CSF Test Results in Infectious Meningitis
Patients with bacterial meningitis usually present with the sudden onset of headache, fever, nuchal rigidity, and photophobia. CSF analysis typically reveals neutrophilic pleocytosis, elevated protein level and a low glucose level. Viral meningitis is typically characterized by an elevated CSF white blood cell count, with a predominance of lymphocytes, and increased protein level.
In patients with cryptococcal meningitis, findings in the CSF are highly variable, and up to 40% of such patients have a normal CSF profile. The CSF white-cell count is generally relatively low, with the cells composed predominantly of lymphocytes; the median count is approximately 20 per cubic millimeter, and only 25% of patients have a count of more than 100 per cubic millimeter. CSF white-cell counts can often be higher in patients receiving antiretroviral therapy. The CSF glucose level may be low or normal, and the CSF protein level is sometimes elevated. Diagnostic tests for cryptococcal meningitis include cryptococcal antigen testing, India ink staining, and culture. The most sensitive test is a cryptococcal antigen lateral flow assay. India ink staining is commonly used in resource-limited settings, but the sensitivity is only 80%. If the patient has fewer than 1000 yeast cells per milliliter of CSF, sensitivity is only 40%. In addition, 5 to 10% of persons with cryptococcal meningitis present with early disseminated cryptococcal infection, with detectable cryptococcal antigen in the peripheral blood and a possibly abnormal CSF profile but with a negative CSF test for cryptococcal antigen.
Tuberculous meningitis has features similar to those of cryptococcal meningitis. Findings in the CSF are similar to those seen in cryptococcal meningitis, although pleocytosis and a low glucose level are often present (Table 2). As compared with cryptococcal meningitis, tuberculous meningitis generally causes a more pronounced increase in the CSF white-cell count. WHO recommends the use of a nucleic-acid amplification test such as the Xpert MTB/Rif assay (Cepheid) as the initial diagnostic assay for tuberculous meningitis. However, sensitivity is highly dependent on an adequate volume of CSF, and a negative test does not rule out tuberculous meningitis.
| Test | Ref Range | Bacterial | Tuberculous | Cryptococcal | Viral |
| CSF Protein Median | 15-45 mg/dL | 250 | 100-200 | 80 | 75 |
| CSF:Plasma Glucose | 0.55-0.70 | 0.20 | 0.28 | 0.40 | 0.65 |
| WBC median | <5 cells/uL | 500-2500 | 200 | 20 | 80-100 |
| %Neutrophils | 0 | 90 | 35-40 | <15 | <35 |
| %Clear color | 100 | 2 | 60 | >95 | >95 |
| Stain | Gram | AFB | India Ink | ||
| Sensitivity | 60-90 | <15 | 80 | ||
| Best Test | Culture | NA amplify | Crypto Ag | PCR | |
| Sensitivity | >95% | 30-70% | >95 | Varies |
Reference
Boulware DR, Makadzange AT. Case 8-2017 – A 39-Year-Old Zimbabwean Man with a Severe Headache. N Engl J Med 2017;376:1065-71.
