Q: I would like to know if active extrapulmonary tuberculosis of the brain and spinal cord could still be diagnosed after 3 months of treatment. How soon after starting of therapy are sputum and spinal cultures negative?

A:The treatment of extrapulmonary tuberculosis differs from that of pulmonary (lung) tuberculosis in several ways, largely because of the difficulty of diagnosis, which often leads to empirical treatment. However, diagnosis made only on clinical grounds leads to over-diagnosis and unnecessary treatment of a large number of patients. The problems of diagnosis are compounded by a lack of diagnostic resources in developing countries. Extrapulmonary tuberculosis is usually paucibacillary (i.e. less number of bacteria on smears), and any treatment regimen effective in pulmonary tuberculosis is likely to be effective in the treatment of extrapulmonary tuberculosis as well. AFB smears on CSF are positive in 10 to 90 percent of patients. CSF culture for M. tuberculosis is positive in 45 to 90 percent of cases but takes four to six weeks by the routine method and 2-3 weeks by the rapid method. Molecular tests like PCR on CSF for M. tuberculosis have a sensitivity of 56 percent and a specificity of 98 percent, and therefore should not be used to exclude tuberculous meningitis. The average time for smears / cultures to become negative on treatment vary from 2 weeks to 2 months. Active TB of the central nervous system after 3 months of treatment may represent treatment failure and could be due to multidrug resistant (MDR) strain or atypical mycobacteria.