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The BACTEC™ MGIT™ 960 SIRE Kit is a rapid qualitative procedure for susceptibility testing of Mycobacterium tuberculosis, from culture, to streptomycin (STR), isoniazid (INH), rifampin (RIF) and ethambutol (EMB). The BACTEC™ MGIT™ 960 STR 4.0 Kit and the BACTEC™ MGIT™ 960 INH 0.4 Kit are for testing at higher drug concentrations.

The BACTEC™ MGIT™ 960 susceptibility test kits are used with the BACTEC™ MGIT™ 960 and BACTEC™ MGIT™ 320 Systems.

Antimycobacterial susceptibility testing is valuable in the proper treatment of patients with tuberculosis. The treatment of tuberculosis is commonly through a multiple drug regimen that includes the antimycobacterial drugs streptomycin, isoniazid, rifampin and ethambutol. It is important that the antimycobacterial drugs prescribed show appropriate activity against Mycobacterium tuberculosis, i.e., susceptibility of the isolate to the drug.

Multi-drug resistant Mycobacterium tuberculosis (MDR-TB) has recently become a serious public health problem.¹ Resistance to any of the primary drugs, streptomycin (STR), isoniazid (INH), rifampin (RIF) and ethambutol (EMB), makes the disease more difficult and expensive to treat. The rapid detection of these resistant isolates is critical to effective patient management.

Two methods have been widely used for antimycobacterial susceptibility testing. The first method, known as the Method of Proportion,² uses Middlebrook and Cohn 7H10 Agar. It compares colony counts on drug-containing and drug-free media. Resistance to a drug is detected when 1% or more of the bacterial population is resistant to the drug concentration under test. Results are generally available after 21 days of incubation. The second method, known as the BACTEC™ 460TB radiometric susceptibility method,³ generally takes from 4 to 12 days. It is based on the production of radioactive ¹⁴C-labeled carbon dioxide by the growing mycobacteria, manifested by a Growth Index increase in the system.

Historically, the Method of Proportion (MOP) procedure has included susceptibility testing of M. tuberculosis using two concentrations of antimicrobials. The Clinical and Laboratory Standards Institute (CLSI, formerly NCCLS) continues to recommend that the MOP test procedure include two concentrations of the primary drugs for testing except rifampin. The recommended low concentrations for the MOP procedure are the critical concentrations for these drugs. The critical concentration is defined as the drug concentration that allows the interpretation of a result as either resistant or susceptible. An isolate is determined resistant if 1% or more of the test population grows in the presence of the critical concentration of the drug. The high drug concentration is used to profile the degree of resistance within the population. This result provides information to the physician to assist in determining whether a modification to the therapy regimen is necessary.

The BACTEC MGIT 960 SIRE test provides the susceptibility result in approximately the same time frame as the BACTEC 460TB System. In addition, this method is nonradiometric and allows appropriate susceptibility results to be reported earlier, in most cases, than the MOP procedure.

The BACTEC MGIT 960 SIRE test was developed with critical concentrations for streptomycin, isoniazid, rifampin and ethambutol that are slightly lower than the critical concentrations used in the MOP in order to avoid false susceptibility. This is most apparent for streptomycin where many isolates are close to the recommended critical concentration as performed by the MOP. For this reason, a second, higher drug concentration was developed for streptomycin and isoniazid. A susceptible result at the critical concentration can be reported and no other testing is necessary. Isolates that are resistant at the critical concentration of streptomycin, isoniazid and/or ethambutol, should be tested at a higher drug concentration either in the BACTEC MGIT system or using an alternate method. In this case, a final result of resistant at the critical concentration may be reported, with notification that an additional test at a higher concentration is being performed.

Testing of resistant isolates at a higher concentration is important to identify those that exhibit low-level resistance, i.e., resistant at the critical concentration and susceptible at the high concentration. The high concentrations in the BACTEC MGIT system were designed to be lower than the concentrations used in the MOP. This design of the BACTEC MGIT system is such that a resistant result, especially for streptomycin, may not always correlate to a resistant result at the high concentration in MOP. In the event that a streptomycin result is obtained that is resistant at the high concentration, an alternate method of testing at this concentration should be performed.

Upon receipt of a new shipment or lot number of BACTEC MGIT 960 SIRE Kit vials, it is recommended that the control organism shown below be tested. The control organism should be a pure culture and the culture should be prepared according to the "INOCULUM PREPARATION" instructions.

The quality control (QC) AST Set should be prepared according to the "Inoculation Procedure for Susceptibility Test" instructions for the drug kits being tested. Important considerations when preparing the QC AST Set are the proper reconstitution of the lyophilized drugs and the proper dilution of the QC organism for the Growth Control and drug tubes.

It is important to add the appropriate drug to the corresponding labeled tube. The use of the pan-sensitive QC organism will not detect incorrect drug pipetted into AST Set tubes.

Observation of the proper results, as shown below, within 4 – 13 days indicates that the BACTEC MGIT 960 SIRE Kits are ready for use in testing patient isolates. If the proper results are not observed, repeat the test. If, after repeating the test, the proper results are still not observed, do not use the product until you have contacted Technical Services at (800) 638-8663 (United States only).

The same control organism should be run as batch QC once each week when susceptibility testing is performed. If the batch QC fails, do not report patient results for the drug(s) that failed for that testing period. Repeat the QC for the drug(s) and patient isolates affected by the initial QC failure. If the repeat QC does not perform as expected, do not report patient results. Do not use the product until you have contacted Technical Services at (800) 638-8663 (United States only).

During the external evaluation of the BACTEC MGIT 960 SIRE Kits, the most common causes of QC failure were contaminated QC cultures, over/under inoculated AST Sets, drug not added to appropriate tubes and instrument error conditions.

*Adjusted and/or supplemented as required to meet performance criteria.

For in vitro Diagnostic Use.

POTENTIALLY INFECTIOUS TEST SPECIMEN: Pathogenic microorganisms, including hepatitis viruses and Human Immunodeficiency Virus, may be present in clinical specimens. "Standard Precautions"^4-7 and institutional guidelines should be followed in handling all items contaminated with blood and other body fluids.

Working with M. tuberculosis growth in culture requires Biosafety Level (BSL) 3 practices, containment equipment and facilities.

Read and follow directions contained in all appropriate package inserts including the BBL MGIT 7 mL Mycobacteria Growth Indicator Tube.

Prior to use, the user should examine the tubes and vials for evidence of contamination or damage. Discard any tubes or vials if they appear unsuitable. Dropped tubes should be examined carefully. If damage is seen, the tube should be discarded.

In the event of tube breakage: 1) Close the instrument drawers; 2) Turn off the instrument; 3) Vacate the area immediately; 4) Consult your facility/CDC guidelines. An inoculated leaking or broken tube may produce an aerosol of mycobacteria; appropriate handling should be observed.

Autoclave all inoculated MGIT tubes prior to disposal.