Meningitis CSF Findings — Exam Revision

Meningitis is inflammation of the meninges surrounding the brain and spinal cord. The arachnoid and pia border the subarachnoid space containing cerebrospinal fluid. Infection triggers inflammatory-cell recruitment and changes permeability at the blood–brain barrier. The resulting CSF cell differential, protein concentration and glucose concentration help distinguish broad categories of disease, while organism-specific tests establish the cause.

For examination revision, read a CSF result as a pattern rather than a single number. Acute pyogenic disease typically produces neutrophils, low glucose and increased protein. Viral disease usually produces lymphocytes with preserved glucose. Tuberculous and fungal disease often produce lymphocytes, low glucose and increased protein. Opening pressure and the clinical time course add useful context but do not independently identify an organism.

Keep meningitis separate from encephalitis, which involves the brain parenchyma. Altered behaviour, seizures or focal deficits may point towards encephalitis or meningoencephalitis, although severe meningitis can also impair consciousness. A stem that supplies red cells, focal neurological changes or a characteristic brain-imaging abnormality may require more than choosing a row from a basic meningitis table.

A typical adult CSF sample is clear, has very few white cells and contains much less protein than serum. A review of meningoencephalitis gives a normal white-cell count below five cells per microlitre and protein below forty-five mg/dL. Laboratory reference intervals, age and sampling conditions matter; adult values should not be imposed unchanged on newborn samples.

Measure blood glucose close to the lumbar puncture. CSF glucose depends on the circulating glucose concentration, so the CSF-to-blood glucose ratio provides context that an isolated CSF glucose value lacks. NICE specifically recommends measuring blood glucose immediately before lumbar puncture. A low CSF glucose in a patient who is also systemically hypoglycaemic requires different interpretation from a disproportionate CSF reduction with normal blood glucose.

Request the opening pressure where appropriate, white-cell count and differential, protein, glucose, Gram stain, culture and pathogen-directed molecular tests. A traumatic sample can introduce peripheral blood and complicate cell and protein interpretation. Record previous antibiotics because they can reduce microbiological yield. No mathematical correction should be used as a stand-alone reason to dismiss a clinically plausible infection.

The simplest comparison is bacterial versus viral: both can increase cells and protein, but preserved glucose makes a typical viral pattern more likely. The next comparison is viral versus TB or fungal: all can be lymphocytic, but a reduced glucose shifts attention towards TB, fungi and other causes. Never choose viral meningitis solely because the differential shows lymphocytes.

The table deliberately avoids universal cell-count bands. Counts overlap across infections and vary with timing, immune status and treatment. A low white-cell count does not always exclude infection, especially in a severely immunocompromised host. Similarly, an atypical differential should prompt reassessment and confirmation rather than forcing the patient into whichever textbook row looks closest.

An acutely ill patient with fever, headache or neck stiffness and a neutrophilic, low-glucose, high-protein CSF pattern raises concern for pyogenic meningitis. Inflammatory injury increases vascular permeability, permitting protein to enter CSF and contributing to cerebral oedema. The immune response can impair cerebral perfusion and increase intracranial pressure. Severe disease may therefore produce seizures, focal deficits or reduced consciousness.

Culture provides organism identification and susceptibility information, while molecular testing can help when conventional culture yield is reduced. A negative Gram stain does not exclude meningitis. The absence of every classic clinical sign also does not rule it out. In a stem that asks for the next step, treatment urgency can be more important than waiting for a definitive stain or culture result.

For neonatal revision, group B streptococcus, E. coli and Listeria form the core association. Poor feeding, lethargy, temperature instability or a bulging fontanelle may be more useful than a classic neck-stiffness history. Beyond the neonatal period, pneumococcus and meningococcus become prominent. Hib remains relevant where immunisation is incomplete, but a fixed historical ranking should not be copied into every modern setting.

An older or immunocompromised adult needs a broader differential, including Listeria. Recent neurosurgery, trauma or a CSF device shifts attention towards healthcare-associated organisms. TB exposure or a subacute course suggests mycobacterial disease. Immunosuppression and cryptococcal risk can be more important than age when the CSF reaction is limited. These are associations that guide tests and initial treatment, not age-based certainty.

