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Drug
Concentration of drug
after reconstitution *
Volume added to
MGIT tube
Section III: Drug Susceptibility Testing
STR 83 µl/ml 100 ml 1.0 µl/ml
INH 8.3 µl/ml 100 ml 0.1 µl/ml
RIF 83 µl/ml 100 ml 1.0 µl/ml
EMB 415 µl/ml 100 ml 5.0 µl/ml
Final concentration in
MGIT tube
* The drugs should be reconstituted using 4 ml sterile deionized or distilled water to
achieve the indicated concentrations.
Calculations of the dilution factor for MGIT medium: 7.0 ml of medium + 0.8 ml of SIRE
Supplement + 0.5 ml of inoculum = 8.3 ml. Addition of 0.1 ml of the drug solution in 8.3 ml
of the medium = 1:83 dilution.
• Aseptically add 0.5 ml of the well-mixed culture suspension (inoculum) into each of
the drug containing tubes using a pipette. Do not add to the control.
• For the control, first dilute the test culture suspension 1:100 by adding 0.1 ml of the test
culture suspension to 10.0 ml of sterile saline. Mix well by inverting the tube 5-6
times. Use this diluted suspension to add 0.5 ml into the growth control tube.
• Tighten the caps and mix the inoculated broth well by gently inverting the tube
several times.
• Susceptibility test “Set Carriers” are provided in different numbers of drug
combinations. For a routine SIRE test with critical concentration, a Set Carrier of
five tubes is used (refer to BACTEC MGIT 960 User’s Manual for details). Place
labeled tubes in the correct sequence in the set carrier (GC, STR, INH, RIF, EMB).
• Enter the susceptibility set carrier into the BACTEC MGIT 960 instrument using the
susceptibility test set entry feature. (Refer to the BACTEC MGIT 960 User’s Manual,
AST Instructions.) Ensure that the order of the tubes in the AST Set Carrier conforms
to Set Carrier definitions. For example, GC, STR, INH, RIF, EMB for the SIRE
standard testing.
• If you need to check purity of the inoculum, streak the test culture suspension
onto a blood agar plate. If blood agar is not available, use chocolate agar or BHI
agar. Incubate at 35 ºC + 1ºC for 48 hours and check if there is any growth. If
growth appears, do not set up the susceptibility test. It may be important to establish
the purity of culture before setting up susceptibility test, particularly if contamination is
suspected.
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