose

The concentration of glucose in the CSF is approximately 70% of the glucose concentration in the blood. This means that to be able to interpret the glucose concentrations in the CSF, you should also take a blood sample to compare the two levels. If the concentration of glucose in the CSF is lower than expected, increased glucose metabolism due to the presence of bacteria is likely. It is worth remembering that glucose in the CSF tends to be normal in viral meningitis. Low glucose concentrations may also occur when there are increased white cells or in malignant infiltration of the CSF because of an increase in metabolism.

Protein

When measuring the level of protein in the CSF, it is important that the sample is not contaminated with blood as this may give a false positive reading. If the CSF has been contaminated by blood while doing the lumbar puncture, there will be fewer red blood cells in sequential bottles. By contrast, the CSF is often uniformly bloodstained following a subarachnoid haemorrhage. After centrifuging a CSF sample following subarachnoid haemorrhage, the fluid becomes xanthochromic (yellow) after 24 hours because the haemoglobin breaks down.

The concentration of protein in CSF may be increased in meningitis, multiple sclerosis, Guillain-Barré syndrome, and intracranial tumours, as meningeal inflammation may result in an increase in capillary permeability. For example, in bacterial meningitis the increase in capillary permeability may be due to vasoactive eicosanoids and inflammatory cytokines, such as tumour necrosis factor. In many brain tumours, the vessels have a less well developed blood-brain barrier allowing protein to leak into the CSF.

On other occasions, you may see increased protein in the CSF below a spinal cord compression (Froin's syndrome). It may also occur in Guillain-Barré syndrome, but in this instance lymphocytes do not increase. On the other hand, a slightly increased protein with lymphocytosis--increased levels of white blood cells--supports the clinical diagnosis of multiple sclerosis. A more sensitive test in most patients with multiple sclerosis involves polyacrylamide gel electrophoresis of CSF, which produces IgG oligoclonal bands. These bands may also be seen in neurosyphilis, subacute sclerosing panencephalitis associated with measles, and neuroAIDS.

In addition, you can tell whether clear fluid leaking from the nose after trauma or surgery is CSF by testing for tau protein, a normally soluble microtubule binding protein that helps form the cytoskeleton.

Meningitis in your country

Meningitis is treated differently in different countries due to the varying prevalence of infective organisms, availability of investigations, and treatment options. Please send in a rapid response to tell us how meningitis is treated in your country.

Summary of normal CSF results

Guidelines: We intend publishing within 24 hours all responses that contribute substantially to the topic under discussion. To avoid points that have already been made please read other people's responses before posting your own. All responses will be eligible for publication in the paper studentBMJ, providing that current appointment and place of work are given. Name and email address are required to send a response and will be published with the response.

Please note: When writing a respnse to a response, please enter the original title of the article and the author of the response you are responding to.