Comparison of Performances of MTD Gene-Probe® Test, BACTEC 960™ System and Löwenstein-Jensen Culture Methods in the Diagnosis of Smear-Negative Tuberculosis Cases

Abstract

The aim of this study was to compare the results of nucleic acid amplification-based MTD (Mycobacterium tuberculosis direct test) Gene-Probe® method in samples obtained from acid-fast bacilli (ARB) smear-negative patients with suspected tuberculosis (TB), with the culture results obtained from automated BACTEC 960™ (MGIT) system and Löwenstein-Jensen (Lj) medium. In addition, the contribution of molecular methods in early diagnosis of pulmonary TB and the effect of radiological prevalence of the disease associated with or without cavity to the molecular diagnosis and/or growth time in culture media have been evaluated. A total of 107 patients (86 male, 21 female; mean age: 49.89 ± 17.1 years, age range: 18-81 years) who were clinically and radiologically suspected of having pulmonary TB and/or TB pleurisy, were included in the study. Of the samples 65 (60.7%) were sputum, 32 (29.9%) were bronchial aspiration, 5 (4.7%) were pleural fluid, and 5 (4.7%) were transthoracic fine needle aspiration biopsy materials. Patient samples were cultured in solid LJ media and liquid-based BACTEC 960 system (Becton Dickinson Co., USA) in the same working day. Meanwhile, MTD Gen-Probe test (Gen-Probe Inc., USA) was studied in two separate working days of the week as specified by the laboratory. The samples were incubated until positivity was determined in BACTEC 960 system and/or growth was detected in LJ medium. Negative cultures were incubated for 42 days and were finalized. When mycobacterial growth was determined in the culture, identification of M.tuberculosis complex (MTBC) and differentiation from nontuberculous mycobacteria were performed by conventional methods and BACTEC 460 NAP test. Forty five (42%) patients were diagnosed as pulmonary paranchimal TB (40 were active pulmonary TB, 1 was miliary TB and 4 were culture-negative pulmonary TB), while 4 (3.7%) patients diagnosed as extrapulmonary TB and 58 (57.9%) patients were diagnosed as other pulmonary diseases unrelated with TB. LJ cultures yielded positive results in 32 of 45 (71 %) pulmonary TB patients, and BACTEC 960 were found positive in 84.4% (38/45) of those patients. On the other hand the positivity rate of MTD Gen-Probe test was detected as 37.4% (40/107). The sensitivity, specificity, positive and negative predictive values for MTD Gen-Probe test were estimated as 89%, 100%, 100% and 93%, respectively. Those values for BACTEC 960 system were found as 82%, 98%, 97% and 88%, and for LJ culture method as 71%, 100%, 100% and 83%, respectively. Average periods to make a decision for diagnosis of TB by MTD Gen-Probe, BACTEC 960 (MGIT) and LJ culture methods were calculated as 2.36 days, 20.11 days and 32.49 days, respectively. In comparison of the methods in terms of turnaround ti-mes, MTD Gen-Probe test was found superior to LJ culture method, however the turnaround times for BACTEC 960 and LJ culture methods were similar. When the clinical data were evaluated, no effect of radiological density of lesion was identified on the diagnosis time of molecular test and time of growth in liquid based automated BACTEC system and/or LJ culture method. However, LJ culture demonstrated earlier reactivity in patients with cavitary lesions. As a result, MTD Gene-Probe test was observed as a reliable and rapid method for the early diagnosis of pulmonary TB patients, early initiation of therapy, prevention of disease progression and transmission.