The utility of Xpert MTB/RIF performed on bronchial washings obtained in patients with suspected pulmonary tuberculosis in a high prevalence setting

In this study, performed in a population with a high pre-test probability for pulmonary tuberculosis, we found Xpert MTB/RIF to have a sensitivity, specificity and negative predictive value of around 90 %, and a positive predictive value of approximately 80 %. Xpert MTB/RIF had a very high diagnostic efficiency of 89.3 %. The majority of Xpert MTB/RIF positive, culture negative results were observed in cases with very low positive Xpert MTB/RIF results and a past history of pulmonary tuberculosis.

The sensitivity observed in the present study is on par with sensitivities recently reported in two studies (ranging from 82–93 %), but the specificity is marginally lower than the reported 96–100 % [7, 8]. The studies reported by Lee et al. and Theron et al. included similar numbers of patients diagnosed with pulmonary tuberculosis, and utilised comparable diagnostic criteria for the diagnosis of pulmonary tuberculosis [7, 8].

We identified 9 cases of Xpert MTB/RIF positive culture, negative cases, of which three were treated for pulmonary tuberculosis. Current evidence suggests that pulmonary tuberculosis treatment within the preceding 5 years can potentially RIF/RIF [14, 15]. Up to 27 % of patients have been reported to remain sputum Xpert MTB/RIF positive 26 weeks after successful anti-tuberculous treatment was initiated [16, 17]. Due to the nature of the polymerase chain reaction test, Xpert MTB/RIF amplifies any DNA whether it originates from alive or dead bacilli. Therefore it cannot be assumed, solely on the basis of the test, that a positive result equates to active disease [12, 18]. It is also unclear whether the bacterial load in bronchoscopy samples in patients with latent infection or recent exposure would reach this limit and potentially result in a very low positive Xpert MTB/RIF test result. More evidence is needed on the interpretation of Xpert MTB/RIF in the setting of recently treated tuberculosis, which may be the most important cause of false positive results.

No cross-reactivity has been reported between Xpert MTB/RIF and multiple bacteria, viruses, fungi or mycobacteria species other than tuberculosis and should therefore not be the reason for false positive results [19, 20]. We observed a single case of false positive Xpert MTB/RIF in a patient with confirmed Mycobacterium avium intracellulare infection. It is likely that dual pathology (including undiagnosed previous tuberculosis) and laboratory contamination may have been present in this case, as well as the other cases not treated as pulmonary tuberculosis, a finding that has been reported in similar settings with a high HIV prevalence [21-23].

It should be noted that 7 of the 9 Xpert MTB/RIF positive culture negative cases had very low positive test results. Some correlation between Xpert MTB/RIF generated quantitative information and bacterial load and disease severity has been described [21, 22]. Xpert MTB/RIF’s calculated analytical limit of detection has been reported as 131 cfu/ml (95% CI 106.2–176.4). The relationship between Xpert MTB/RIF generated cycle threshold values, bacterial load and smear microscopy grades has shown that very low positive test results (cycle threshold values >28 cycles) correlate with negative microscopy and low positive values (22–28) with scanty positive smear microscopy [7, 21, 22]. This also correlated to the lower end of time-to-positivity of cultures. More research is required to aid in the interpretation of Xpert MTB/RIF with very low positive results, which in clinical practice, may be interpreted in the same light as a negative or, at best, scanty positive smear microscopy result. Our observations suggest that very low positive results require confirmation by culture or other means in cases with a low pre-test probability. Cut-off values for positivity should potentially vary between high and low endemic tuberculosis areas with lower cycle threshold values accepted as positive in high endemic areas. Moreover, clinicians practicing in high burden settings should be aware that past tuberculosis and dual pathology should be considered in cases with unexpected positive Xpert MTB/RIF.

The present study has several limitations. The retrospective nature limited the data collection to cases with complete clinical, radiological and microbiological data, and precluded any analyses on the excluded patient population. Some cases with false negative cultures may have gone undetected, and the true sensitivity may be different. We were also not able to document the true impact of Xpert MTB/RIF performed on bronchial lavages on the time to diagnosis or treatment, which is arguably of even greater clinical relevance.