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The BACTEC Radiometric Technique is used for antimicrobial susceptibility testing of primary anti-mycobacterial drugs (streptomycin, isoniazid, rifampin, and ethambutol).^1-4 In this technique, ¹⁴CO₂ produced by mycobacteria from the metabolism of the ¹⁴C-labeled substrate present in BACTEC 12B Medium is quantitatively measured. If an antimicrobial is added to the medium, mycobacterial isolates which are susceptible to the antimicrobial are inhibited and the amount of ¹⁴CO₂ generated is reduced.

Butler and co-workers⁵ proposed an improved conventional method for susceptibility testing of PZA that required a special 7H10 Agar Medium at pH 5.5. A limitation of this method is that at a pH of 5.5, many isolates of M. tuberculosis either fail to grow or grow poorly.

BACTEC 12B Medium cannot be used for susceptibility testing of PZA since this drug is active only at lower pH values.⁵ Heifets et al. suggested a radiometric method for testing PZA susceptibility by using BACTEC 12B Medium at pH 5.5. This method required lowering the pH of each individual bottle from 6.8 to 5.5 once the growth was initiated in the vial.⁶ At pH 5.5 several isolates failed to grow in 12B Medium although overall results obtained in 12B pH 5.5 were better than those obtained by using 7H10 pH 5.5. Later, addition of egg yolk was suggested to improve growth of mycobacteria.⁷ This resulted in more reportable results but the addition of fresh egg-yolk was found to be cumbersome and not practical for a routine clinical laboratory. Salfinger and Heifets, after further studies, reported that the MIC of PZA increased with an increase in pH of medium from 5.5 to 6.8.⁸ Using a modification of the above published radiometric susceptibility procedures, Salfinger et al. recommended the use of BACTEC 12B Medium at pH 5.9 to 6.0 (BACTEC PZA Test Medium) with a concentration of PZA higher than that used in the conventional solid medium (pH 5.5).⁹

DO NOT use vials past their expiration date.

Do not use vials that exhibit any cracks or defects. Discard vials in the appropriate manner.

For information on quality control for the BACTEC 460TB instrument, refer to the Operation and Maintenance Manual for the BACTEC 460TB and 460.

Like any other drug susceptibility test, the quality of the inoculum is very critical. Use only actively growing cultures in 12B Medium with appropriate dilution.

Under ideal circumstances, the control should read GI ≥ 200 within 4 to 7 days. If a GI of 200 is achieved in the control in less than 4 days, it indicates that the inoculum was too heavy. Unless the results are clear cut, the test should be repeated. If a GI of 200 is achieved in the control in more than 7 – 10 days, it indicates poor quality of inoculum (i.e., low viability, low initial GI reading of the inoculum or poor dilution) and the results may be erroneous. The GI in the control and the drug vials should be followed carefully until clear cut results are achieved. When in doubt, repeat the test. If the GI in the drug vial is very close to 10% of that in the control, test the vials for an additional 1 – 3 days. This will help in making a judgment on the susceptibility pattern of the test culture. If the results are not interpretable, repeat the test with an isolate grown in fresh BACTEC 12B Medium (Catalog No. 442004). Always use H37Rv (ATCC™ 27294) as a susceptible control and H37Rv PZA-R (ATCC 35828) as a resistant control.

Quality control requirements must be performed in accordance with applicable local, state and/or federal regulation or accreditation requirements and your laboratory's standard Quality Control procedures. It is recommended that the user refer to pertinent NCCLS guidance and CLIA regulations for appropriate Quality Control practices.

For in vitro Diagnostic Use.

This product contains dry natural rubber.

Pathogenic microorganisms, including hepatitis viruses and Human Immunodeficiency Virus, may be present in clinical specimens. "Standard Precautions"^10-13 and institutional guidelines should be followed in handling all items contaminated with blood and other body fluids.

Procedures should be performed in a suitable safety cabinet and properly ventilated area. Inoculated vials must be tested on a BACTEC 460TB instrument equipped with a BACTEC 460TB hood which provides HEPA filtered air and negative pressure in the instrument test area.

Prior to use, each vial should be examined for evidence of damage, deterioration, or contamination such as cloudiness, bulging or depressed septum. DO NOT USE any vial showing evidence of contamination. A contaminated vial could contain positive pressure. Vial contamination may not be readily apparent. Vials that exhibit evidence of damage should be discarded prior to inoculation. On rare occasions, the glass bottle may be cracked and the neck may break during removal of the flip-off cap or in handling. Also, on rare occasions a vial may not be sealed sufficiently. In both cases, the contents of the vials may leak or spill especially if the vial is inverted. If the vial has been inoculated, treat the leak or spill with caution, as pathogenic organisms/agents may be present. Before discarding, sterilize all inoculated vials by autoclaving.

To minimize the potential of leakage during inoculation of specimen into culture vials, use syringes with permanently attached needles or Luer-Lok™Brand Tips.