Xpert(®) MTB/RIF for detection of Mycobacterium tuberculosis from frozen string and induced sputum sediments

Although it is now widely used for tuberculosis (TB) diagnosis, Xpert(®) MTB/RIF availability remains inadequate in low-resource settings. Moreover, its accuracy in testing stored samples from non-expectorating patients has not been evaluated.

To assess the performance of Xpert in frozen samples of induced sputum (IS) and sputum from string test (ST) from non-expectorating individuals with presumed TB.

This was a laboratory-based study of 377 ST and IS samples collected between March 2010 and March 2013 at a referral hospital in Uganda. Samples were decontaminated, centrifuged and cultured, and the resultant samples were frozen at -20°C before Xpert evaluation.

TB was detected in ST and IS samples from 19/163 (11.7%) children and 63/201 (29.4%) adults using culture. Xpert sensitivity in frozen sediments from children was 37.5% (95%CI 8.5-75.5) in ST and 41.7% (95%CI 15.2-72.3) in IS samples, with specificities of respectively 100% (95%CI 94.9-100) and 98.6% (95%CI 92.7-100). In adults, sensitivity was respectively 50% (95%CI 31.3-68.7) and 48.5% (95%CI 30.8-66.4) in ST and IS samples, with specificities of respectively 100% (95%CI 95.5-100) and 98.6% (95%CI 92.4-100).

Given these results, and particularly the high specificity, the use of Xpert on frozen ST and IS sediment samples from both children and adults is promising.