False-Positive Xpert MTB/RIF Results in Retested Patients with Previous Tuberculosis: Frequency, Profile, and Prospective Clinical Outcomes

Abstract

Globally, Xpert MTB/RIF (Xpert) is the most widely used PCR test for the diagnosis of tuberculosis (TB). Positive results in previously treated patients, which are due to old DNA or active disease, are a diagnostic dilemma. We prospectively retested sputum from 238 patients, irrespective of current symptoms, who were previously diagnosed to be Xpert positive and treated successfully. Patients who retested as Xpert positive and culture negative were exhaustively investigated (repeat culture, chest radiography, bronchoscopy with bronchoalveolar lavage, long-term clinical follow-up). We evaluated whether the duration since previous treatment completion, mycobacterial burden (the Xpert cycle threshold [ ] value), and reclassification of Xpert-positive results with a very low semiquantitation level to Xpert-negative results reduced the rate of false positivity. A total of 229/238 (96%) of patients were culture negative. Sixteen of 229 (7%) were Xpert positive a median of 11 months (interquartile range, 5 to 19 months) after treatment completion. The specificity was 93% (95% confidence interval [CI], 89 to 96%). Nine of 15 (40%) Xpert-positive, culture-negative patients reverted to Xpert negative after 2 to 3 months (1 patient declined further participation). Patients with false-positive Xpert results had a lower mycobacterial burden than patients with true-positive Xpert results ( , 28.7 [95% CI, 27.2 to 30.4] versus 17.6 [95% CI, 16.9 to 18.2]; < 0.001), an increased likelihood of a chest radiograph not compatible with active TB (5/15 patients versus 0/5 patients; = 0.026), and less-viscous sputum (15/16 patients versus 2/5 patients whose sputum was graded as mucoid or less; = 0.038). All patients who initially retested as Xpert positive and culture negative ("Xpert false positive") were clinically well without treatment after follow-up. The duration since the previous treatment poorly predicted false-positive results (a duration of ≤2 years identified only 66% of patients with false-positive results). Reclassifying Xpert-positive results with a very low semiquantitation level to Xpert negative improved the specificity (+3% [95% CI, +2 to +5%]) but reduced the sensitivity (-10% [95% CI, -4 to -15%]). Patients with previous TB retested with Xpert can have false-positive results and thus not require treatment. These data inform clinical practice by highlighting the challenges in interpreting Xpert-positive results, underscore the need for culture, and have implications for next-generation ultrasensitive tests.

Evaluation of the Xpert MTB/RIF Ultra Assay for Direct Detection of Mycobacterium tuberculosis Complex in Smear-Negative Extrapulmonary Samples

“…It is important to note that these results were obtained from a limited number and variety of samples, as well as a few NTM species. In other studies in which GX-Ultra was used, the authors indicated that the increase in sensitivity with this new version came at the expense of a decrease in specificity, specifically in patients with a history of tuberculosis (13,27) and patients with meningitis (17). They hypothesized that the presence of DNA from MTUBC or intact bacilli could explain the reduction in specificity.…”

Xpert MTB/RIF Ultra assay for the diagnosis of pulmonary tuberculosis in children: a multicentre comparative accuracy study

“…One child in the probable TB group might have been correctly identified as an active TB case only by Xpert Ultra. In a second child, who was classified as "not TB" but had been treated for TB three years ago, Xpert Ultra might have detected MTB DNA only as remnant of a previous active TB episode 14,24 . However, cross-contamination or sample mix-up cannot be excluded.…”

Retrospective Analysis of False-Positive and Disputed Rifampin Resistance Xpert MTB/RIF Assay Results in Clinical Samples from a Referral Hospital in Hunan, China

“…However, concerns about Xpert's specificity have been rising, as previous studies have reported false-positive errors in M. tuberculosis (11,12) and RIF resistance (13)(14)(15) detection in clinical practice. In some cases, false-positive RIF resistance results have been attributed to silent mutations within the RRDR of rpoB (13,14), or to a large cycle difference in the cycle threshold (C T ) value between any two probes (defined as ΔC T maximum values) (13,15).…”