Characterisation of Mycobacterium tuberculosis isolates lacking IS6110 in Viet Nam

SETTING: The molecular diagnosis of tuberculosis (TB) in Viet. Nam is often based on the detection of insertion sequence (IS) 6110 in Mycobacterium tuberculosis. However, 8-11% of M. tuberculosis strains in South-East Asia do not contain this target and this undermines the validity of these molecular tests. OBJECTIVE: We quantified the frequency of M. tuberculosis strains lacking IS6110 in rural Viet Nam and studied their epidemiological and clinical characteristics. DESIGN: Consecutively diagnosed adult TB patients in rural Southern Viet Nam submitted two sputum samples for culture, IS6110 restriction fragment length polymorphism (RFLP) spoligotyping and 15-loci variable number tandem repeat typing. Polymerase chain reaction (PCR) was performed to confirm the absence of IS6110 elements in strains lacking IS6110 hybridisation in RFLP. RESULTS: Among 2664 TB patient isolates examined, 109 (4.1%) had no IS6110 element. Compared to other strains, these no-copy strains were less often resistant to anti-tuberculosis drugs, particularly to streptomycin (adjusted OR 0.2, 95%CI 0.1-0.5), and showed significant geographic variation. No associations with TB history or demographic factors were found. CONCLUSIONS: Strains without the IS6110 target pose a problem in Viet Nam as regards false-negative molecular TB diagnosis in PCR. Compared to other strains circulating in Viet Nam, no-copy strains arc more susceptible to anti-tuberculosis drugs.