Summary
Background
Systematic screening for tuberculosis using Xpert Ultra generates “trace” results of uncertain significance. Additional microbiological testing in this context is often negative, but untreated individuals might still progress to culture-positive disease. We aimed to estimate the two-year risk of tuberculosis among screening participants with trace-positive sputum.
Methods
We screened 31,505 people for tuberculosis in Uganda using sputum Xpert Ultra as an initial test, through event-based and door-to-door screening. We enrolled 128 participants with trace-positive sputum (PWTS), 139 Ultra-negative controls into a prospective cohort, and 110 Ultra-positive (>trace) controls for cross-sectional comparison. All participants underwent extensive initial evaluation, and untreated PWTS and negative controls were followed with re-testing for up to 24 months. We estimated cumulative hazards of tuberculosis among PWTS versus negative controls, using two definitions of tuberculosis: one incorporating clinician judgment (primary) and one based strictly on microbiological results (secondary). We then compared hazards between subgroups of PWTS.
Findings
Of 128 PWTS, 79 (62%) were male and 19 (15%) HIV-positive. Forty-five (35%) PWTS were recommended for treatment upon enrollment, eight lost to follow-up within three months, and 75 followed for median 706 (interquartile range 344-714) days, of whom 19 were recommended for treatment during follow-up. The cumulative hazard of tuberculosis among PWTS not treated at baseline was 0.24 (95% confidence interval: 0.15-0.40) at one year and 0.33 (0.21-0.54) at two years, versus 0.03 (0.01-0.10) at two years for negative controls. Hazards were similar for microbiologically defined tuberculosis (0.36 [0.22-0.58] at 2 years). Tuberculosis diagnosis during follow-up was strongly associated with abnormal baseline chest X-ray (hazard ratio 14.6 [3.3-63.8]) but not with baseline symptoms.
Interpretation
Individuals with trace-positive sputum during screening have a substantial two-year risk of tuberculosis, even when extensive initial evaluations do not confirm disease. Treatment should be considered for most screening participants with trace-positive sputum and abnormal chest imaging.
Funding
National Institutes of Health and Gates Foundation
Research in context
Evidence before this study
Recent advances in tuberculosis research have shifted the disease framework from a binary classification of latent versus active tuberculosis to a continuum of disease states. They have also led to a better understanding of the dynamic disease course of early tuberculosis, which can either progress to culture-positive disease or regress spontaneously over time. “Trace” results from Xpert MTB/RIF Ultra (“Ultra”) are sometimes perceived as false positives in individuals who subsequently test negative on additional diagnostic assays. However, some of these individuals may have early tuberculosis that falls below the detection threshold of existing diagnostic tests and could progress to microbiologically detectable disease over time. In screening contexts, trace-positive Ultra results constitute up to half of all positive results, and it is critical to understand how to interpret this result from screening participants, particularly among individuals without further evidence of tuberculosis on additional testing. To investigate this, we searched PubMed for studies published in English from inception to up to February 7, 2025, using the terms “tuberculosis” AND (“Xpert” OR “Xpert Ultra” OR “Ultra”) AND “Trace” and also reviewed the reference lists of relevant search results. In two prevalence surveys that each used a combination of symptom and chest X-ray as the screening test with Ultra as a confirmatory test, the proportion with positive cultures among those with trace-positive sputum ranged from 20% to 38%. In a study conducted in Uganda where Ultra was used as an initial screening test, only 14% of individuals with a trace-positive result had positive sputum cultures. However, prior studies were limited to a one-time evaluation of individuals following a trace result. None longitudinally followed these individuals after a negative initial work-up to assess their long-term risk of tuberculosis and its predictors, and appropriate management strategy for such individuals remains unknown.
Added value of this study
In this study, individuals with Ultra trace-positive screening results who were not started on treatment after extensive diagnostic testing were followed for up to two years with repeated testing. The 2-year cumulative hazard of tuberculosis disease was substantial at 0.33 (95% confidence interval 0.21-0.54), compared to 0.03 (0.01-0.10) among individuals with Ultra-negative screening results. Those who had a normal chest X-ray at enrollment were at significantly lower risk of developing tuberculosis. Two-year tuberculosis risk was similar between those who reported symptoms at the time of enrollment and those who did not.
Implications of all the available evidence
The high two-year risk of tuberculosis observed among people with trace results in this study, even when Ultra was used without any prior screening step, support provision of treatment for tuberculosis disease to most individuals who receive trace results during tuberculosis screening interventions. These results also demonstrate that X-ray could be a useful tool to guide treatment decision-making for individuals with trace-positive sputum.