Resistant TB, Detected Too Far From Care

Approximately 500,000 people develop drug-resistant tuberculosis each year, yet only one-third are diagnosed and enrolled on appropriate treatment. The gold-standard method — culture-based phenotypic drug susceptibility testing (DST) — takes 2-6 weeks, requires BSL-3 laboratory infrastructure, and is unavailable at the peripheral health facilities where most patients first present. GeneXpert MTB/RIF detects only rifampicin resistance, providing no guidance for selecting alternative regimens when resistance to isoniazid, fluoroquinolones, or newer drugs like bedaquiline is present. The result: most patients with drug-resistant TB are either never diagnosed or are treated with ineffective regimens for weeks before resistance is identified.

Drug-resistant TB kills approximately 150,000 people per year. MDR-TB treatment takes 9-18 months and costs 10-100x more than drug-susceptible TB treatment. Delayed or incorrect treatment drives ongoing transmission of resistant strains, creating a positive feedback loop. The WHO TPP identifies four priority drugs for peripheral DST — rifampicin, isoniazid, fluoroquinolones, and bedaquiline — because knowing resistance status for these four enables selection of the appropriate treatment regimen. No existing test covers all four at the peripheral level.

GeneXpert MTB/RIF (2010) was a breakthrough for rifampicin resistance detection but tests only one drug. GeneXpert MTB/XDR (2021) expands coverage to isoniazid, fluoroquinolones, and second-line injectables with ~93% sensitivity and 98% specificity for fluoroquinolone resistance, but runs on the same infrastructure-dependent platform. Whole genome sequencing offers comprehensive resistance profiling but costs >$100/test, requires specialized equipment, and has incomplete knowledge of resistance mutations for newer drugs. Line probe assays (Hain GenoType) cover first- and second-line drugs but require BSL-3 facilities and trained molecular biologists. Critically, none of these approaches delivers results fast enough for same-day treatment decisions: the WHO TPP optimal target is <30 minutes for detection plus DST.

Two approaches show promise: (1) microfluidic chip-based phenotypic DST, which has demonstrated growth-based resistance detection in 12 hours for seven drugs including bedaquiline and levofloxacin, and (2) targeted next-generation sequencing panels that could cover all priority resistance mutations on a portable platform. The WHO TPP sets the bar: instrument cost <$5,000 (optimal), time to result <30 minutes (optimal) or <6 hours (minimum), deployable at peripheral health centers. Bridging this gap requires simultaneous advances in microfluidics, sample processing, and cost reduction.

A team could prototype a microfluidic DST chip optimized for the four WHO priority drugs, using fluorescence-based readout of mycobacterial growth in drug-containing vs. drug-free chambers. The key engineering challenge is reducing incubation time from 12 hours to <6 while maintaining sensitivity. Alternatively, a team could develop a portable, low-cost targeted sequencing panel covering known resistance mutations for rifampicin, isoniazid, fluoroquinolones, and bedaquiline, benchmarking cost and turnaround against GeneXpert XDR. Relevant disciplines: microfluidics, biomedical engineering, molecular biology, bioinformatics.