# Bacterial vs Viral Meningitis (USMLE)

> How do you distinguish bacterial from viral from fungal meningitis on CSF?

The CSF numbers that separate bacterial, viral, fungal and tuberculous meningitis, plus the organisms, empiric regimens and CT-before-LP rule behind them.

Bacterial meningitis gives a neutrophil-predominant CSF with thousands of cells, high protein, and low glucose. Viral meningitis gives a lymphocytic pleocytosis in the hundreds with normal glucose. Fungal and tuberculous meningitis give lymphocytes, elevated protein, low glucose, and high opening pressure. Glucose is the fastest discriminator -- low glucose means bacteria, fungi, or tuberculosis, essentially never a virus.

## How do you distinguish bacterial from viral from fungal meningitis on CSF?

**Read every CSF panel in the same order: opening pressure, cell count, differential, protein, glucose, then the organism-specific test.** Bacteria in the subarachnoid space trigger a massive neutrophil influx within hours: a cell count in the thousands, over 80% neutrophils, protein above 100 mg/dL, and glucose below 40 mg/dL.

**CSF glucose is the most discriminating value on the panel.** Normal CSF glucose runs about two-thirds of the simultaneous serum glucose, so send a serum glucose with the tap and use the **CSF-to-serum ratio below 0.4**. Bacteria, fungi, and mycobacteria consume glucose and impair its transport across inflamed meninges; viruses do not. A lymphocytic pleocytosis with **normal** glucose is viral; with **low** glucose it is fungal, tuberculous, or partially treated bacterial disease.

**Tempo narrows it further.** Bacterial meningitis evolves over hours, viral over days, and fungal or tuberculous meningitis over weeks with cranial nerve palsies. More organism-level patterns sit in the [Infectious Disease](/topics/infectious-disease) hub.

## CSF findings in bacterial, viral, fungal and tuberculous meningitis

**Four profiles cover nearly every meningitis vignette on the exam.**

| CSF parameter | Bacterial | Viral | Fungal (cryptococcal) | Tuberculous |
| --- | --- | --- | --- | --- |
| **Opening pressure** | ↑ (200-500 mm H₂O) | Normal or mildly ↑ | ↑↑ markedly elevated (hallmark) | ↑ |
| **White cell count** | 1,000-5,000/µL | 10-500/µL | 20-200/µL, may be near-normal | 100-500/µL |
| **Predominant cell** | Neutrophils (>80%) | Lymphocytes | Lymphocytes | Lymphocytes |
| **Protein** | ↑↑ (>100 mg/dL) | Normal to mildly ↑ | ↑ | ↑↑↑ highest of the four |
| **Glucose** | ↓↓ (<40 mg/dL) | Normal | ↓ | ↓↓ |
| **Confirmatory test** | Gram stain and culture | HSV and enterovirus PCR | Cryptococcal antigen; India ink | Acid-fast stain, culture, CSF NAAT |

**Gram stain morphology names the organism before culture returns:** gram-positive diplococci are *Streptococcus pneumoniae*, gram-negative diplococci are *Neisseria meningitidis*, and gram-positive rods are *Listeria monocytogenes*. Drill the [gram-positive versus gram-negative framework](/blog/gram-positive-vs-gram-negative-bacteria-usmle) until those are automatic.

## Common organisms by age group and host

**Age and immune status predict the organism.**

| Patient | Most likely organisms | Stem clue |
| --- | --- | --- |
| **Neonate (0-1 month)** | Group B *Streptococcus*, *E. coli*, *Listeria* | Bulging fontanelle, poor feeding |
| **Children and adolescents** | *S. pneumoniae*, *N. meningitidis*; *H. influenzae* type b if unvaccinated | Preceding otitis; missed vaccines |
| **College students, military recruits** | *N. meningitidis* | Dormitory or barracks; petechial rash |
| **Adults over 50** | *S. pneumoniae*, *N. meningitidis*, *Listeria*, gram-negative rods | Deli meats or soft cheese; ataxia |
| **Immunocompromised (HIV, transplant, steroids)** | *Cryptococcus neoformans*, *Listeria*, *M. tuberculosis* | CD4 below 100; weeks of headache |

## Empiric treatment by age, plus the three specific therapies

**Empiric antibiotics cover the age-appropriate organisms, then narrow with culture.** Neonates get **ampicillin plus gentamicin or cefotaxime** -- ceftriaxone is avoided because it displaces bilirubin and causes biliary sludging. Children and adults under 50 get **vancomycin plus ceftriaxone**, the vancomycin covering resistant pneumococcus. Adults over 50 and any immunocompromised patient get **vancomycin plus ceftriaxone plus ampicillin**, because cephalosporins have no activity against *Listeria*.

Three specific therapies are tested relentlessly:

- **Dexamethasone** is given **before or with the first antibiotic dose** in suspected pneumococcal meningitis. Steroids blunt the inflammatory burst that follows bacterial lysis, cutting mortality and hearing loss; given afterward it loses that benefit.
- **Intravenous acyclovir** is started immediately for suspected **HSV encephalitis** -- fever, personality change, temporal lobe findings, red cells in the CSF -- without waiting for the PCR.
- **Amphotericin B plus flucytosine** is induction therapy for **cryptococcal meningitis**, followed by fluconazole consolidation, plus **serial lumbar punctures** to drain the elevated opening pressure.

