Performance of the Xpert MTB/RIF Ultra Assay for Determining Cause of Death by TB in Tissue Samples Obtained by Minimally Invasive Autopsies.

This was an ancillary study to a large observational postmortem evaluation (CADMIA study) aimed at validating MIA against CDA for any CoD determination in different age groups in Maputo, Mozambique.3,5 Both the CDA and MIA pathological and microbiological methods have been comprehensively described elsewhere.8,9 In a previous analysis from CADMIA, TB-related lesions were extensively investigated in CDA samples.6 Microbiological methods include acid-fast bacilli smear and two molecular tools: in-house real-time polymerase chain reaction and Xpert Ultra following a pre-specified algorithm.6 CDA diagnosis was considered the gold standard for CoD determination in CADMIA.For this specific analysis, we selected MIA samples from the lung, CNS, cerebrospinal fluid (CSF), and plasma from all the study cases with any TB finding (TB disease or Mycobacterium TB DNA detected in CDA samples). This analysis included a total of 117 patients. In 31 patients, TB was the final CoD, 31 cases had TB disease at death but had died of another CoD, and in 18 cases DNA of M TB was detected but no histological lesions compatible with TB were found. In addition, we included a subset of 37 patients with no TB findings in the CDA and with availability of the four MIA samples.MIA samples were collected in tubes containing 1 mL lysis buffer (ATL buffer, Qiagen), and stored and processed by Xpert Ultra. Lung and CNS tissue samples were processed as described previously,6,10 and 0.5 mL plasma and CSF were mixed with the assay sample reagent buffer in a 1:3 ratio before testing. We determined the performance of Xpert Ultra for each MIA sample and for combinations of samples.We also estimated the number needed to misdiagnose (NNM) as: NNM 1/4 Total/(false positives thorn false negatives). The NNM is the number of patients who need to be tested for one patient to be misdiagnosed. Because specific MIA samples were not available for some patients with TB findings, a sensitivity analysis of the performance of Xpert Ultra in MIA samples was conducted among cases in which all four MIA samples were available.