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Evaluation of a new molecular diagnostic tool for the identification of NTM/MTB in positive MIGIT cultures

S Caldrer(1) A Carrara(1) A Ragusa(1) A Donini(1) E Pomari(1) F Formenti(1) A Angheben(1) F Gobbi(1,2) F Perandin(1)

1:IRCCS Sacro Cuore - Don Calabria Hospital, Negrar di Valpolicella (Verona), Italy; 2:Department of Clinical and Experimental Sciences, University of Brescia, Brescia, Italy

To better control the spread of tuberculosis (TB) and Mycobacterium tuberculosis complex (MTB), WHO has recommended rapid molecular tests as the initial diagnostic step for tuberculosis. In recent years, the spreading of non-tuberculous mycobacteria (NTM) has also increased. In countries with low tuberculosis incidence such as Italy, the rapid differentiation between NTM and MTB is useful for mycobacterial disease management, to allow appropriate and timely therapeutic decisions. 

This study evaluates the diagnostic capability of a new rapid molecular assay (Standard M10 MTB/NTM, SD Biosensor), to detect the presence of MTB, NTM mycobacteria or co-infection (MTB/NTM), in positive liquid medium cultures (MIGIT,BD). The assay was validated using 100 positive and 50 negative MIGIT cultures, all confirmed by a commercial real-time PCR kit (MDR/MTB MGB Kit, ELITechGrop) and Sanger identification. 

After evaluation of the best dilution culture samples to use, and assessment of potential interference from co-occurring MTB/ NTM species, we confirmed excellent sample stability (up to 4 hours) after sample pre-treatment with the indicated reagent, highlighting the flexibility of this test for laboratory organization. In terms of specificity, the M10 MTB/NTM system showed excellent performance (all 50 negative samples were confirmed) and excellent sensitivity, identifying all 50 MTB and 48 on 50 NTM samples. Notably, one M. celatum positive culture was not identified and one M. fortuitum culture sample was identified as a co-infection with MTB. These two discordant cases will be investigated further. Moreover, the "early call" result, could reduce the diagnostic response time being useful for mycobacterial disease management.

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