Comparison of detection of Acid Fast Bacilli in clinical samples by AFB Smear microscopy and Culture in the diagnosis of Tuberculosis in a tertiary care setting

Abstract

Ziehl Neelsen smear is widely used for rapid Diagnosis of Tuberculosis in developing countries but culture on Lowenstein Jensen (LJ) medium is more sensitive and cheaper than modern diagnostic techniques in the present settings.Objective: To evaluate the validity of ZN smear and reliability of acid-fast bacilli culture on LJ medium.Design and settings: This is a comparative study, carried out in PMRC TB Research Centre King Edward Medical University Mayo Hospital Lahore from January, 2006 to December, 2008.Patients and Methods: Study subjects included patients visiting TB OPD Clinic and Wards of Mayo Hospital and other leading hospitals of Lahore. A total of 6738 specimen were processed for both smear and culture. The smears were stained with ZN method using 1% Carbol Fuchsin, 25% Sulphuric Acid and 0.3% Methylene Blue and were observed under 100X oil immersion lens. Cultures were inoculated on LJ medium after digestion and decontamination of clinical specimens.Results: 313 (4.65%) were contaminated in the culture therefore excluded from the study, of remaining 6425 pulmonary and extra-pulmonary specimens 964 (15.01%) were smear positive and 2060 (32.06%) were culture positive. Smear positivity of extrapulmonary specimens were very low (3.97%) as compared to Culture of the same sample (21.18%). 1103 (20.27%) smear negative specimens were found out to be positive by culture on LJ medium.Conclusion: Although AFB smear is rapid, cheap and specific test for diagnosis of TB,its sensitivity is low and culture on LJ medium is more reliable and cheaper thanradiometric and molecular based diagnostic techniques.