Test Catalog

Interpretive Handbook

Test 35994 :
Antimicrobial Susceptibility, Mycobacterium tuberculosis Complex, First Line

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

Primary treatment regimens for Mycobacterium tuberculosis complex often include isoniazid, rifampin, ethambutol, and pyrazinamide. Susceptibility testing of each Mycobacterium tuberculosis complex isolate against these first-line antimycobacterial agents is a key component of patient management.


In vitro susceptibility testing methods are available to assess the susceptibility of Mycobacterium tuberculosis complex isolates to selected antimycobacterial agents. The Clinical Laboratory Standards Institute (CLSI) provides consensus protocols 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. Current recommendations indicate that laboratories should use a rapid broth method in order to obtain Mycobacterium tuberculosis susceptibility data as quickly as possible to help guide patient management. Resistance, as determined by rapid testing, must be confirmed by another method or by another laboratory.


This test uses an FDA-cleared commercial system for rapid broth susceptibility testing of Mycobacterium tuberculosis complex and assesses resistance to the following antimycobacterial drugs at the critical concentrations indicated: isoniazid (0.1 mcg/mL and 0.4 mcg/mL), rifampin (1 mcg/mL), and ethambutol (5 mcg/mL and 8 mcg/mL). A separate test is available for testing of the other first-line agent pyrazinamide.

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

Rapid, qualitative susceptibility testing of Mycobacterium tuberculosis complex isolates growing in pure culture


Affirming the initial choice of chemotherapy for Mycobacterium tuberculosis infections


Confirming the emergence of drug resistance


Guiding the choice of alternate agents for therapy for Mycobacterium tuberculosis infections

Interpretation Provides information to assist in interpretation of the test results

Mycobacterium tuberculosis complex isolates are reported as susceptible or resistant to the aforementioned drugs at the critical concentrations.


Some experts believe that patients infected with strains exhibiting resistance to low levels of isoniazid (0.1 mcg/mL) but not exhibiting resistance to high levels (0.4 mcg/mL) may benefit from continuing therapy with this agent. A specialist in the treatment of tuberculosis should be consulted concerning the appropriate therapeutic regimen and dosages.

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

For resistant organisms, confirmatory testing is automatically performed by minimum inhibitory concentration for isoniazid, rifampin, and ethambutol.

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 susceptible or resistant.

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

1. Blumberg HM, Burman WJ, Chaisson RE, et al: American Thoracic Society/Centers for Disease Control and Prevention/Infectious Diseases Society of America: treatment of tuberculosis. Am J Respir Crit Care Med 2003;167(4):603-662

2. CLSI: Susceptibility Testing of Mycobacteria, Nocardiae, and Other Aerobic Actinomycetes; Approved Standard. CLSI document M24-A (ISBN 1-56238-550-3). CLSI, Wayne, PA, 2003

3. Inderlied CB, Pfyffer GE: Susceptibility test methods: Mycobacteria. In Manual of Clinical Microbiology. Eighth edition. Edited by PR Murray, EJ Baron, JH Jorgensen, et al. Washington, DC, ASM Press, 2003, pp 1149-1177

4. LaBombardi VJ: Comparison of the ESP and BACTEC Systems for testing susceptibilities of Mycobacterium tuberculosis complex isolates to pyrazinamide. J Clin Microbiol 2002;40:2238-2239

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