Annals of Clinical & Laboratory Science 31:279-283 (2001)
© 2001 Association of Clinical Scientists
Comparison of the BACTEC MGIT 960 and BACTEC 460TB Systems for Detection of Mycobacteria in Clinical Specimens
Tsi-Shu Huang1,3,
Chii-Shiang Chen,
Susan Shin-Jung Lee,
Wen-Kuei Huang and
Yung-Ching Liu1,2
1 Section of Microbiology and Infectious Disease, Kaohsiung Veterans General Hospital;
2 Department of Medicine, National Yang-Ming Medical College; and
3 Department of Medical Technology, Foo-Yin Institute of Technology; Kaohsiung, Taiwan, Republic of China
Address correspondence to Yung-Ching Liu, M.D., Section of Infectious Disease, Veterans General Hospital-Kaohsiung, 386 Ta-Chung 1st Rd, Kaohsiung, Taiwan, ROC. tel 886 734 68169; fax 886 734 68296; e-mail tshuang{at}isca.vghks.gov.tw
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Abstract
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The reliability of the Mycobacteria Growth Indicator Tube (MGIT) 960 system for rapid detection of mycobacteria in clinical specimens was evaluated and compared to the radiometric method (BACTEC 460TB) and to mycobacterial culture on Lowenstein-Jensen (LJ) medium. Clinical specimens (n = 590) were tested without selection. A total of 121 (20.5%) isolates of mycobacteria were recovered; 98 (81.0%) of them were recovered with the BACTEC 460TB system, 86 (71.1%) were recovered with the BACTEC MGIT 960 system, and 55 (45.5%) were recovered with LJ medium (MGIT 960 versus BACTEC 640TB, p >0.05; MGIT 960 or BACTEC 460TB versus LJ, p <0.001). The mean time to detection (TTD) was 18 da for BACTEC 460 TB, and 13 da for BACTEC MGIT 960. The mean time to detection in each system, based upon data where both systems were culture positive, was significantly different (16.6 da for BACTEC 460TB and 13 da for BACTEC MGIT 960, p<0.001). The contamination rate of the BACTEC MGIT 960 system was 13.2%, which was intermediate between the BACTEC 460TB system (11.7%) and the LJ medium (14.7%). These data indicate that the fully automated MGIT 960 system is an accurate, non-radiometric alternative to the BACTEC 460TB method for rapid detection of mycobacteria in a clinical setting.
(received 17 February 2001; accepted 1 May 2001)
Keywords: Mycobacterium tuberculosis, automation, radiometric assay, microbiological culture, method comparison
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Introduction
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Tuberculosis remains a major health threat, and the rapid emergence of drug-resistant mycobacteria has strengthened the demand for rapid methods for detection of mycobacteria in clinical samples. Since the prevention of tuberculosis relies on the early detection and cure of infectious cases [1], current efforts are focused upon improving the rapidity of identification of M tuberculosis, allowing prompt initiation of appropriate therapy. Techniques based on the PCR (polymerase chain reaction) for the diagnosis of tuberculosis have been well established [2,3], but culture still remains the "gold standard" [4]. Because M tuberculosis has a doubling time of 16–18 hr, current methods do not provide quick detection and identification; the patients, their contacts, and health-care workers are therefore allowed to remain at risk. Our focus is on shortening the delay for TB confirmation by culture. Use of liquid culture media is one way of shortening the delay considerably. The BACTEC 460TB system (Becton Dickinson Diagnostic Instrument Systems, Towson, Maryland, USA) is widely recognized as a reference method for detection of mycobacteria, combining the advantages of liquid media with semi-automation. However, this system uses a radiometric method for the detection of mycobacterial growth. Disposal of the radioactive waste produced by this system presents a logistical problem and increases costs. The MGIT (mycobacteria growth indicator tube) is a ready-to-use liquid medium produced by Becton Dickinson [5]. The BACTEC MGIT 960 system uses an oxygen-quenching fluorescent sensor technology (BBL MGIT) in conjunction with unique algorithms to determine positivity of the culture tubes. The goal of this study was to evaluate the performance of the BACTEC MGIT 960 system for detection of mycobacteria in comparison to the BACTEC 460TB system.
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Materials and Methods
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Specimen collection and processing.
