The manual MGIT system for the detection of M. tuberculosis in respiratory specimens: an experience in the University Malaya Medical Centre.
A prospective study was conducted on 510 respiratory specimens for the presence of M. tuberculosis detected by direct acid-fast bacilli (AFB) smear examination, culture in the Manual Mycobacteria Growth Indicator Tube (BBL MGIT, Becton-Dickinson) and culture on Lowenstein-Jensen (LJ) medium. From positive BBL MGIT tubes, Ziehl-Neelsen and Gram stains were performed and subcultures were put up on LJ medium. A total of 101 (19.8%) specimens were positive by the BBL MGIT, 60 (11.8%) by primary LJ medium culture, 31 (6.1%) by direct smear examination and 29 (5.7%) by all three methods. Using primary LJ culture as the gold standard, the sensitivity and specificity of the BBL MGIT were 90% and 89.6% respectively but the sensitivity of AFB smear microscopy was only 48.3%. About half (51.1%) of the BBL MGIT false positives were due to contamination by non-AFB bacteria. The remaining false positives comprised specimens that were AFB microscopy positive but LJ culture negative. Of the AFB isolates obtained on LJ primary and sub-cultures, almost all (93.3%) were identified as Mycobacterium tuberculosis complex. The mean time-to-detection was significantly shorter (p < 0.0001) for the BBL MGIT than for LJ culture. For the former, positive results were available within 14 days for both AFB smear-positive and AFB smear-negative specimens. On the average, positive results were obtained 1.8 days earlier for direct AFB smear-positive samples than for AFB smear-negative samples. On the other hand, positive growth on LJ medium appeared after at least 33 days of incubation. These findings suggest that the BBL MGIT system will be a suitable alternative to LJ culture for the routine diagnosis of pulmonary tuberculosis, but a combination of liquid and solid cultures is still required for the highest diagnostic accuracy.
Fadzilah
Mohd Nor
MN
Department of Medical Microbiology, Faculty of Medicine, University of Malaya, Kuala Lumpur, Malaysia.
Ng
Kee Peng
KP
Ngeow
Yun Fong
YF
D001431:Bacteriological Techniques / Q000191:economics / Q000379:methods* / Q000592:standards; D001992:Bronchoalveolar Lavage Fluid; D003470:Culture Media; D006801:Humans; D009169:Mycobacterium tuberculosis / Q000254:growth & development / Q000302:isolation & purification*; D011237:Predictive Value of Tests; D011446:Prospective Studies; D013048:Specimen Handling; D013183:Sputum; D013997:Time Factors; D014396:Tuberculosis, Pleural / Q000175:diagnosis* / Q000382:microbiology; D014397:Tuberculosis, Pulmonary / Q000175:diagnosis* / Q000382:microbiology
D003470:Culture Media