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
- CNS infections including meningitis or encephalitis
- Subarachnoid haemorrhage (as a normal CT-head scan does not exclude SAH, particularly if >6 hours after symptom onset)
- Neurological conditions – e.g. Guillain Barré syndrome, Multiple Sclerosis
- Disorders of intracranial pressure – idiopathic intracranial hypertension (as a diagnostic procedure and therapy)
CSF Lab Requirements
| Tests | Preferred Tap | CSF volume | Sample Tube |
|---|---|---|---|
| Microbiology investigations | Tap #1 and #3 | 1mL in each (25 drops in each) | Plain Universal container |
| Protein | Tap #2 | 0.25mL (6 drops) | Plain Universal container (tubes containing gel or anticoagulant are not suitable for analysis of CSF protein) |
| Glucose | Tap #2 | 0.25mL (6 drops) | Fluoride-oxalate tube (grey top) sent immediately to the laboratory |
| Oligoclonal bands | Tap #4 | 0.5mL (12 drops) | Plain universal container for CSF AND blood in a biochemistry bottle (often gold top) |
| Spectrophotometry for xanthochromia | Tap #4 | 1mL (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). |
| Cytology | Any | 5-10mL | Plain Universal container for CSF Sample has limited stability therefore give the sample directly to a member of the lab team |
| Flow cytometry | No preferred tap | 2mL (40 drops) | Sample has limited stability therefore give the sample directly to a member of the lab team |
CSF Analysis
| Normal | SAH | Bacterial meningitis | Viral meningitis | TB meningitis | |
| Appearance | Clear and Colourless | Blood-stained or yellow (xanthochromia) if >12 hours post symptoms | Turbid / Cloudy in severe cases | Clear | Opaque |
| Opening Pressure | 10-20 cm H2O | Usually elevated | Usually elevated | Normal or elevated | Usually elevated |
| WCC | 0-5 cells/µL | Elevated | Elevated (primarily polymorphonuclear leukocytes) | Elevated (primarily lymphocytes) | Elevated |
| Protein | 0.15-0.45 g/L | Elevated | Elevated | Elevated | Elevated |
| Glucose | 2.8 – 4.2 mmol/L | Normal | Low | Normal | Low |
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
- GeekyMedics CSF interpretation
- Patient.info CSF interpretation
- Provan, A., 2011. Oxford Handbook Of Clinical And Laboratory Investigation. Oxford: Oxford University Press.
- CSF sample requirements
Written by Dr Indira Kenyon (FY1)
Edited by Dr Anna Podlasek (SHO)
