Mycobacterium tuberculosis (MTB) remains one of the world's deadliest communicable diseases. In 2018, an estimated 10 million people developed tuberculosis (TB) globally; of these, more than 1.45 million died from the disease, 251,000 (17.3%) of whom were HIV-positive. Ethiopia has been classified among the 30 high TB, high TB/ human immunodeficiency virus (HIV), and high multidrug-resistant (MDR) TB (MDR-TB) burden countries (1). The causative agent, acid-fast bacilli (AFB), is obligate aerobic, so tuberculosis mainly affects the lung lobes where complete aeration is available (2).
Diagnostic methods of the disease continue to evolve. As of 2016, more than 50 diagnostic methods were developed for the diagnosis of MTB (3, 4). Although less accurate, the smear microscopy method using acid-fast stains and a light microscope is the most commonly used method in most high-burdened and resource-poor countries (5). The sensitivity of this method ranges from 32–94% (6). The sensitivity of fluorescence microscopy (FM) using auramine stain is 52–97% compared to light microscopy (5, 7). There were no significant differences in specificity between the Ziehl-Neelsen (ZN) and FM techniques (98.2% and 98.4%, respectively) (8).
Mycobacterium Growth Indicator Test (MGIT) 960 is a culture method that minimizes the time for MTB detection and drug resistance testing (9). However, this method is not accessible for high-burden countries such as Ethiopia since it requires sophisticated infrastructure and high technical skills. The sensitivity of GeneXpert for the diagnosis of pulmonary TB ranges from 51–81%, with an average of 62%, while the specificity ranges from 97–99%. In pulmonary specimens, sensitivities were 100% and 68.6% for smear-positive and smear-negative specimens, respectively (10). The aggregated sensitivity and specificity of GeneXpert to detect rifampicin resistance (RIF) were 93% and 98%, respectively (11). However, the cost per test is a challenge for the rollout of this method, especially in low-resource settings (12).
Abbott real-time PCR can release a batch of 93 test results in one run, which takes approximately 6–8 hours (13). For the diagnosis of MTB, the assay uses fluorescently labeled primers targeting protein antigen B (PAB) and insertion sequence 6110 (IS6110). IS6110 is a multicopy target to maximize the sensitivity and minimize false negativity due to target gene mutation or deletion (14). The Abbott MTB RIF and Isoniazid (INH) Resistance assay is performed separately or in reflex mode using the remaining eluents from Abbott real-time MTB (15).
The overall invalidity rate of Abbott real-time PCR is 1% when compared to the culture method, which is the gold standard, and the overall sensitivity is 93%. The assay sensitivity increased to 99% for smear positivity and 81% for smear negativity. In contrast, the test has 97% specificity for culture-positive MTB (15). Abbott real-time exhibited INH sensitivity and specificity of 88.3% and 94.3%, respectively. At the same time, RIF demonstrated a sensitivity of 95.8% and a specificity of 100% (16).
In view of the preceding, there is a need to introduce and implement rapid batch testing molecular methods in high-burden countries such as Ethiopia to improve case detection and management at an early stage of the disease. Therefore, this study evaluated the performance of Abbott real-time PCR to produce evidence for policymakers and stakeholders for informed and objective decision-making on the use of the technology for MTB diagnosis.