Brief Description

Antimicrobial susceptibility testing (AST) determines the ability of the four primary antitubercular drugs to inhibit the growth (in vitro) of Mycobacterium tuberculosis complex (MTBC) in a broth macrodilution matrix which utilizes critical concentrations of drug tested. AST is important to the proper treatment and management of patients with tuberculosis (TB). AST guides the physician in the selection of the most appropriate drug treatment regimen and alerts the physician when drug resistance may be a consideration in patient management.

Laboratory Unit



Broth macrodilution using BACTEC™ MGIT™ system

Specimen Collection

Viable isolate of MTBC in pure culture of solid or liquid media

Mixed or non-viable MTBC organism is acceptable for molecular-based AST; however, growth-based AST may not be available.

See “Molecular Detection of Drug Resistance (MDDR) for Mycobacterium tuberculosis complex” testing for molecular-based AST.


Submit isolates using the tuberculosis (TB) Category A carrier provided by the MSPHL in accordance with the Infectious Substance, Category A shipping guidelines.

Acceptable Specimen Type(s)

Submit a slant (L-J, Middlebrook, or Mitchison) or liquid broth culture (MGIT, VersaTREK, or MP).

Test Request Form(s)

Possible Results


Reference Range

Isoniazid (0.1 & 0.4 mcg/mL)
Rifampin (1.0 mcg/mL)
Ethambutol (5.0 mcg/mL)
Pyrazinamide (100 mcg/mL)

Unacceptable Conditions

No name or patient identifier on specimen
Specimen leaked during transport to the laboratory
Mixed or non-viable organism

Interfering Substances


Result Reported

4 - 17 days after MTBC identified



CPT Codes

87188 x5



Additional Information

MTBC showing resistances to any drug are referred to the Centers for Disease Control and Prevention (CDC) for second-line AST.

19 CSR 20-20.080 Duties of Laboratories

"Isolates or specimens positive for [M. tuberculosis complex] must be submitted to the State Public Health Laboratory for epidemiological or confirmation purposes"