Making the diagnosis of tuberculous meningitis in a developing country with limited resources can be difficult, with failure to do so often being fatal for the patient. Thwaites et al developed diagnostic classification trees (CARTs – classification and regression tree) distinguishing patients with tuberculous meningitis from those with bacterial meningitis. The authors are from Centre for Tropical Diseases in Ho Chi Minh City and the University of Oxford in England.
NOTE: The study was performed in Ho Chi Minh City and involved a population with a high prevelance of tuberculosis. The diagnostic resources were limited, and factors such as survival without antituberculous medications were used for categorizing patients.
NOTE: This might be more accurate for distinguishing tuberculous from nontuberculous meningitis, rather than from bacterial meningitis. Classification as bacterial meningitis included sterile blood and CSF cultures with full recovery at 3 months without antituberculous therapy. In theory viral meningitis would not be included because selection criteria included presence of neutrophils and a low glucose in the CSF.
Patients: immunocompetent adults from 16 to 64 years of age
Decision trees:
(1) at time of admission
(2) after 48 hours of broad spectrum antibiotics
Parameters for admission tree:
(1)WBC count per µL in CSF
(2) duration of illness in days
(3) WBC count per µL in peripheral blood
(4) age of the patient in years
|
CSF WBC |
Duration |
Blood WBC |
Age in Years |
Diagnosis |
|
< 760 |
< 6 days |
>= 10,200 |
NA |
non-TB |
|
< 760 |
< 6 days |
< 10,200 |
NA |
tuberculous |
|
< 760 |
>= 6 days |
NA |
NA |
tuberculous |
|
>= 760 |
< 7 days |
NA |
NA |
non-TB |
|
>= 760 |
>= 7 days |
NA |
< 42 |
tuberculous |
|
>= 760 |
>= 7 days |
NA |
>= 42 |
non-TB |
Parameters for 48 hour tree:
(1) duration of illness in days (equivalent to 6 days on admission since 6 + 2)
(2) percent neutrophils in peripheral blood at 48 hours
(3) percent neutrophils in CSF at 48 hours
(4) rise in CSF-to-blood glucose ratio
rise in CSF-to-blood glucose ratio =
= ((ratio after 48 hours) – (ratio at admission)) / (ratio at admission)
|
Duration of Illness (on admission) |
Neutrophils Blood |
Neutrophils CSF |
Rise in CSF-to-Blood Glucose Ratio |
Diagnosis |
|
< 8 days (< 6) |
< 80% |
< 81% |
NA |
tuberculous |
|
< 8 days (< 6) |
< 80% |
>= 81% |
NA |
non-TB |
|
< 8 days (< 6) |
>= 80% |
NA |
NA |
non-TB |
|
>= 8 days (>= 6) |
NA |
NA |
>= 100% |
non-TB |
|
>= 8 days (>= 6) |
NA |
NA |
< 100% |
tuberculous |
Performance:
• The admission tree showed a sensitivity of 99% and specificity of 93% (with prospective test data sensitivity was 88% and specificity was 70%).
• The 48 hour tree showed a sensitivity of 93% and specificity of 95%.
Specialty: Infectious Diseases, Neurology