Specimens were prospectively collected from patients suspected of having tuberculosis or who were being monitored for treatment with anti-tuberculosis drugs at the Kaohsiung Veterans General Hospital, Taiwan, for a 1-mo period. A total of 590 specimens received in that interval was investigated without selection, including specimens from the respiratory tract, body fluids, tissues, wound, pus, and skin. Specimens that could not be processed immediately were stored at 2–6°C for no longer than 72 hr. Specimens from nonsterile sites were decontaminated, digested with 3% NaOH in 0.5% N-acetyl-L-cystine, and concentrated. Specimens from sterile sites were not digested with NaOH prior to concentration. Following concentration, all specimens were inoculated to BACTEC 12B, MGIT, and a Lowenstein-Jensen (LJ) slant. The incubation took place for 8 wk at 37°C.
Smear examination.
Acid-fast smears of the digested and decontaminated specimens were stained with Kinyoun cold acid-fast stain and examined by standard procedures [4].
Culture by BACTEC MGIT 960.
The BACTEC MGIT 960 culture tube contained 7 ml of Middlebrook 7H9 broth base, to which was added an enrichment supplement containing oleic acid, albumin, dextrose, and catalase (BBL MGIT OADC), and an antibiotic mixture of polymyxin B, amphotericin B, nalidixic acid, trimethoprim, and azlocillin (BBL MGIT PANTA). After inoculation of each tube with 0.5 ml of the processed specimen, the tubes were incubated at 37°C in the BACTEC MGIT 960 instrument and were monitored automatically every 60 min for an increase of fluorescence. A series of algorithms programmed in the instrument was used to determine presumptive positivity and to alert the operator to the presence and locations of the positive tubes by means of indicator lights on the front of the instrument, tube status displayed on the video display screen, and light-emitting diodes (LEDs) positioned at the appropriate tube station. Any sample that was identified as positive was removed from the instrument, and a smear was prepared and examined for acid-fast bacilli (AFB). The time to detection (TTD) of mycobacteria was based on the date of the earliest instrumenal indication of positivity, which correlated with AFB smear positivity.
Culture by BACTEC 460.
BACTEC 12B culture vials were prepared according to the manufacturer’s instructions and inoculated with 0.5 ml of the processed specimen. Vials were monitored with the BACTEC 460TB System 2x–3x/wk for the first 2 to 3 wk, and 1x/wk thereafter through the next 6 wk. Any vial that was identified as having a growth index (GI) 
10 was considered presumptively positive and was tested daily until a GI of 50 was observed, whereupon an AFB smear was prepared and examined. The TTD was based on the first date that the GI was 50, which correlated with AFB smear positivity.
Culture by solid media.
All specimens were inoculated onto conventional solid media (ie, the LJ slant). Tubes were examined after the first, third, sixth, and eighth wk. Appearance of colonies on the slant surface was confirmed with an acid-fast smear.
Statistical analysis.
The data for rate of recovery were analyzed by McNemar’s chi-square test. The data for time to detection were analyzed by paired t-test.
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Results
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A comparison of the MGIT 960 system with the BACTEC 460TB system for detection of mycobacteria in 590 specimens from 297 patients is shown in Table 1
. Ninety-eight (16.6%) specimens were positive by the BACTEC 460TB system, including 65 M tuberculosis complex, 18 Mycobacterium other than M tuberculosis (MOTT), and 15 acid-fast bacilli (AFB) (nonviable acid-fast bacilli detected by microscopy, assumed to be mycobacteria). Eighty-six (14.6%) specimens were culture-positive by MGIT. Twenty specimens were culture-positive by the MGIT 960 but not detected by BACTEC 460TB system. Thirty-two specimens were culture-positive by the BACTEC 460TB system but were not detected by BACTEC MGIT 960, including 14 M tuberculosis complex, 8 MOTT, and 10 AFB isolates.
The comparison of recovery rates is shown in Table 2. A total of 121 isolates of mycobacteria was recovered from all 3 media. The 2 liquid media were significantly more sensitive than LJ medium (p <0.001). When a combination of a liquid plus a solid medium was considered, there was no significant difference (p >0.05) between the use of the BACTEC 460TB system plus the LJ medium and the BACTEC MGIT 960 system plus the LJ medium. Forty-three samples were positive by only 1 method: 16 by the BACTEC 960 system, 24 by the BACTEC 460TB system, and 3 by LJ medium (Table 2). The detection rates in each specimen type using the 2 liquid media were not statistically different ( p >0.05, Table 3).
