Abstract: Objective To determine when the susceptibility of the colony forming on the slants performed by absolute concentration method the cause analysis of the inconsistency in the outcomes of drug susceptibility test for Mycobacterium tuberculosis by different report time. Methods  Thirteen isolates on the INH containing medium from 11 patients with colony forming after six weeks were collected and the strains on the drug free L-J medium were also collected.All 24 collected strains were performed PCR species identification, MIRU and spoligotying gene typing, drug susceptibility tests (DST), INH resistant gene and randomly selected 3 strains （7019H0.2,7285H0.2 and 7325H1）to do DST for Simulation mixed population of INH susceptible and resistant strains.Observe the  results in differert report time (4 and 6 weeks). All the patients were recived telephone follow-up about treatment and drug resistance results. Results  All 13 isolates from INH contain medium were INH resistant when the proportion method was performed after subculture while all 11 strains from drug free medium were all INH susceptible.All the 13 isolates from INH containing medium and 11 isolates from control medium were identified as Mycobacterium tuberculosis complex (MtbC). Of 13 isolates from INH resistant strains, 10 strains (10/13,76.92%)had mutation in the katG gene (S315T). Among the 11 patients, 3 patients were proved to be INH resistant in the following DST.Three patients owned the different genotype between the drug containing medium and control medium.The genetype were Beijing and T2,Beijing and H3, Beijing and T2.Three strains mixed with H37Rv grown after 6 weeks instead of 4 weeks when the proportion was less than 1;128 (clinical isolate:H37Rv). Conclusion Case of mixed infection of different drug sensitivity caused by Mycobacterium tuberculosis, when the proportion of resistant strains is low,it may cause inconsistent results in different report time.

Key words: Mycobacterium tuberculosis, Microbial sensitivity tests, Uncertainty