Long-Term Storage at −80°C: Effect on Rate of Recovery of Mycobacterium tuberculosis From Direct Acid-Fast Bacilli Smear-Positive Sputum Samples

Abstract

Background: The aim of this study was to evaluate the difference in the rate of recovery of Mycobacterium tuberculosis (MTB) from routinely cultured sputum and long-term stored sputum specimens (at −80°C) using Löwenstein–Jensen (LJ) media, Mycobacterium Growth Indicator Tube (BBL MGITTM), and Middlebrook 7H11 (MB 7H11) agar. Methods: Direct acid-fast bacilli smear-positive sputum specimens (both before and after storage [n = 136]) were studied (after culturing on LJ media, BBL MGITTM, and MB 7H11 agar) and the performances were compared. Results: For the detection of MTB, BBL MGITTM and MB 7H11 agar (before storage) demonstrated a sensitivity, specificity, positive predictive value (PPV) of 98.28%, 30.77%, 92.68%, 66.67%, and negative predictive value (NPV) of 97.41%, 30.77%, 92.62%, 57.14%, respectively, when compared to LJ media (before storage). Similarly, BBL MGITTM and MB 7H11 agar (after storage) demonstrated a sensitivity, specificity, PPV, and NPV of 95.5%, 38.89%, 90.6%, 58.33%, and 95.5%, 66.67 %, 94.64%, 70.59%, respectively, when compared to LJ media (after storage) for the detection of MTB. Conclusion: None of the culture techniques independently (both before and after storage) detected growth of MTB from all the sputum specimens studied. However, BBL MGITTM system and LJ media combination (both before and after storage) effectively detected the growth of MTB from sputum specimens when compared to other culture technique combinations.