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Table 2. Recovery rates for mycobacteria by the BACTEC MGIT 960 system, by the BACTEC 460TB system, and by culture on LJ medium.
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Mycobacteria were isolated by any method from 121 of the 590 samples (20.5%). An acid-fast stain was positive in 25 samples and negative in 96 samples (Table 4). The BACTEC 460TB system identified 98 (81.0%) mycobacteria, of which 18 were acid-fast stain-positive; the MGIT 960 detected 86 (71.1%) mycobacteria, of which 23 were acid-fast stain-positive. Of the 24 acid-fast stain-positive samples, growth of mycobacteria occurred on the BACTEC 460TB alone in 1 case, whereas 6 cases grew only on the MGIT 960. Of the 94 acid-fast stain-negative samples, 27 cases showed growth of M tuberculosis on the BACTEC 12B alone, and 13 cases on the MGIT 960 alone.
For the smear-negative specimens, 80 and 63 specimens were culture-positive on the BACTEC 460TB and MGIT 960 systems, respectively. The smear-negative category contributed to a higher recovery rate with BACTEC 460TB. Overall, the BACTEC MGIT 960 and 460TB systems detected 95.8% and 75.0% of the smear-positive specimens, and 67.0% and 85.2% of the smear-negative specimens, respectively.
In Table 5, the means and ranges of times to detection (TTD) were 13 (1–38) da for the BACTEC MGIT 960 system and 18 (4–46) da for the BACTEC 460TB system. The mean detection times, based upon data where both systems were culture-positive for acid-fast bacilli, were 13 and 16.6 da for the BACTEC MGIT 960 system and BACTEC 460TB system, respectively. For smear-positive specimens, the mean values were 8.2 and 9.9 da (p >0.05); for smear-negative specimens, the means were 14.9 and 19.3 da (p <0.05). Overall, the mean TTD was shorter for the MGIT 960 system than for the 460TBsystem (p <0.001).
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Table 5. Microscopy and detection times for AFB smear-positive and smear-negative isolates in BACTEC MIGT 960 and BACTEC 460TB systems. TTD = time-to-detection (da).
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The contamination rate for the BACTEC MGIT 960 system was 13.2%, which was intermediate between the contamination rates of the BACTEC 460TB system (11.7%) and the LJ medium (14.7%).
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Discussion
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In this study, the number of mycobacteria recovered by the BACTEC MGIT 960 system was lower than the number recovered by the BACTEC 460TB system, but the difference was not significant. The advantage of the BACTEC 460TB system was almost exclusively in smear-negative samples. This may be attributed to the greater sensitivity for detection of the radiolabeled materials used in the BACTEC 460TB system.
With the use of either BACTEC system in combination with a solid medium, LJ, there was no statistical difference in the recovery rates of mycobacteria. Moreover, the mycobacteria detection rates were significantly higher than when LJ was used alone. This finding suggests that the solid medium, LJ, should not be used alone.
Based on previous studies [5–10], large variations in recovery rates and contamination rates are observed at different test sites, owing to different conditions of specimen quality, transport time, and other operational factors. In the present study, the recovery rates were lower and the contamination rates higher than previously reported [6–10]. The high contamination rates may have contributed to the low recovery rates.
Because of the high contamination rates, investigations were made in our institute, including an evaluation of the sputum quality. In a total of 186 sputum specimens submitted for mycobacteria culture, 61.8% were evaluated as unacceptable (squamous epithelial cell 
25/low power field). The contamination rates of acceptable and unacceptable sputum specimens were 13.9% and 7.0%, respectively (unpublished data). Prompted by these findings, we evaluated the methods of specimen collection and the patient instructions that were used in each ward and we gave lectures on sputum collection to the nurses.
The average TTD with the BACTEC MGIT 960 system was shorter than with the BACTEC 460TB system, especially in smear-negative, culture-positive specimens.
The overall performance of the BACTEC MGIT 960 system is comparable to the well-established BACTEC 460TB system. The MGIT 960 system has the advantages that it is easy to use and has a shorter detection time, higher capacity, and provides fully automated, continuous monitoring for mycobacteria from clinical samples, which are all important considerations in selecting a microbiology system.
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References
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Abstract
Introduction
