4.2. Methods based on real-time genetic amplification technology RT-PCR GeneXpert MTB/RIF
The Xpert MTB/RIF assay is an automated, cartridge-based, easy-to-use system with a closed amplification system to prevent cross-specimen contamination. This test can be easily performed by laboratory technicians without the need for advanced biosafety equipment. In February 2015, the FDA approved the expanded use of the Xpert MTB/RIF test to reduce the isolation period of patients with suspected tuberculosis. According to the new guidelines, one or two negative Xpert MTB/RIF test results are sufficient to rule out pulmonary tuberculosis, in contrast to previous CDC recommendations that required isolation until three consecutive negative AFB smear results. from sputum, to rule out contagious tuberculosis [
16]. The GeneXpert system is an innovative molecular diagnostic platform used for the rapid detection of M. tuberculosis. This system features an interface that allows users to enter data, interpret results, and monitor test progress. The MTB/RIF cartridge contains the necessary elements for the amplification and detection of the specific DNA of the bacterium M. tuberculosis and for the identification of mutations associated with rifampicin resistance. Through the cartridge, the MTB/RIF test can be performed quickly and without requiring any preparation laborious, which makes it suitable for use in field laboratories or in resource-limited areas, contributing to the diagnosis and management of the disease in an efficient manner.
The Xpert MTB/RIF test is a fully automated PCR test based on PCR in real time that de tects M. tuberculosis and mutations associated with resistance to rifampicin (RIF), the 81-bp basic region of the rpoB gene [
73]. The analytical sensitivity of the Xpert MTB/RIF test is five genomic copies of purified DNA and 131 CFU·ml
−1 of M. tuberculosis in sputum. It was not no cross-reactivity with non-tuberculous mycobacteria (NTM) detected [
74].
Multinational evaluations have confirmed the feasibility, accuracy, and effectiveness of the Xpert MTB/RIF test in healthcare facilities in tuberculosis TB-endemic countries in Africa, Asia and Latin America. These results have led the World Health Organization to support the expanded use of this technology. Initially, WHO recommended the use of the Xpert MTB/RIF test for patients with suspected multidrug-resistant tuberculosis MDR-TB and those with HIV co-infection.
Recently, WHO recommended that programs move away from the use of smear microscopy and prioritize the initial use of the Xpert MTB/RIF test. The sensitivity and specificity of the Xpert MTB/RIF test for the diagnosis of pulmonary tuberculosis in adults are approximately 90% and 98%, respectively, using culture as the reference standard [
75].
Several studies have investigated the possibility of using the Xpert MTB/RIF test instead of direct smear microscopy as the primary screening method for urgent clinical samples in a context characterized by a low prevalence of tuberculosis [
73].
GeneXpert MTB/RIF, a test subject to numerous studies and validations in various clinical settings, has been shown by a meta-analysis in low- and middle-income countries to have a high aggregated sensitivity and specificity as an initial test replacing smear microscopy. With a sensitivity of 89% and a specificity of 99% it has proven effectiveness. However, the sensitivity of the test was higher in cases with positive results on microscopy than in those with negative results. Among people without HIV, the sensitivity was 86, while for those with HIV it was 79% [
72].
In the context of extrapulmonary tuberculosis, a condition affecting organs and tissues outside the lungs, the performance of the GeneXpert MTB/RIF system was evaluated. The study included a considerable number of patients with various forms of extrapulmonary tuberculosis, such as nodular, peritoneal, bone-articular and genito-urinary tuberculosis. The GeneXpert MTB/RIF system was compared to the standard reference method, which involves culturing the bacterium and identifying it by traditional laboratory methods [
76].
Extrapulmonary tuberculosis (EPTB) accounts for approximately one fifth of the total cases of tuberculosis in immunocompetent patients. The incidence of EPTB is significant increased in HIV-positive people, exceeding 50% of all tuberculosis cases associated with this condition. Despite the fact that molecular methods generally do not reached the expected level in the diagnosis of tuberculosis, the polymerase chain reaction technique (PCR) is proving to be a particularly useful tool in the diagnosis of EPTB and may, by also be used to identify drug-resistant strains [
77,
78].
In a study conducted at the Bacteriology Department of the Mohammed Military Hospital, Morocco, 714 patient samples were analyzed. The mean age was 47.21 ± 19.98 years, with the majority being male (66.4%). Of the total of 714 samples, 285 came from the lungs and 429 from other areas of the body. The positive detection rate by microscopy was 12.88%, by GeneXpert MTB/RIF it was 20.59%, and by culture it was 15.82%. For lung samples, positive detection rates were higher, 18.9% by microscopy, 23.85% by GeneXpert MTB/RIF, and 20.35% by culture. For extrapulmonary samples, the rates were lower, 9.71% by microscopy, 18.41% by GeneXpert MTB/RIF, and 12.82% by culture. GeneXpert MTB/RIF showed a sensitivity of approximately 78.2% and a specificity of 90.4% in both sample types, while for extrapulmonary samples these figures were 79.3% for sensitivity and 90, 3% for specificity [
79].
