Information for Neurologists on PCR for TB
Direct Smears of CSF for acid-fast bacilli (AFB), although virtually diagnostic, is usually positive in less than 10% of cases of Tubercular Meningitis.
Culture for M. Tuberculosis takes up to 8 weeks and is also often negative. The sensitivity compared to pulmonary samples is less than 20% in case of CSF samples.
ELISA based method of detection is rapid, but its sensitivity at best is reported not to be higher than 55%. Also the biggest drawback is the high rate of false positivity, 20-30%, in both antigen and antibody detection assays in high prevalence regions like India. This makes results of ELISA very difficult to interpret for the physicians.
PCR is a molecular diagnostic tool. Though there are various methods in molecular diagnostics for the detection of Mycobacterium tuberculosis, (PCR, SDA, TMA, NASBA), PCR continues to be most standardized and widely accepted test worldwide. It is a very rapid test and the sensitivity is reported to be as high as 95%. Auroprobe Laboratories has made this test highly sensitive and specific by choosing DNA target MPB 64 which is very well represented in the Indian strains of Mycobacterium tuberculosis.
Comparison Of Various Diagnostic Assays For Tuberculosis Infection
Type of Test |
Advantage |
Disadvantage |
| ZN SMEAR |
Economical, early result, easy availability |
Poor sensitivity (<30%) |
| RADIOMETRIC CULTURE (BACTEC 460 TB SYSTEM) |
Differentiation between M.tuberculosis and MOTT, Drug sensitivity can be performed |
Result in 1 week (Avg), very expensive instrument, not easily available everywhere |
| ELISA |
Rapid, gives global diagnosis by antibody detection |
High false positive rate in endemic countries, low sensitivity |
| AUROPROBE TB PCR (ATP 1): |
Highly sensitive (>98%), early result |
Needs expertise and elaborate set up including class 100 clean rooms |
Information on collection of sample for Neurologists
Clinical Indications |
Specimen |
Collection |
Recommended Volume |
Storage |
Transportation |
Meningitis |
CSF |
In sterile container |
2ml |
*At 40C |
**In gel pack box |
Tubecruloma |
CSF |
In sterile container |
2ml |
*At 40C |
**In gel pack box |
Paraspinal abcess |
CSF |
In sterile container |
2ml |
*At 40C |
**In gel pack box |
| * |
In middle shelf of refrigerator |
| ** |
Special transportation box available from Auroprobe Laboratories |
| *** |
Since there is intermittent shedding of M. tuberculosis bacilli in the CSF a negative PCR result may be found in spite of the presence of tuberculoma in the brain tissue. Therefore, a positive PCR for M. tuberculosis in CSF is confirmatory of Tuberculosis CNS infection, whereas a negative report may be a false negative due to intermittent shedding of the bacteria in the CSF |
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