Advances in TB Diagnostics: Portable Tests, Ultrasensitive DNA Detection, and Rapid Drug-Resistance Kits

Three recent developments in tuberculosis diagnostics are advancing the ability to detect the disease more quickly, accurately, and in settings where testing has traditionally been difficult or unavailable.

A study published in the New England Journal of Medicine reported that a portable device called the MiniDock MTB delivers laboratory-quality results in under thirty minutes, using either a sputum sample or a simple tongue swab. Researchers from UC San Francisco, UC Irvine, and Makerere University College of Health Sciences in Uganda enrolled nearly 1,400 patients across seven countries in Africa and Asia. The test met the World Health Organization's accuracy targets and proved easy enough to use that health workers learned the procedure from the printed instructions alone. The device is battery-powered and costs about $300, with each test running $3 to $4. It uses molecular technology to detect DNA from Mycobacterium tuberculosis. For patients who cannot produce sputum, including children, elderly people, and those living with HIV, the tongue swab option is transformative. In March, the World Health Organization issued its first formal recommendation for near-point-of-care nucleic acid amplification tests, along with guidance on tongue swab specimens and a sputum pooling strategy. The test performs best in patients with established infections and may miss early cases with low bacterial loads. It also cannot distinguish ordinary TB from drug-resistant strains, so a follow-up test is still needed to guide treatment.

Separately, researchers from the Postgraduate Institute of Medical Education and Research, Chandigarh; the National Institute of Tuberculosis and Respiratory Diseases, New Delhi; and others have developed a Quick DNA kit that significantly reduces the time required to test for multidrug-resistant tuberculosis. At the heart of this diagnostic breakthrough is the Trans-Filter, a membrane designed to filter out TB bacteria and turn hazardous biological samples into safe, shippable data. The researchers screened over 1,800 patients to test the new system. The process begins with a patient's sputum sample, which is liquefied and passed through the Trans-Filter device. The filters are sterilized and air-dried, locking the bacterial DNA in the filter paper. During testing, the membranes were stored at temperatures as high as 50°C (122°F) for up to four weeks with the bacterial DNA remaining preserved. Once the filter reaches a central laboratory, the Quick DNA kit uses heat lysis — heating the filter in a buffer solution at 80°C for five minutes — to release the bacterial DNA, which is then analyzed using a Line Probe Assay to detect mutations indicating resistance to antibiotics such as rifampicin or isoniazid. The kit reduces DNA extraction from a seven-step process taking nearly an hour to a single step taking five minutes. The study focused primarily on smear-positive samples with high bacterial concentration; further research may be needed to determine sensitivity for smear-negative patients. A small subset of samples involving a specific gene, inhA, showed lower sensitivity.

In a third line of research, a highly sensitive molecular test detected Mycobacterium tuberculosis DNA in a surprising proportion of hospitalized patients in the United States, according to a study published in Nature Communications. Using an ultrasensitive assay that can detect TB at levels below the limits of standard diagnostic tools, a team led by Boston University researchers tested a set of 146 anonymized respiratory samples collected at two Boston hospitals from May to September 2013 and 50 control samples collected from May to July 2014. They also conducted a longitudinal clinical study with 101 samples collected from February to June 2014. The Totally Optimized PCR TB assay found TB DNA in 12.3% of an initial set of samples, compared with 2% of control samples. In the follow-up cohort, 15.8% of samples were positive. An incidence rate of 12% to 16% is far higher than expected, given Boston's low rate of TB cases. Seventy-five percent of patients who tested positive for TB DNA were age 50 or older, fitting with US patterns in which TB is more common in older adults. Four young patients tested positive, three of whom shared a diagnosis of acute chest syndrome, a life-threatening complication of sickle cell disease. The researchers hypothesize the results may point to a previously unrecognized form of TB called paucibacillary TB disease, in which bacterial levels are too low to be detected by conventional tests. The findings may be evidence of the "iceberg principle," in which diagnosed cases represent the visible tip while many more cases remain hidden. The authors noted that the results require confirmation in larger prospective studies.