A study was performed using two diagnostic methods: ZN smear and GeneXpert MTB/RIF test. The research focused on tuberculous meningitis (TBM), a severe form of tuberculosis. The GeneXpert MTB/RIF test was evaluated for the diagnosis of TBM in a large group of patients in Vietnam. Although the Ziehl-Neelsen smear remained the most sensitive technique, the GeneXpert MTB/RIF test made an important contribution to the early diagnosis of tuberculous meningitis [
80,
81,
82].
Another study conducted in Korea analyzed the effectiveness of the Xpert MTB/RIF Test and a MTB nested PCR in the identification of Mycobacterium tuberculosis. Clinical lung samples and non-pulmonary were collected from 171 patients with suspected tuberculosis. The results showed that the sensitivity, specificity, value positive predictive value (PPV) and negative predictive value (NPP) of the Xpert Test MTB/RIF for the diagnosis of tuberculosis with Mycobacterium tuberculosis positive at culture were 86.1%, 97.8%, 91.2%, and 96.4%, respectively. In comparison, values of the nested PCR were 69.4%, 94.1%, 75.8%, and respectively 92.0%. In addition, the Xpert MTB/RIF test demonstrated a significantly longer response time short compared to nested PCR, with a median of 0 [0–4] days versus 4 [1–11] days, respectively (p under 0.001) [
83].
Another study from Malaysia published in 2021 was carried out to reduce the underdiagnosis of pulmonary tuberculosis with negative ZN smear results, the clinical and radiological characteristics of patients with this form of tuberculosis were evaluated. The research included 235 patients from a clinic in Luyaun, between September 2016 and June 2017. Of the 50 cases of pulmonary tuberculosis with smear-positive results, 49 samples were confirmed positive by the Gene-Xpert MTB/RIF test and by cultivation (MTB). In contrast, of the 185 cases with pre-summative negative smear results, Gene-Xpert MTB/RIF test identified 21 positive cases. These results were confirmed by MTB cultivation. Compared with the traditional method of detecting acid-fast bacilli in sputum, the Gene-Xpert MTB/RIF test showed higher sensitivity and specificity with almost complete accuracy. This research highlights the importance of using the Gene-Xpert MTB/RIF test in the rapid and accurate diagnosis of pulmonary tuberculosis, thereby contributing to the reduction of underdiagnosis and the initiation of early treatment for this disease [
84,
85].
Also, research from the comparative study] analyzed the methods of rapid diagnosis of tuberculosis recurrence. Detection of recurrence can be challenging, given that M. tuberculosis specific DNA can be persistently present in sputum and bronchopulmonary samples, even when the disease is not active [
86]. Further development of molecular tests included lowering the threshold of detection (via Xpert MTB/RIF Ultra technology) and additional gene analysis associated with resistance, resulting in significant improvement in diagnosis tuberculosis including in children [
87,
88].
In a study, the diagnostic accuracy of the detection of M. tuberculosis-specific DNA by either the Gene-Xpert MTB/RIF technique and the M. tuberculosis-specific ELISPOT method in bronchoalveolar lavage (BAL) samples was compared with the results of M. tuberculosis culture from sputum or bronchopulmonary samples in patients with suspected recurrence of pulmonary tuberculosis. Among the 44 patients with a history of tuberculosis and suspected recurrence of the disease, only 4 of them (9.1%) were confirmed to have recurrent tuberculosis by the culture method. As for the Gene-Xpert MTB/RIF method, it was able to detect M. tuberculosis DNA in bronchoalveolar lavage in 1 of 4 patients with recurrence (25%), as well as in 2 of 40 patients (5%) with previous tuberculosis without recurrence. In contrast, the BAL-ELISPOT technique, using a threshold of >4,000 target-specific early antigenic lymphocytes 6 or culture-filtered protein-specific interferon γ 10, provided positive results for all 4 patients with recurrence (100%) and for 2 of 40 patients (5%) with previous TB without recurrence [
89].
In a study carried out at the Department of Bacteriology, Mohammed V Military Teaching Hospital / Faculty of Medicine and Pharmacy University Mohamed V, Rabat, a total of 714 samples were examined, 285 were taken from the lungs, and 429 from other areas of the body. The diagnostic methods used, microscopic examination (ZN), GeneXpert MTB/RIF and bacterial culture, had variable infection detection rates, depending on the type of samples (pulmonary or extrapulmonary). Positive detection rates for microscopy, ZN, GeneXpert MTB/RIF and culture were 12.88%, 20.59% and 15.82%, respectively. In detail, for lung samples, these rates were 18.9%, 23.85% and 20.35%, and for extrapulmonary samples, they were 9.71%, 18.41% and 12.82%. The GeneXpert MTB/RIF assay showed close sensitivity and specificity in pulmonary (78.2% and 90.4%) and extrapulmonary (79.3% and 90.3%) samples [
79].
