Evaluation of the Simple One-Step (SOS) stool method for Truenat MTB plus and Xpert MTB/XDR assay

Stool is a recommended sample for the diagnosis of Mycobacterium tuberculosis (MTB) using Xpert MTB/RIF (Ultra) (GXU) in children, and the Simple One-Step (SOS) stool method is one of the recommended processing methods. We investigated modifications to the protocol of the SOS stool method to make it fit for testing stool with Truenat MTB Plus and MTB-RIF Dx (Truenat) and Xpert MTB/XDR (GXX). Experiments were conducted using stool spiked with different MTB concentrations and using modified versions of the SOS stool processing method. The established protocol was then validated on routine clinical stool samples from presumptive TB patients, comparing the performance of Truenat and GXX against GXU. The concordance for MTB detection between GXU and Truenat using 50, 100, and 150 mg of stool was 100%. Truenat did not provide valid results at 300 mg. In routine stools, concordance for MTB detection was 89.3% and 90.6% when comparing Truenat-100 mg to GXU-100 and 600 mg, respectively. For rifampicin detection, 44.8% of results were indeterminate on Truenat, but none on GXU (except Trace calls). GXX stool testing gave valid results, although for low bacterial loads, GXX less often detected MTB. We conclude that stool can be tested on Truenat to detect MTB using the SOS stool Xpert method adapted to the test composition of Truenat. In addition, the SOS protocol, as originally developed for GXU, can be used for GXX. This provides opportunities to expand access to rapid molecular testing to detect TB and resistance to first- and second-line anti-TB drugs for children and adults who cannot produce sputum.IMPORTANCEFollowing World Health Organization recommendations, countries commenced stool testing on GeneXpert as the primary test to improve tuberculosis (TB) case finding among children who cannot produce sputum. To further expand access to rapid molecular testing, we adapted the Simple One-Step (SOS) stool method for GeneXpert to the test kit composition of the Truenat platform through a series of laboratory experiments. Truenat is being introduced on a large scale by countries for near point-of-care testing as it can operate better in more remote settings. As part of our experiments, we also concluded that stool can be tested with the Xpert MTB/XDR cartridge using the original SOS protocol. Our findings further expand access to rapid molecular testing to detect TB and resistance to diverse anti-TB drugs for children and adults who cannot produce sputum. Increased access to TB testing in these vulnerable populations will support TB case finding efforts and timely and appropriate treatment.