Infectious Disease Tests
- CSF gram stain for direct observation of microorganisms under a microscope. A sample of CSF is centrifuged and the concentrated portion is placed on a slide and treated with a special stain for examination under the microscope. If bacteria or fungi are present on a CSF gram stain, then the patient probably has bacterial or fungal meningitis.
- 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 affected person and prophylaxis (preventive treatment) of 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 health practitioner strongly suspects a central nervous system infection, then additional testing may be ordered. This may include one or more of the following:
- Detection of viruses – there are multiple ways to detect viruses in CSF. Most commonly, polymerase chain reaction (PCR) testing is used to detect viral genetic material (DNA, RNA) but viral antigen tests and cultures may also be used.
- CSF Cryptococcal antigen – in addition to culture, this test may be done to detect a fungal infection caused by the yeast Cryptococcus neoformans.
- 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 testing may be positive with Mycobacterium tuberculosis and with other mycobacteria. Molecular tests specific for Mycobacteria tuberculosis may be performed when tuberculosis is suspected.
- Parasites may also be detected in CSF with laboratory tests such as molecular tests or culture. Parasitic meningitis or encephalitis are rare and can be lethal. One example is an infection caused by the free-living amoeba, Naegleria fowleri, a single-cell parasite that can be found in warm water lakes and rivers. Infection occurs when the parasite enters the respiratory system through the nose of a person swimming in contaminated water.
- CSF syphilis testing (VDRL) may be positive with neurosyphilis, infection of the brain by syphilis; a negative result does not necessarily rule out brain involvement.