Despite the progress made, challenges are still encountered in efforts to develop more accurate methods for the diagnosis of tuberculosis. More than 50 diagnostic tests are currently in development, but rigorous evaluation of the entire diagnostic process faces difficulties, such as the absence of quality control reagents. It is crucial to make progress in the development of appropriate phenotypic testing methods and robust quality assurance systems [
90].
The collection and transport of M. tuberculosis specimens continues to be a challenge in settings where tuberculosis is widespread and the necessary infrastructure to maintain specimen integrity is lacking. A study addressed this issue, and PrimeStore Molecular Transport Medium (MTM) was developed, which not only rapidly inactivates M. tuberculosis, but also preserves genomic DNA under high temperature conditions, thus facilitating subsequent molecular analysis, which provides information essential for a correct diagnosis and adequate treatment of tuberculosis patients [
91].Molecular testing by Xpert MTB/RIF for tuberculosis could bring significant savings to health systems in high-income countries by decreasing the need for patient isolation and the total length of hospitalization [
92].
One of the essential criteria for performance evaluation is participation laboratories to theExternal Quality Assessment (EQA) for the Xpert MTB/RIF test, which it is integrated into the quality assurance system necessary for clinical practice and laboratory. This system includes the pre-analytical, analytical and post-analytical processes, with the aim of guaranteeing the continuous quality of the tests [
93].
4.3. Loop-Mediated Isothermal Amplification (LAMP) technology
Other molecular techniques include ligase chain amplification, for identifying drug resistance mutations, and LAMP, which is faster and less expensive than PCR [
94,
95].
LAMP is a fast and simplified NAAT platform developed by Eiken Chemical Co., Ltd(Tokyo, Japan). The technology uses four different primers specifically designed to recognize six distinct regions of the target gene, and the reaction process takes place at a constant (isothermal) temperature using the strand displacement reaction. A simplified amplification test technique was used in one study loop mediated isotherm (LAMP). The procedure was performed on a semisolid gel of polyacrylamide of dimensions 6 × 8, using a prototype deviceaccessible from the point of financial view. Each serving on the gel contains a small amount of only 670 nanoliters, thus reducing the need for large amounts of chemicals. DNA amplified is identified by means of the fluorescence of the LCGreen Plus+ dye, embedded in the gel along with other reagents [
96].
Amplification and detection of gene products can be completed in one step by incubating the mixture of samples, primers, DNA polymerase with strand displacement activity and substrates at a constant temperature. Amplification efficiency is high, and DNA can be amplified 10^9-10^10 times in 15-60 min. Due to its high specificity, the presence of the amplified product can indicate the presence of the target gene. Currently, there is limited evidence regarding the accuracy of LAMP for TB detection.
A study conducted in Gambia, compared various methods of detecting tuberculosis using sputum samples, both in patients with symptoms suggestive of TB and in patients confirmed with this disease. The loop-mediated amplification assay for TB (TB-LAMP) was evaluated in comparison with other techniques such as smear microscopy with ZN, MGIT culture and GeneXpert MTB/RIF. The reference standard was culture. TB-LAMP showed an overall sensitivity of 99% and a specificity of 94%. In the latent class analysis, TB-LAMP had a sensitivity of 98.6% and a specificity of 99%, while GeneXpert had the highest sensitivity (99.1%) but the lowest specificity (96 %). Both TB-LAMP and GeneXpert showed high sensitivity and specificity in detecting TB, regardless of age or strain of infection. These findings underline the utility of both methods, GeneXpert and TB-LAMP, in the diagnosis of TB. However, although TB-LAMP requires less infrastructure, it cannot detect drug-resistant strains, making it more suitable for initial testing of new TB cases in medical clinics [
97].
However, widespread implementation of these techniques in developing countries is still limited by lack of infrastructure, high costs, and lack of skilled personnel. In addition, the need for adequate transport and storage of samples is another important challenge.
Another study investigated the effectiveness of parallel tests for the diagnosis of pulmonary tuberculosis in patients whose smear results were negative. In this study, 258 patients were included, and different testing methods, including culture, GenXpert MTB/RIF, and SAT-TB, were compared. The results revealed that the use of parallel tests resulted in significantly higher sensitivity compared to individual testing. The parallel testing model demonstrated a significant improvement in diagnostic efficacy for smear-negative PTB. Thus, this method should be considered in clinical practice when PTB is suspected but smear results are negative [
98].
The same LAMP is rapid assay to prove rifampicin and isoniazid resistance of TB Isolates [
99].