Infectious disease tests

In addition to chemistry tests, such as protein and glucose, other routine tests may be performed to look for microorganisms if meningitis or encephalitis is suspected.

  • CSF gram stain for direct observation of microorganisms under a microscope. There should be no microorganisms in CSF fluid. If bacteria or fungi are present on a CSF gram stain, then the patient may have a bacterial or fungal meningitis or encephalitis.
  • CSF culture and sensitivity is used to detect any microorganisms, which will grow in the culture. If bacteria are present, they can be tested in the laboratory to predict the best choices for antimicrobial therapy for the patient and for taking prophylactic measures in close contacts, if needed. If there are no microorganisms present, it does not rule out an infection; they may be present in small numbers or unable to grow in culture due to prior antibiotic therapy.

If any of the initial tests are abnormal or if the doctor strongly suspects a CNS infection, then additional testing may be ordered. This may include one or more of the following:

  • Detection of viruses - detection of viral genetic material (DNA, RNA) by PCR testing; for example, herpes virus and enteroviruses. Positive PCR tests for viral DNA or RNA, antigen tests, and growth on viral cultures indicate that the patient has a viral infection and may have viral encephalitis or meningitis. The presence of viral antibodies and their increase over time indicates a recent infection by that virus.
  • CSF Cryptococcal antigen - to detect a specific fungal infection
  • Other CSF antigen tests - depending on which organism(s) are suspected
  • Specific CSF antibody tests - depending on which organism(s) are suspected

Other CSF tests for infectious diseases that are less commonly ordered include:

  • CSF AFB smear and culture may be positive with tuberculosis and with other mycobacteria
  • CSF molecular tests for Mycobacteria tuberculosis when tuberculosis is suspected
  • CSF syphilis testing (VDRL) positive with neurosyphilis (involvement of the brain by syphilis); a negative does not rule out brain involvement

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