Test Catalog

Test ID: TB1LN    
Antimicrobial Susceptibility, Mycobacterium tuberculosis Complex, First Line, Varies

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 M tuberculosis infections


Confirming the emergence of drug resistance


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

Testing Algorithm Delineates situations when tests are added to the initial order. This includes reflex and additional tests.

When this test is ordered, the additional test will always be performed and charged separately.

If resistance to a first line antimicrobial agent is detected reflex testing for confirmation of resistance and second line agents will be performed and charged.

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 M 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 M 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 Mm tuberculosis complex susceptibility testing results. Current recommendations indicate that laboratories should use a rapid broth method in order to obtain M 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 M tuberculosis complex and assesses resistance to antimycobacterial drugs at the critical concentrations.

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.

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.

Clinical Reference Recommendations for 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

Special Instructions Library of PDFs including pertinent information and forms related to the test