Half with Xpert Ultra ‘Trace’ go on to get TB within a year

New molecular tests are changing how tuberculosis (TB) is found, but they also introduce new puzzles for doctors. Xpert Ultra is a highly sensitive test that can detect very small amounts of TB DNA in sputum. When the test returns a result labeled “Trace,” it means DNA was detected at extremely low quantities, and it can be hard to tell whether that indicates active disease or only a tiny leftover signal. This uncertainty makes clinical decisions and the evaluation of new diagnostic ideas more complicated. To better understand what a Trace result means for patients, Ronit R. Dalmat and colleagues carried out a multi-site observational study in ambulatory clinics in South Africa and Uganda. The team enrolled people aged 15 and older who had a sputum Xpert Ultra Trace result and had not yet started TB treatment. Each participant received a full clinical assessment and repeated standard sputum testing. When clinicians could not be certain about active TB after evaluation, they deferred treatment and the team followed those people regularly to see whether they were later diagnosed with TB, for up to 24 months.

The study followed 311 participants who had a sputum Xpert Ultra Trace result; half were male, 57% were PLHIV, and 37% had been treated for TB within the prior five years. Participants underwent repeated sputum testing including culture and sputum Xpert, and some had tongue swabs and chest x-rays as part of the evaluation. Clinicians deferred treatment when TB status remained uncertain after the initial workup, and untreated participants were followed until TB diagnosis or treatment started. Within twelve months, 24% of participants were diagnosed by culture, 37% were positive by sputum Xpert or culture, and 54% were diagnosed by culture or by clinical diagnosis. When the researchers excluded people who were diagnosed with TB at the initial visit, three baseline findings predicted higher risk during follow-up: recent TB history (hazard ratio 2.6), abnormal chest x-ray (hazard ratio 2.4), and a positive tongue swab (hazard ratio 4.5). The risk of being diagnosed was highest in the first three months after a negative baseline evaluation (0.22 per person-month, 95% CI 0.19–0.26) and then fell to 0.01 per person-month (95% CI 0–0.02) in both the 3–6 and 6–12 month intervals.

These findings show that a sputum Xpert Ultra Trace result is not harmless: about half of people with that result were diagnosed with TB within a year, and many were diagnosed within the first three months. Even after an initial clinical evaluation that did not trigger treatment, the risk of later diagnosis stayed higher than typical background TB incidence for the population during follow-up. The study highlights that Trace results create a window of uncertainty in which close monitoring can catch disease that becomes apparent soon afterward. For clinicians and health systems using Xpert Ultra, the message is clear: a Trace result should prompt careful follow-up, particularly when the patient has a recent TB history, an abnormal chest x-ray, or a positive tongue swab. These observations also underline how Trace results affect the assessment of new diagnostic strategies, because very low-level detections may predict later disease rather than being irrelevant noise.