## When to image the head before the lumbar puncture

**Order a non-contrast head CT before the tap only when mass effect could make herniation likely.** The five classic indications are **papilledema, a focal neurologic deficit, altered mental status, immunocompromise, and new-onset seizure** -- the lateralizing signs covered in [stroke localization](/blog/stroke-localization-usmle) are the most common trigger.

**Antibiotics are never delayed for the scanner.** The correct sequence is: draw **blood cultures**, give **dexamethasone and antibiotics**, obtain the **CT**, then perform the **LP**. Pretreatment sterilizes the CSF culture over hours, but cell count, protein, and glucose stay abnormal, and blood cultures often identify the organism.

> **10-second exam strategy:** Check glucose first. Normal glucose with lymphocytes is viral. Low glucose with neutrophils is bacterial. Low glucose with lymphocytes is fungal or tuberculous -- and an enormous opening pressure in an AIDS patient is cryptococcus until the antigen says otherwise.

## How this is tested on the exam

- **College freshman, fever, stiff neck, spreading petechial rash, hypotension** -> *N. meningitidis* with Waterhouse-Friderichsen syndrome; ceftriaxone plus vancomycin, with rifampin or ciprofloxacin prophylaxis for contacts.
- **72-year-old who eats soft cheese, now febrile, confused, and ataxic** -> *Listeria*; add **ampicillin**.
- **HIV patient with CD4 of 40, three weeks of headache, opening pressure 40 cm H₂O, encapsulated yeast** -> cryptococcal meningitis; amphotericin B plus flucytosine with serial LPs.
- **Fever, aphasia, and a temporal lobe seizure with red cells in the CSF** -> HSV-1 encephalitis; start acyclovir before the PCR returns.

## Common wrong-answer traps

- **Trap: calling partially treated bacterial meningitis "viral."** Prior oral antibiotics sterilize the Gram stain and shift the differential toward lymphocytes, but **glucose stays low and protein stays high**, which keeps the diagnosis bacterial.
- **Trap: excluding viral meningitis because neutrophils predominate.** In the first 24 to 48 hours viral meningitis is often neutrophil-predominant before converting to lymphocytes. **Normal glucose** keeps it viral.
- **Trap: excluding cryptococcal meningitis because the CSF looks bland.** Advanced AIDS leaves too few CD4 cells to mount a pleocytosis, so the cell count may be nearly normal. The **markedly elevated opening pressure and a positive cryptococcal antigen** make the diagnosis.

## Sources

- [CDC meningitis overview](https://www.cdc.gov/meningitis/index.html)
- [IDSA practice guidelines](https://www.idsociety.org/practice-guideline/practice-guidelines/)

CSF panels become pattern recognition once you have worked through enough stems -- [Practice meningitis vignettes on StepGenie](https://dashboard.stepgenie.app/sign-up) and drill the age-based regimens, the CT-before-LP rule, and these traps.

## Frequently asked questions

### What CSF glucose level suggests bacterial meningitis?

Bacterial meningitis classically drops CSF glucose below 40 mg/dL, and more reliably drops the CSF-to-serum glucose ratio below 0.4. Because normal CSF glucose is roughly two-thirds of the serum value, always draw a serum glucose at the same time as the tap. Low glucose also occurs in fungal and tuberculous meningitis, but essentially never in uncomplicated viral meningitis.

### Is dexamethasone given before or after the first antibiotic dose in meningitis?

Dexamethasone is given before or together with the first antibiotic dose in suspected pneumococcal meningitis. Antibiotics lyse bacteria and release cell wall components that drive a sudden inflammatory surge in the subarachnoid space, and steroids only blunt that surge if they are already on board. Giving dexamethasone after antibiotics forfeits the reduction in mortality and hearing loss, and it is stopped if cultures exclude pneumococcus.

### Which patients need a head CT before lumbar puncture for suspected meningitis?

Image first when a mass lesion could make herniation likely: papilledema, a focal neurologic deficit, altered mental status, immunocompromise, or new-onset seizure. Everyone else can be tapped directly. Critically, the CT never delays treatment. Draw blood cultures, give dexamethasone and empiric antibiotics, then get the scan and perform the lumbar puncture afterward, since the cell count, protein and glucose remain diagnostic despite pretreatment.

### What empiric antibiotics cover meningitis in an adult over 50?

Adults over 50, and immunocompromised patients of any age, receive vancomycin plus ceftriaxone plus ampicillin. Vancomycin covers penicillin-resistant Streptococcus pneumoniae, ceftriaxone covers pneumococcus and Neisseria meningitidis, and ampicillin is the piece students forget because cephalosporins have no activity against Listeria monocytogenes. Add dexamethasone when pneumococcal meningitis is suspected, and narrow the regimen once culture and susceptibility results return.

### How does tuberculous meningitis differ from cryptococcal meningitis on CSF?

Both give a subacute lymphocytic pleocytosis with low glucose. Tuberculous meningitis produces the highest protein of any meningitis, often with cranial nerve palsies and basilar involvement, and is confirmed by acid-fast stain, mycobacterial culture or CSF nucleic acid testing. Cryptococcal meningitis is defined by a markedly elevated opening pressure in an AIDS patient, sometimes with a nearly normal cell count, and is confirmed by cryptococcal antigen.

---

Canonical page: [Bacterial vs Viral Meningitis (USMLE)](https://www.stepgenie.app/blog/meningitis-bacterial-viral-fungal-usmle)
Topic hub: [Infectious Disease](https://www.stepgenie.app/topics/infectious-disease)