From the Research
CB NAAT is not typically used for blood specimens to detect tuberculosis because TB is primarily a pulmonary disease with low levels of mycobacteria in the bloodstream, and the test has reduced sensitivity in paucibacillary specimens, as shown in a study published in 2024 1. The test was specifically designed and validated for respiratory specimens like sputum, where bacterial load is higher. When TB bacteria enter the bloodstream (extrapulmonary TB), they are present in very low concentrations, often below the detection threshold of CB NAAT. The limitations of CB NAAT include:
- Reduced sensitivity in paucibacillary specimens (those with low bacterial counts), which makes blood testing unreliable
- Presence of inhibitors in blood that can interfere with the amplification process
- Limited ability to detect drug resistance beyond rifampicin
- Inability to differentiate between live and dead bacteria (potentially causing false positives in treated patients)
- Higher costs compared to conventional microscopy For suspected TB bacteremia, blood culture remains the preferred diagnostic method despite its longer turnaround time, as it can detect the small numbers of mycobacteria that may be present in blood, as supported by a study published in 2005 2. However, a more recent study published in 2023 3 highlights the usefulness of CBNAAT in the early diagnosis of extrapulmonary TB cases, but this does not apply to blood specimens. It is essential to consider the most recent and highest-quality study, which in this case is 1, to guide clinical decision-making and ensure the best possible outcomes in terms of morbidity, mortality, and quality of life.