Interpreting CSF Results

Understanding how to do an LP and interpret the results is an essential skill for most doctors, particularly those planning on a hospital-based specialty. You will often be handed over to review the results and act accordingly during ward cover shifts.

As with any handover, ensure when you are asked to chase the results that you understand what has been sent, the clinical context and what you are expected to do with the results. For example, chasing results for a suspected meningitis may be more urgent than a stable neurological disorder but if the treatment for meningitis has already been initiated empirically (as it should be), then this is far less concerning.

Most common indications for LP

  1. CNS infections including meningitis or encephalitis
  2. Subarachnoid haemorrhage (as a normal CT-head scan does not exclude SAH, particularly if >6 hours after symptom onset)
  3. Neurological conditions – e.g. Guillain Barré syndrome, Multiple Sclerosis
  4. Disorders of intracranial pressure – idiopathic intracranial hypertension (as a diagnostic procedure and therapy)

CSF Lab Requirements

TestsPreferred TapCSF volumeSample Tube
Microbiology investigationsTap #1 and #31mL in each (25 drops in each)Plain Universal container
ProteinTap #20.25mL (6 drops)Plain Universal container (tubes containing gel or anticoagulant are not suitable for analysis of CSF protein)
GlucoseTap #20.25mL (6 drops)Fluoride-oxalate tube (grey top) sent immediately to the laboratory
Oligoclonal bandsTap #40.5mL (12 drops)Plain universal container for CSF AND blood in a biochemistry bottle (often gold top)
Spectrophotometry for xanthochromiaTap #41mL (25 drops)Plain Universal container for CSF and blood in a biochemistry bottle (often gold top) for liver function tests to compare. CSF sample MUST be protected from light (wrap sample in foil or hand towel after labelling).
CytologyAny5-10mLPlain Universal container for CSF Sample has limited stability therefore give the sample directly to a member of the lab team
Flow cytometryNo preferred tap2mL (40 drops)Sample has limited stability therefore give the sample directly to a member of the lab team

CSF Analysis

 NormalSAHBacterial meningitisViral meningitisTB meningitis
AppearanceClear and ColourlessBlood-stained or yellow (xanthochromia) if >12 hours post symptomsTurbid / Cloudy in severe casesClearOpaque
Opening Pressure10-20 cm H2OUsually elevatedUsually elevatedNormal or elevatedUsually elevated
WCC0-5 cells/µLElevatedElevated (primarily polymorphonuclear leukocytes)Elevated (primarily lymphocytes)Elevated
Protein0.15-0.45 g/LElevatedElevatedElevatedElevated
Glucose2.8 – 4.2 mmol/LNormalLowNormalLow

Appearance

Typical appearance of  CSF is a clear fluid. There may be a trace of blood in case of traumatic tap or acute subarachnoid haemorrhage (SAH). Alternatively, it may be yellow (xanthochromic) in SAH, if the LP is performed >12 hours after the event. Finally, if there is an infection, the CSF may appear turbid.

Opening Pressure

Opening pressure is normal if between 10-20 cm H2O, but above 25 cm H2O is more concerning of raised ICP. It might be increased in the case of idiopathic intracranial hypertension, space-occupying lesions in the brain or patients with anxiety (by tense abdominal muscles and subsequently increased central venous pressure). The opening pressure is low in case of obstruction in a spinal subarachnoid space.

Protein

Normal is around 0.15-0.45g/L. Protein will be raised in meningitis (viral, bacterial or TB) as the result of blood-brain barrier disruption.

Glucose

Normal glucose level should be the 2/3 of serum glucose value (both samples should be taken at the same time). It level would be low in bacterial or TB meningitis as the bacteria take up glucose for energy. This is not the case for viral meningitis, where the glucose level is normal.

Cytology

Normally there are minimal cells in CSF: white blood cells <5/µL and red blood cells <10/mm3. WBC is raised in all forms of meningitis. In bacterial you would expect this to be predominantly polymorphs (neutrophils), whereas in viral meningitis this will be mainly lymphocytes. Eosinophils could be seen in the case of tumours, parasites, or foreign body reactions.

Red cell count is important for the diagnosis of subarachnoid haemorrhage, where you expect high red cell count. If there is a traumatic ‘bloody’ tap, there may be thousands of them. If so, then white cells should be expected in the CSF, but in similar proportions to the peripheral blood.

Additional tests:

Xanthochromia

Xanthochromia is the yellow appearance of CSF resulting from red cell breakdown and is tested for using spectrophotometry. It is positive in SAH. It is important to remember, that it is photosensitive. Immediately after labelling you should cover it with a foil or hand towel and deliver to the laboratory.

CSF PCR

Important test when querying meningitis in order to establish organism

CSF culture and gram stain

Again an important test when querying meningitis in order to establish organism

Oligoclonal bodies

For the diagnosis of Multiple Sclerosis.

The table below summarises the expected findings in the most common reasons for which LP is performed. Note that normal ranges differ between labs and so always use your hospital lab’s ranges.

References

Written by Dr Indira Kenyon (FY1)
Edited by Dr Anna Podlasek (SHO)

How useful was this post?

Click on a star to rate it!

Average rating 4.4 / 5. Vote count: 5

No votes so far! Be the first to rate this post.

As you found this post useful...

Follow us on social media!

We are sorry that this post was not useful for you!

Let us improve this post!

Tell us how we can improve this post?

Leave a Comment

Your email address will not be published.

Related Posts

Interpreting Blood Films
Interpreting Blood Films
A blood film looks at our three cell types (erythrocytes, leukocytes...
Drowsy
Reduced GCS
As a doctor, you will frequently be called for a drowsy patient....
Anaemia
Anaemia
Perhaps the most common blood test you will review daily will...

Follow us

Our Newsletter

Trending Now

Junior Doctor Pay Calculator
We’ve created a junior doctor pay calculator which will help you better understand your salary,...
Hyponatraemia
Hyponatraemia (serum Sodium <135 mmol/L) is one of the most common electrolyte abnormalities you will...
ePortfolio
Your eportfolio is a tool to store and record evidence that demonstrates your progress, clinical competencies...
How to take a psychiatric history
Psychiatry, as a specialty is unique in that diagnostic methods, rely very heavily on symptomatology,...
Preparing for the Situational Judgement Test
Preparing for the Situational Judgement Test (SJT) exam can be quite daunting. It makes up 50% of your...
Ranking Foundation Jobs
If you’re worried about not getting your top choice, you shouldn’t worry. It doesn’t...
Audits & Quality Improvement Projects (QIPs)
Audits & QIPs are a way to identify issues, drive changes and assess the effects they have. It is...

Sign up for our awesome resources

Join over 25,000 users who have signed up for our free weekly webinars, referral cheat sheet & other amazing content!