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Section III: Drug Susceptibility Testing
A. Primary Drug Susceptibility Testing (SIRE)
1. Introduction
Antimicrobial susceptibility testing is critical in prescribing an effective drug regime for a
tuberculosis patient, especially in areas where drug resistance incidence is high. It is also
important in the follow-up of patients who are on antimicrobial therapy but are not
responding to therapy. Drug susceptibility testing is an integral part of the WHO DOTS-Plus
program.
The BACTEC MGIT 960 susceptibility testing for Streptomycin (S), Isoniazid (I), Rifampin
(R) and Ethambutol (E), called SIRE, is a rapid and qualitative procedure for establishing
susceptibility of M. tuberculosis for the four drugs using critical test concentrations. In
addition, higher test concentrations for S, I and E (high E not available in US) are also
available in case of testing against higher concentrations is indicated.
2. Principles of the test
Isolated cultures from TB patients are subjected to growth in the presence of a known
concentration of a test drug. A control is also included with no addition of the drug. If the
patient’s isolate grows in the control but does not grow in presence of the drug, it is
considered susceptible. On the other hand, if it grows in both the tubes, then it is considered
to be resistant to that drug.
There are several methods for susceptibility testing but the most common one is the
proportion method. In the proportion method, resistance is established at the 1% level for
most of the drugs. This means that if 1% or more of the total test bacterial population is
resistant to a drug it is considered as resistant for clinical purposes. Historically, proportion
method has used Middlebrook agar solid medium. After 3 to 4 weeks of incubation, the
percentage of colonies on the drug medium, as compared with the drug-free medium, is
calculated to establish resistance.
In 1980, a broth-based proportion method known as BACTEC 460TB radiometric
susceptibility testing was introduced. In this method, a radiometric medium, BACTEC 12B,
with a C 14 -labeled substrate is used. The bacterial inoculum in the control is a hundredfold
less than the inoculum in the drug-containing medium. The 14 CO2 produced during growth
and metabolism of mycobacteria in this medium is measured and designated as the Growth
Index (GI). Once the GI in the control reaches 30, usually after 4-6 days incubation
(maximum 12 days), comparison of GI values in the drug-containing and drug-free medium
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establishes the proportion of resistance.
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