Viral meningitis typically causes a mononuclear or lymphocytic pleocytosis, normal glucose and normal or increased protein. Nonpolio enteroviruses are major causes. Other viruses include herpesviruses, mumps and geographically relevant arboviruses. PCR on CSF can identify several pathogens. The exact panel should match the clinical setting rather than assuming that every viral cause is included in a routine multiplex test.

A key exception is neutrophilic predominance early in viral meningitis. StatPearls warns that this pattern is not confined to the first day and is not a reliable stand-alone discriminator. Therefore, “neutrophils equal bacteria” is too absolute. The safer reasoning is that neutrophils increase concern for bacterial disease, while glucose, protein, microbiology and the overall presentation determine how that concern is investigated.

The term aseptic meningitis is broader than viral meningitis. It can describe a syndrome with meningeal inflammation and negative routine bacterial culture, including nonviral causes. Previous antibiotics may make a bacterial infection culture-negative. A question that uses the word aseptic still needs interpretation of the clinical course and investigations; it is not automatically asking for an enterovirus.

Prominent parenchymal features require consideration of encephalitis. HSV encephalitis may produce CSF red cells, but a traumatic puncture is another possible source of blood. Neither red cells nor a negative routine culture identifies HSV on its own. Use the neurological presentation and appropriate molecular testing, and recognise that the emergency management question differs from a simple CSF-pattern identification question.

Tuberculous meningitis is often subacute, with lymphocytes, low glucose and increased protein. Mycobacterial infection can produce basal inflammatory exudates. These may encase cranial nerves, involve blood vessels and obstruct CSF circulation, explaining cranial neuropathies, infarction and hydrocephalus. Those anatomical complications connect the clinical vignette with the biochemical pattern more effectively than memorising “lymphocytes” alone.

Confirmatory evaluation includes mycobacterial nucleic-acid testing and culture, with acid-fast microscopy where available. These methods have limitations, and a negative result does not automatically exclude TB meningitis. Clinical suspicion should incorporate TB exposure, immune status, time course and imaging. Avoid relying on CSF chloride or an old descriptive appearance as if either were a modern confirmatory test.

Cryptococcal meningitis is especially relevant in immunocompromised hosts. CSF may show lymphocytes, reduced glucose and increased protein, but the cellular response can be limited. Raised opening pressure is an important management issue. Cryptococcal antigen testing on CSF supports diagnosis; culture provides further confirmation. India ink remains a classic demonstration but has lower diagnostic sensitivity and should not displace antigen-based testing when available.

TB and fungal patterns overlap. Distinguish them using the patient context and pathogen-specific tests, not an invented biochemical boundary. A stem describing immunosuppression and cryptococcal antigen points towards cryptococcosis; one describing basal exudates and TB exposure points towards TB. If both are plausible, the appropriate answer may be to investigate both while addressing the immediate clinical risk.

NICE recommends lumbar puncture before antibiotics when it is safe and does not cause a clinically significant delay. Do not routinely perform neuroimaging before every lumbar puncture. A patient with shock, respiratory compromise, uncontrolled seizures or a bleeding risk needs stabilisation first. Features suggesting a mass lesion or raised intracranial pressure require appropriate evaluation before sampling.

When imaging or sampling is delayed, take blood cultures, begin treatment and stabilise the patient rather than waiting. A normal scan does not itself exclude raised intracranial pressure or guarantee lumbar-puncture safety. The examination decision therefore depends on clinical risk as well as imaging. The general rule is prompt diagnosis without sacrificing timely treatment for suspected bacterial meningitis.

• Start with ABC assessment and identify shock, seizures or reduced consciousness.
• Obtain blood cultures and measure blood glucose; perform lumbar puncture promptly when safe.
• Send CSF for cells, glucose, protein, Gram stain, culture and relevant molecular tests.
• Start empiric antibacterial therapy promptly if bacterial meningitis is suspected; adapt coverage to age, risk and local guidance.
• Add appropriate Listeria coverage when risk factors warrant it, and use corticosteroids according to the bacterial-meningitis protocol.
• Review microbiology, clinical progress and complications, including hydrocephalus, seizures and hearing impairment.