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CSF Culture and Antigen Testing

CSF bacterial cultures yield the bacterial cause in 70-85% of cases. The yield diminishes significantly in patients who have received antimicrobial therapy. In these cases, some experts advocate the use of a CSF bacterial antigen assay. This is a latex agglutination technique that can detect the antigens of HIB, S pneumoniae, N meningitidis, E coli K1, and S agalactiae. Its theoretical advantage is the detection of the bacterial antigens even after microbial killing, as is observed following antibacterial therapy.

Others, however, have shown that the CSF bacterial antigen assay may not be better than the Gram stain. It is specific (a positive result indicates a diagnosis of bacterial meningitis), but a negative finding on the bacterial antigen test does not rule out meningitis (50-95% sensitivity).

C neoformans may be cultured from the CSF in cryptococcal meningitis. Other methods of identification have included India ink preparation and the detection of CSF cryptococcal antigen. India ink has a sensitivity of only 50%, but it is highly diagnostic if positive.

Because of the low sensitivity of the India ink preparation, many centers have adapted the use of CSF cryptococcal antigen determination, a test with a sensitivity of greater than 90%. However, the CSF cryptococcal antigen determination is not universally available. In instances when the India ink results are negative but the clinical suspicion for cryptococcal meningitis is high, the CSF specimen may be sent to reference laboratories that can perform CSF cryptococcal antigen determination to confirm the diagnosis. In addition, the titer of the antigen could serve to monitor the response to treatment.

Obtain blood cultures and serum cryptococcal antigen to determine if cryptococcal fungemia is present.

In syphilitic meningitis, isolating T pallidum from the CSF is extremely difficult and time consuming. The spirochete could be demonstrated using dark-field or phase-contrast microscopy on specimens collected from skin lesions (eg, chancres and other syphilitic lesions). The diagnosis is usually supported by the CSF Venereal Disease Research Laboratory (VDRL) test, which has a sensitivity of 30-70% (a negative result on the CSF VDRL test does not rule out syphilitic meningitis) and a high specificity (a positive test result suggests the disease). Always take care to not contaminate the CSF with blood during spinal fluid collection (eg, traumatic tap).

The culture for B burgdorferi has a low yield. The recent availability of the CSF Lyme PCR assay offers a rapid, sensitive, and specific method of diagnosis. This assay is gaining popularity as the method of choice for diagnosis of Lyme meningitis.

The culture for Mycobacterium usually takes several weeks and may delay definitive diagnosis. M tuberculosis detection assays involving nucleic acid amplification have become available and have the advantage of a rapid, sensitive, and specific method of tuberculosis detection. The need for mycobacterial growth in cultures remains because this offers the advantage of performing drug susceptibility assays.

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