Related Experiment Video
Updated: Sep 15, 2025
Diagnosing Pulmonary Tuberculosis with the Xpert MTB/RIF Test
Published on: April 9, 2012
Prognostic Value of Time-to-Positivity and Cycle Threshold (CT) Values in Predicting Tuberculosis Outcomes and
Raunak Bir1, Urvashi B Singh2, Hitender Gautam2
1Department of Microbiology, Employees' State Insurance Corporation (ESIC) Medical College and Hospital, Faridabad, IND.
Background:
Relapse and default cases of pulmonary tuberculosis (PTB) present a significant challenge due to higher bacillary loads and the increased risk of developing multidrug-resistant tuberculosis (MDR-TB). Conventional diagnostic methods, such as smear microscopy, lack sensitivity and timeliness. This study evaluates the prognostic value of time to positivity (TTP) from liquid culture and cycle threshold (CT) values from GeneXpert Mycobacterium tuberculosis/rifampicin (Cepheid, Sunnyvale, CA) in predicting treatment outcomes.
Objectives:
This study aimed to assess the utility of baseline mycobacteria growth indicator tube (MGIT) TTP and GeneXpert CTvalues as early biomarkers for prognosis and the risk of MDR-TB development in smear-positive relapse and default PTB patients.
Methods:
This cross-sectional study with a prospective arm enrolled 72 adult PTB patients (relapse, default, or treatment failure). Sputum samples underwent smear microscopy, MGIT liquid culture, and GeneXpert testing. Baseline and three-month follow-up results were compared. TTP and CT values were correlated with microbiological outcomes.
Results:
MGIT culture was positive in 52.8% (38/72) of patients (mean TTP: 26.5 days). In comparison, GeneXpert detected M. tuberculosis in 70.8% (51/72) (mean CT: 24.4). At follow-up, 7.8% (3/38) remained culture-positive (mean TTP: 29.7 days); one isolate was MDR. Patients with follow-up culture positivity had significantly lower baseline TTP (19.7 ± 3.4 vs. 25.8 ± 3.8 days). CT values trended lower among poor responders but did not reach statistical significance. Logistic regression suggested that a TTP threshold of ≤20.89 days might predict poor outcomes (model accuracy: 70%).
Conclusion:
Baseline MGIT TTP demonstrates significant prognostic potential in identifying patients with PTB at risk of treatment failure or MDR development. In contrast, GeneXpert CT values, while useful diagnostically, were less predictive of outcomes. Incorporating TTP into routine clinical practice could enable earlier intervention and improve TB control strategies in high-burden settings.
Related Concept Videos
01:26Pulmonary Tuberculosis IV
Several diagnostic approaches are used to detect TB. The conventional method is the Tuberculin Skin Test (TST), also known as the Mantoux test. However, this method has...
01:28Pulmonary Tuberculosis V
Latent tuberculosis infection occurs when TB bacteria are present in a person's body, but are not causing illness or symptoms. It is not contagious, and preventive treatment is crucial to avoid the...
01:31Pulmonary Tuberculosis III
The first classification is based on the development of the disease, and it includes the following categories:
01:29Pulmonary Tuberculosis I
Causative Organism
The primary infectious agent causing tuberculosis is Mycobacterium tuberculosis, a slow-growing, acid-fast, aerobic rod that exhibits sensitivity to heat and ultraviolet light. Instances of Mycobacterium bovis and Mycobacterium avium contributing to the development of TB infection are rare.
Mode of...
01:28Pulmonary Tuberculosis II
Here is a detailed explanation of its pathophysiology:
Transmission: The process begins when a person inhales droplet nuclei containing M. tuberculosis. These are typically released into the air when an individual with pulmonary or...