Diagnosing Tuberculosis: What Do New Technologies Allow Us to (Not) Do?

Abstract

New tuberculosis (TB) diagnostics are at a crossroads: their development, evaluation, and implementation is severely damaged by resource diversion due to COVID-19. Yet several technologies, especially those with potential for non-invasive non-sputum-based testing, hold promise for efficiently triaging and rapidly confirming TB near point-of-care. Such tests are, however, progressing through the pipeline slowly and will take years to reach patients and health workers. Compellingly, such tests will create new opportunities for difficult-to-diagnose populations, including primary care attendees (all-comers in high burden settings irrespective of reason for presentation) and community members (with early stage disease or risk factors like HIV), many of whom cannot easily produce sputum. Critically, all upcoming technologies have limitations that implementers and health workers need to be cognizant of to ensure optimal deployment without undermining confidence in a technology that still offers improvements over the status quo. In this state-of-the-art review, we critically appraise such technologies for active pulmonary TB diagnosis. We highlight strengths, limitations, outstanding research questions, and how current and future tests could be used in the presence of these limitations and uncertainties. Among triage tests, CRP (for which commercial near point-of-care devices exist) and computer-aided detection software with digital chest X-ray hold promise, together with late-stage blood-based assays that detect host and/or microbial biomarkers; however, aside from a handful of prototypes, the latter category has a shortage of promising late-stage alternatives. Furthermore, positive results from new triage tests may have utility in people without TB; however, their utility for informing diagnostic pathways for other diseases is under-researched (most sick people tested for TB do not have TB). For confirmatory tests, few true point-of-care options will be available soon; however, combining novel approaches like tongue swabs with established tests like Ultra have short-term promise but first require optimizations to specimen collection and processing procedures. Concerningly, no technologies yet have compelling evidence of meeting the World Health Organization optimal target product profile performance criteria, especially for important operational criteria crucial for field deployment. This is alarming as the target product profile criteria are themselves almost a decade old and require urgent revision, especially to cater for technologies made prominent by the COVID-19 diagnostic response (e.g., at-home testing and connectivity solutions). Throughout the review, we underscore the importance of how target populations and settings affect test performance and how the criteria by which these tests should be judged vary by use case, including in active case finding. Lastly, we advocate for health workers and researchers to themselves be vocal proponents of the uptake of both new tests and those - already available tests that remain suboptimally utilized.

Keywords: Diagnosis; Non-invasive testing; Rapid tests; Tuberculosis.

Fig. 1

Selected minimal and optimal characteristics of the WHO target product profiles for TB triage and confirmatory tests. Current and future technologies should be benchmarked against these criteria, which are needed to advance the diagnostic status quo. Although serious attempts to meet the TPP criteria are underway, these criteria are aspirational, and, if a particular test or technology falls short in some domains it should not be discarded. Importantly, these criteria are almost a decade old, and, although they remain key to guide developers, they urgently require update to include lessons learnt from the rapid development and scale-up of SARS-CoV-2 detection technologies. Adapted from the WHO [17].

Fig. 2

Selected current and future triage and confirmatory tests according to specimen type (green, sputum; red, blood; yellow, urine; blue, other). Black shading on the outer ring indicates WHO-endorsed; grey indicates on pathway to the WHO review; light grey indicates still at early developmental stages. Image sources include https://www.fujifilm.com/products/medical/digital_radiography/fdr_xair/, https://www.delft.care/cad4tb/, https://www.fujifilm.com/products/medical/data/Lunit_INSIGHT_CXR_Medical_White_Paper.pdf, http://www.stoptb.org/wg/new_diagnostics/assets/documents/TB%20diagnostics%20pipeline%20needs%20and%20solutions.pdf).