Abstract
Tuberculosis (TB) is a major public health concern, particularly in third-world nations, where the prevalence is reportedly as high as 40 % (Bhargava et al., 2001). Being the most common opportunistic infection among HIV-positive subjects, TB has re-emerged as a global emergency especially in the Indian subcontinent (Kumarasamy et al., 1995). India reportedly has more TB cases than any other country in the world (Kumarasamy et al., 2003). HIV has a substantial influence on the incidence, clinical manifestations, treatment and disease outcome of TB. In addition, the dual epidemic is a cause for concern in countries where these two infections are prevalent in epidemic proportions (Ramachandran et al., 2003).
Prompt diagnosis of TB is crucial for the success of ensuing treatment in any community setting. Conceptually, most of the available standard laboratory and mycobacteriology guideline texts advocate at least three consecutive sputum specimens to detect acid-fast bacilli (AFB) and performing sputum culture on patients suspected to have the disease (Nelson et al., 1998). Considering the huge economic burden imposed on the HIV/AIDS community to avail of expensive antiretroviral drugs and other diagnostic tests, it may be unmanageable for the patients to provide three consecutive specimens. In such a scenario, the usefulness of a single AFB smear in TB diagnosis needs further evaluation. Therefore, we compared the value of microscopy of a single AFB smear in TB diagnosis with a radiometric culture method as the gold standard.
We compared the results of a single AFB sputum smear with those from AFB culture from 121 HIV-positive patients suspected to have TB during a 4-year period from June 2002 to May 2006. Sputum specimens were collected and processed following the standard procedure recommended by the BACTEC 460TB (Becton Dickinson) operations manual. Briefly, after inoculating the treated specimens in BACTEC 12B vials (Becton Dickinson), an initial smear was prepared, stained by the Ziehl–Neelsen method and observed under oil immersion for AFB. The inoculated vials were incubated at 37 °C in the presence of 5–10 % CO2 and read in the BACTEC 460TB following the manufacturer's instructions. Following primary isolation, the M. tuberculosis complex was identified by the p-nitro-α-acetylamino-β-hydroxy-propiophenone differentiation test. Analysis of data was carried out using the Statistical Package for Social Sciences (SPSS, Chicago, IL, USA) version 13.0. Particulars on the diagnostic test procedures used, viz. sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV), were calculated to compare the AFB smear microscopy with culture.
The single AFB smear method showed a sensitivity of 53.3 %, specificity of 89.5 %, PPV of 68 % and NPV of 82 % (Table 1).