Test Catalog

Interpretive Handbook

Test 34550 :
Susceptibility, Mycobacterium tuberculosis Complex, Second Line

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

The Clinical and Laboratory Standards Institute (CLSI) provides a consensus protocol for the methods, antimycobacterial agents, and critical concentrations of each agent to be tested in order to permit standardized interpretation of Mycobacterium tuberculosis complex susceptibility testing results. CLSI guidelines suggest that second-line agents should be tested when an isolate of Mycobacterium tuberculosis complex is resistant to rifampin, is monoresistant to the critical concentration of isoniazid and the physician intends to use a fluoroquinolone for therapy, or is resistant to any combination of 2 first-line agents.


This test uses a broth microdilution method for susceptibility testing of Mycobacterium tuberculosis complex against second-line agents. Agents tested are amikacin, cycloserine, ethionamide, kanamycin, moxifloxacin, ofloxacin, p-aminosalicylic acid, rifabutin, and streptomycin. In contrast to other Mycobacterium tuberculosis susceptibility methods which test 1 or 2 critical concentrations of a drug, this method examines a range of drug concentrations and produces an minimal inhibitory concentration result.

Useful For Suggests clinical disorders or settings where the test may be helpful

Determination of Mycobacterium tuberculosis complex minimal inhibitory concentrations to second-line antimicrobial agents

Interpretation Provides information to assist in interpretation of the test results

Results are reported as minimal inhibitory concentrations (MIC) in mcg/mL and tentative interpretations of susceptible or resistant are provided.



MIC Range Tested


MIC Tentative Interpretations (mcg/mL)(1)





< or =4.0




< or =32.0




< or =5.0

>5 .0



< or =5.0




< or =2.0




< or =2.0


Para-aminosalicylic acid


< or =2.0




< or =0.5




< or =2.0




     < or = 0.25




< or = 4




< or = 1


1. Laboratory-derived tentative interpretations based on MIC breakpoints established relative to the indirect agar proportion method; consensus breakpoint interpretations are not available at this time.(Hall L, Jude KP, Clark SL, et al: Evaluation of the Sensititre MycoTB plate for susceptibility testing of the Mycobacterium tuberculosis complex against first- and second-line agents. J Clin Microbiol 2012;50:3732-3734)


*This test is used as an alternative to TB1LN / Antimicrobial Susceptibility, Mycobacterium tuberculosis Complex, First Line when reagents are not available to perform the TB1LN test.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Consensus guidelines for interpretive criteria using this method are not available at this time. Breakpoints were established by Mayo Clinic by comparison to the critical concentration for each drug and are considered tentative until consensus guidelines are established.


In vitro susceptibility does not guarantee clinical response. Therefore, the decision to treat with a particular agent should not be based solely on the antimicrobial susceptibility testing result.


Drug susceptibility testing should be performed on pure culture isolates of Mycobacterium tuberculosis complex.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

Results are reported as minimal inhibitory concentration (MIC) values with units of mcg/mL and tentative interpretations of susceptible or resistant are provided.

Clinical References Provides recommendations for further in-depth reading of a clinical nature

1. Centers for Disease Control and Prevention. Treatment of Tuberculosis, American Thoracic Society, CDC, and Infectious Diseases Society of America. MMWR  2003;52(No. RR-11):1-79

2. Woods GL, Lin S-Y G, Desmond EP: Susceptibility test methods: Mycobacteria, Nocardia and other Actinomycetes. In Manual of Clinical Microbiology. 10th edition. Edited by J Versalovic, KC Carroll, G Funke, et al: Washington, DC, ASM Press, 2011, pp 1215-1238

Tell Us What You Think