Accurate, rapid, and cost-effective screening tests recommended by WHO are the need of the hour to meet goals for the End TB strategy. Here is a rollout of current available diagnostics for TB, especially the Xpert MTB/RIF, the only WHO-recommended test for detecting TB and drug resistance in low level health systems.

Significant improvement in the understanding of molecular biology of Mycobacterium tuberculosis has led to the development of newer diagnostic techniques of tuberculosis yet the gap between global estimates of incidence and new case notifications is 4:1.

While new point-of-care tests may take a while to be available, public health systems should adopt and optimise current WHO-recommended screening tests to improve case detection. Here is a rollout of the current diagnostics that should be made available for tuberculosis (TB).

Until newer rapid technologies become available, public health systems have to use the current WHO-recommended diagnostic techniques by accelerating and optimising their use across the tiered network of TB laboratories for the detection of the disease and resistant M. tuberculosis isolates.

Rapid TB diagnostics and drug susceptibility testing (DST) 

The current recommendations of WHO states that rapid TB diagnostics and drug susceptibility testing (DST) should not only be reserved for higher risk patients of MDR-TB and-or HIV associated TB but they should be offered to everyone with TB symptoms and signs. WHO divides universal access to DST into rapid DST for rifampicin and further testing for fluoroquinolones and second-line injectable agents in patients exhibiting rifampicin resistance. 

Phenotypic methods

The ability to grow M. tuberculosis in culture media that contains a critical concentration of specific anti-TB agents helps to check for their resistance or susceptibility. First-line agents (isoniazid and rifampicin) and chosen second-line anti-TB drugs (kanamycin, amikacin, ofloxacin, levofloxacin) are used in phenotypic DST and these have proven to be dependable and consistent in various scenarios.

WHO now recommends under certain conditions newer drugs, bedaquiline and delamanid be added to the core MDR-TB regimen. For the shorter standardized MDR-TB regimen, linezolid and clofazimine are also being recalled. Rifampicin and pyrazinamide are being recognized for the ability to accurately detect resistance-conferring mutations. The precision of sequencing of different genes involved with drug resistance are being reviewed by WHO in order to be able to recommend the use of genotypic DST methods.

Molecular (genotypic) methods

As per WHO recommendations, as specific DNA mutations cause resistance to specific anti-TB drugs in the M. tuberculosis genome, molecular (genotypic) methods are being used since 2008 to detect drug resistance to rifampicin alone or in combination with isoniazid. The Xpert MTB/RIF assay (Cepheid, Sunnyvale, CA, USA) and two commercial line probe assays (LPAs), the MTBDR plus assay (Hain Lifescience, Nehren, Germany) and the Nipro NTM + MDRTB detection kit 2 (Nipro Corporation, Tokyo, Japan) are examples of such tests.

Xpert MTB/RIF assay

The Xpert MTB/RIF has been suggested by WHO as a replacement for the initial diagnostic microscopy and culture for all patients with symptoms and signs of TB. Challenges seen with the Xpert MTB/RIF include funding, quality assurance, training and maintenance of the instrument. A complete implementation plan as well as a steady and continuous electrical supply is also required for the Xpert MTB/RIF to serve as a diagnostic option across the different healthcare levels.

Cepheid, USA is developing a newer platform called the GeneXpert Omni, which is expected to be more compact, lighter and more economical and ideal for point-of-care nucleic acid detection to fill the diagnostic gap at lower levels of the laboratory network. A built-in 4-h battery will power the GeneXpert Omni and the option of a 12 h of run time auxiliary battery also available.

For sputum smear–positive specimens (direct testing), and in cultured isolates of MTBC, the WHO recommends commercial LPAs for detecting rifampicin and isoniazid resistance. Earlier and proper treatment is possible with the earlier detection of drug susceptibility with the use of LPAs which can materialize into better health outcomes. However, in sputum smear–negative specimens, accuracy is lowered.

For patients with confirmed rifampicin-resistant TB or MDR-TB, instead of using phenotypic culture-based DST, the WHO has recommended that technicians use second-line LPAs (SL-LPAs) as the initial test for the detection of resistance to fluoroquinolones and second-line injectable drugs (SLID). Second-line LPAs are very accurate in detecting fluoroquinolone or SLID resistance which brings confidence in treating MDR-TB patients without these drugs.

Phenotypic culture- based DST may still be required if the second-line LPAs results are negative and we are in a setting of pre-test high fluoroquinolones and/or SLIDs resistance probability. The Xpert MTB/RIF can simultaneously detect TB and rifampicin resistance and is the only WHO recommended diagnostic test suitable for use at lower tiers of the laboratory network. Up to 48 samples can be processed simultaneously using the high-throughput technology of LPA.

Tiered TB laboratory network diagnostic techniques

The national TB control programme (NTP) may assign and manage a national TB reference laboratory (NRL) for diagnostic testing. When they work together, this can ensure good coordination between them so that priorities, strategies and activities can be synergized. Presently, less than 1 in 4 patients are diagnosed with DR-TB since most national TB programs do not presently offer DST to all patients with signs and symptoms of TB.

Perspectives for new tools for the diagnosis of DR-TB

Stronger political pledge, together with coordinated health system efforts, as per our country needs is required to overcome the challenges of the slow scale-up of DR-TB detection technologies. Better monetary allocation, faster policy implementation of TB diagnosis, qualified human resources, better laboratory and health system infrastructure, proper equipment maintenance and bio-safety are all factors which need to be addressed. Newer advanced nucleic acid amplification tests will pave the way forward.

Microarray-based multiplexing diagnostic platforms and next-generation sequencing for the simultaneous detection of numerous resistance-conferring mutations will be possible with these. It is vital that product developers align the needs of the end users as per certain high priority target product profiles (TPPs). Non-sputum-based biomarker tests and next-generation microscopic DST for diagnosing and managing TB are urgent priorities that may become game changers.

Reference: Current tools available for the diagnosis of drug-resistant tuberculosis. C Gilpin, A. Korobitsyn, and K. Weyer. Ther Adv Infectious Dis. 2016, Vol. 3(6) 145 –151