Application on GeneXpert on bronchoscopic samples in the clinical management of patients suspicious of Tuberculosis
Date: 2017-11-13
Author: Dr. TO Kin Wang
Summary: Background: There is limited experience on clinical use of GeneXpert on bronchoalveolar lavage (BAL) fluid samples obtained from patients clinically suspected of pulmonary tuberculosis (TB) in intermediate burden settings. Methods: sputum acid-fast-bacilli (AFB) smear negative patients were offered bronchoscopy. BAL fluid was collected for AFB smear, TB culture, Cobas Taqman TB polymerase chain reaction (PCR) and for GeneXpert. Cases were diagnosed as TB if any one of the tests was positive. Results: From December 2014 to February 2017, 227 samples were collected. Patients’ mean (SD) age was 60.7 (15) years, 143 patients were males and 84 were females. Cough and haemoptysis were the presenting symptoms in 70% and 37.4% respectively. Apical shadows in chest X-ray (CXR) and apical cavitations in computed tomography (CT) were more common in GeneXpert positive cases (p=0.01 and 0.02 respectively). Sensitivities and specificity of GeneXpert was 80% and 98% respectively. The positive predictive value and negative predictive value was 92.3 and 95.1% respectively. There were 9 false negative GeneXpert samples (8 were Cobas Taqman TB PCR negative): 6 were diagnosed by BAL culture, 2 by biopsy and one by Cobas Taqman TB PCR. There were 3 false positive cases with negative culture, 2 were put on empirical treatment with favorable clinical responses, while one defaulted followed up. Conclusion: GeneXpert in BAL samples has high specificity and similar performance as Cobas Taqman PCR to rule in TB for initiating early treatment in clinical suspicious cases. However, it cannot replace other investigations as the only test for diagnosing pulmonary TB.
Optimizing MGIT pyrazinamide susceptibility testing using a reduced inoculum
Date: 2016-07-31
Author: Dr. YAM Wing Cheong
Summary: Pyrazinamide (PZA) is considered an essential component of first-line TB therapy, it is important that laboratories should adopt a successful algorithm to provide rapid and accurate susceptibility results for PZA. In this study, testing 160 M. tuberculosis isolates indicated no significant difference between standard and reduced inoculums for MGIT960 PZA susceptibility test (p >0.05). The failure rate was higher for reduced inoculums (6.9%) than the standard inoculums (1.8%). Further investigation on isolates using PCR-sequencing indicated all PZA resistant strains exhibited pncA mutations (known or novel) with the most common G162D genotype as described previously in Hong Kong. The PCR-sequencing is highly sensitive to detect all pncA mutations among our PZA resistant isolates collected over 10 years (2003-2013) in Hong Kong. Another 20 confirmed PZA susceptible isolates exhibited pncA wild types, indicating PZA resistance is mainly associated with pncA mutations in our locality. This is further verified by pyrazinamidase (PZase) activity among our PZA susceptible and resistant isolates. As M. tuberculosis isolates from all Hospital Authority Microbiology Laboratories are centralized in the TB reference Laboratory (PHLSB) for DST using MGIT960 system, we propose retesting all PZA-resistant isolates to provide accurate and reliable susceptibility results. Any M. tuberculosis clinical isolate reported as PZA resistant by the MGIT960 should be confirmed by PZase assay and pncA gene mutation analysis using PCR-sequencing.