Sunday, April 04, 2010

TB meningitis diagnostic test pearls

H/t Wendy Ziai and John Lewin Neurol Cl May 2008

1. CXR and ppd may be negative in half of cases, and typical  CSF profile may not be present especially in immunosuppressed patients. 

2. AFB in CSF is positive in only 30 % (maybe able to increase to 70 % with meticulous and repeated sampling), culture is only positive in 40-70 %, and may require weeks to have a result.

3.  CSF adenosine deaminase (ADA) activity is a biochemical marker that may help.  At a cutof of 6.97 iu/L it is fairly sensitive and specific  (85 % and 88 % respectively).  ADA is useful in third world countries and poorly equipped labs  (Gautam et al., Nepal Med Coll, 2007). 

4.  Molecular  nucleic acid amplification kits   have sensitivity 60-83 % and specificity of 98 % and should be first line to rule out (Dinnes et al. Health Care Tech  2007)

5. PCR is fastest and most sensitive tests but is not good enough to rule out TB meningitis.

6.  Measurement of interferon gamma  in CSF compared with PCR is more sensitive (70 v 65 %) and has specificity of 94 %.  Interferon gamma plus PCR has 80 % sensitivity.

7.  In general careful bacti is as good as molecular in initially diagnosing TB meningitis although molecular stays positive longer with treatment.

8.  TB is prevalent in indigent urban nonwhite populations with a high rate of HIV infection (Arch Int Med 1996).  Presentation is fever, malaise, headache and personality changes, leading in 2-3 weeks to classic signs such as headache, meningismus, vomting, confusion and focal neurologic findings.  Occassionally it presents like acute bacterial infefctions.

9. MRI classic triad for TB meningitis is basal meningeal enhancement, hydrocephalus, and supratentorial and brainstem infarctions; hydrocephalus can be communicating or noncommunicating.

10.  Outcome scoring system is called Weisfelt system and is calculated one hour after admission based on six variables: age, heart rate, GCS, cranial neuropathies, CSF WBC, Gram stain findings.

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