Synopsis
Select, prepare and perform lumbar puncture safely, preserve diagnostic yield and interpret opening pressure and cerebrospinal-fluid results in relation to timing, serum measurements and clinical probability.
- Define the question before puncture: infection, inflammation, subarachnoid haemorrhage, malignant infiltration, pressure disorder and therapeutic drainage require different timing, tubes, paired blood tests and laboratory handling.
- Obtain consent covering purpose, alternatives, discomfort, bleeding, infection, post-dural-puncture headache, failure and the rare risk of neurological deterioration when contraindications are missed.
- Check observations, conscious level, focal signs, optic discs where competent, anticoagulants, antiplatelets, platelet count and coagulation context, spinal anatomy, local infection and cardiorespiratory ability to tolerate positioning.
- Brain imaging before LP is selective, not routine. Shock, airway compromise or strong mass-effect risk changes the order; imaging itself does not make puncture safe if clinical contraindications persist.
- Use an aseptic technique and an atraumatic small-gauge needle when feasible. Position the patient laterally for a valid opening pressure, relaxed with legs no more flexed than needed after the interspace is entered.
- Measure opening pressure at equilibrium with the manometer zeroed at the needle, recording position, sedation, straining and technical limitations. A number obtained seated is not interchangeable with standard lateral measurement.
- Plan sample allocation before starting and label tubes in sequence. Typical studies include paired glucose, protein, cells and differential, Gram stain and culture, with PCR, cytology, oligoclonal bands or specialist tests added for the actual syndrome.
- CSF glucose must be interpreted against a near-simultaneous blood glucose. A low CSF-to-blood relationship can support bacterial, tuberculous, fungal or malignant disease but is not exclusive to one cause.
- Neutrophilic pleocytosis, high protein and low glucose suggests bacterial meningitis, yet early viral infection, partially treated disease and immunosuppression can produce atypical patterns; microbiology and clinical severity govern action.
- For suspected aneurysmal subarachnoid haemorrhage after negative CT, follow NICE NG228: CT timing matters and, when LP is indicated, allow at least twelve hours from symptom onset before spectrophotometric bilirubin analysis.
- A falling red-cell count across tubes does not reliably distinguish a traumatic tap from subarachnoid bleeding. Use bilirubin spectrophotometry, CT timing and the whole clinical pathway rather than visual inspection alone.
- Communicate urgent samples directly to the laboratory, record antimicrobial timing and transport requirements, review every final culture and PCR result and arrange a plan for unresolved or incidental abnormalities.
Key red flags
Raised opening pressure, turbid fluid, neutrophilic pleocytosis, increased protein and reduced CSF glucose relative to blood supports bacterial meningitis, but prior antibiotics, early disease and immunosuppression can alter every component.
Lymphocytic pleocytosis with moderately increased protein and usually preserved glucose supports viral infection. Early viral disease may be neutrophilic, and HSV encephalitis can have red cells, so PCR and phenotype are essential.
Raised pressure, lymphocytic cells, high protein and low glucose can suggest tuberculosis or fungal disease. Yield depends on volume, repeated sampling and specialist microbiology, and empirical decisions may precede confirmation.
CSF bilirubin detected by validated spectrophotometry at the correct interval can support subarachnoid haemorrhage after appropriate CT assessment. Oxyhaemoglobin alone is vulnerable to artefact from traumatic sampling.
A reproducibly raised lateral opening pressure with an otherwise appropriate CSF profile can support intracranial hypertension, but obesity, Valsalva, pain, sedation, flexion and positioning can falsely elevate the reading.
Investigation priorities
Measure subarachnoid pressure for suspected intracranial hypertension, infection and selected other syndromes.
Management branches
Fever, headache, meningism, altered behaviour, reduced consciousness, focal deficit, rash or seizure creates concern for meningitis or encephalitis.
- Perform ABCDE, obtain blood cultures and other immediate samples and start guideline-concordant empirical antibiotics and, when indicated, aciclovir and adjunctive therapy without waiting for imaging or LP if delay would result.
- Assess cardiorespiratory stability, conscious level, focal signs, papilloedema risk, recent seizure and haemostasis to decide whether puncture can occur now or needs CT and specialist review first.
- Send planned CSF urgently with paired glucose, document pre-treatment timing and refine isolation, antimicrobial and public-health actions with microbiology, infection and national guidance.