Prevalence and interpretation of Xpert ® Ultra trace results among presumptive TB patients.

Background: The "trace call" results on Xpert® Ultra indicates extremely low TB levels and may be difficult to interpret. The prevalence of trace results among presumptive TB patients in high TB-HIV infection settings is unknown, as is the significance of divergent "trace call" result interpretations.
Methods: Presumptive TB patients attending a public health facility in Lusaka, Zambia, were prospectively enrolled. Participants underwent several TB investigations, including sputum smear microscopy, Ultra testing, and culture. The diagnostic accuracy of Ultra (culture-based reference) and the number of patients recommended for TB treatment was assessed according to several different interpretation criteria for "trace call" results.
Results: Among the 740 participants, 78 (10.5%) were Ultra-positive and an additional 37 (5.0%) had a "trace call" result. The prevalence of trace results did not differ according to HIV status (5.3% vs. 4.8%) or prior TB status (5.6% vs. 4.9%). Differing interpretations of trace results had modest effects on Ultra's sensitivity (range 79.3-82.6%) and specificity (range 94.3-99.2%), but increased the number of patients recommended for treatment by up to 44.9%.
Conclusions: Ultra trace results were common in this setting. The interpretation of trace results may substantially impact TB case yield.
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