INT J TUBERC LUNG DIS 12(12):1449–1455 © 2008 The Union
Drug susceptibility testing of Mycobacterium tuberculosis against second-line drugs using the Bactec MGIT 960 System C. Rodrigues,* J. Jani,† S. Shenai,* P. Thakkar,* S. Siddiqi,‡ A. Mehta* * Department of Microbiology, P D Hinduja National Hospital and Medical Research Centre Tertiary Care Hospital, Mumbai, † Becton Dickinson Diagnostic Systems, Becton Dickinson, Mumbai, India; ‡ Becton Dickinson Diagnostic Systems, Sparks, Maryland, USA SUMMARY OBJECTIVE:
To establish the critical test concentrations for seven second-line anti-tuberculosis drugs in the Bactec Mycobacterial Growth Indicator Tube (MGIT) 960 TB system and to evaluate its efficacy compared to the Bactec 460 TB system. D E S I G N : This study was carried out in three phases. In Phase I, pan-susceptible strains were tested to establish the minimum inhibitory concentration; in Phase II, mostly resistant strains were tested to determine a critical test concentration; and in Phase III, actual clinical isolates were tested to validate the optimal critical concentrations established in Phases I and II. R E S U LT S : The critical concentrations established for
seven second-line drugs with the Bactec MGIT 960 system are amikacin 1.0 μg/ml, capreomycin 2.5 μg/ml, kanamycin 2.5 μg/ml, ofloxacin 2.0 μg/ml, moxifloxacin 1.0 μg/ml, ethionamide 5.0 μg/ml and para-amino salicylic acid 4.0 μg/ml. C O N C L U S I O N : The Bactec MGIT 960 System is an accurate and reliable method for rapid drug susceptibility testing (DST) of Mycobacterium tuberculosis against second-line drugs. In the present study, few of the strains were resistant to fluoroquinolones and further DST for this group is required. K E Y W O R D S : drug susceptibility testing; MGIT 960; India
GLOBAL DRUG RESISTANCE is of increasing concern.1–4 The recently described extensively drug-resistant tuberculosis (XDR-TB, defined as multidrug-resistant TB [MDR-TB] with additional resistance to any fluoroquinolone and to at least one of three injectable secondline anti-tuberculosis drugs used in treatment: capreomycin [CPM], kanamycin [KM] or amikacin [AMK]), and the high death rates among these patients has increased the importance of conducting in vitro drug susceptibility testing (DST) not only of first-line drugs but also of second-line drugs.5,6 India is a known TB endemic area, and drug resistance is expected to be very high. Before a thorough countrywide investigation is carried out to establish the extent of MDR-TB and XDR-TB prevalence, it is essential to validate available new procedures for performing second-line DST. It is well known that broth-based DST methods provide significantly rapid and reliable results. The radiometric Bactec 460 TB System (Becton Dickinson Diagnostic Systems, Sparks, MD, USA) is considered the gold standard for DST for both first- and secondline anti-tuberculosis drugs. The procedures for DST using the broth-based Bactec 460 TB system are well established;7,8 however, there are increasing concerns about the disposal of the radioactive material with
the use of this system. The recently introduced nonradiometric Bactec Mycobacterial Growth Indicator Tube (MGIT) 960 System (Becton Dickinson Diagnostic Systems) has been reported to be equivalent to the radiometric system in performance. Many studies have been performed using the Bactec MGIT 960 system in the detection and recovery of mycobacteria from clinical specimens and DST of Mycobacterium tuberculosis isolates for the first-line drugs isoniazid, rifampicin, ethambutol and pyrazinamide, as well as streptomycin.9–16 In countries with a high prevalence of tuberculosis (TB) and drug resistance, it is important to establish Bactec MGIT 960 test procedures and critical test concentrations for secondline anti-tuberculosis drugs, both for long-established drugs and for newer drugs that have recently been introduced in these countries. Once established, these procedures will help the main clinical mycobacteriology laboratories to provide physicians with clinically relevant DST data in a timely manner. It will also aid in better patient management and in the prevention of further drug resistance. Two major studies have been published on using Bactec MGIT 960 for DST against second-line drugs in Europe.17,18 The primary aim of the present study was
Correspondence to: Camilla Rodrigues, Department of Microbiology, P D Hinduja National Hospital and Medical Research Centre Tertiary Care Hospital, Mahim, Mumbai 400 016, India. Tel: (+91) 22 2444 7784. Fax: (+91) 22 2444 2318. e-mail:
[email protected] Article submitted 31 December 2007. Final version accepted 15 July 2008.
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to evaluate whether the procedures for second-line DST established in the European studies are also applicable in Indian settings. Hinduja is a tertiary care hospital for chronic TB patients. The purpose of our study was to establish critical test concentrations for second-line drugs in our laboratory settings. Our study is also expected to provide guidelines for rapid broth-based DST for the second-line drugs currently used in India.
MATERIALS AND METHODS Study site The study was carried out at the P D Hinduja National Hospital and Medical Research Centre, a tertiary care hospital in Mumbai, India. Antimicrobial agents All the test drugs for DST were obtained in a chemically pure form. The majority of the test drugs—AMK, KM, CPM, ofloxacin (OFX), ethionamide (ETH) and para-amino salicylic acid (PAS)—were obtained from Sigma Aldrich, St Louis, MO, USA, while moxifloxacin (MXF) was obtained from Cipla Ltd, Mumbai, India. Stock solutions for all drugs were prepared as per the standard procedures for DST. AMK, KM, CPM and PAS were dissolved in deionised (DI) water; OFX and MXF in 0.1 N NaOH with subsequent dilutions in DI water; and ETH in ethylene glycol with subsequent dilution in DI water. Stock solutions were sterilised using a 0.22 μm polycarbonate filter membrane, except for ETH. As most of the filters may initially retain some of the drug, the first 20% of the filtered solutions were discarded. All stock solutions were stored at –70°C in small aliquots in appropriate tubes. The frozen drug solutions were used immediately after thawing and the remainder was discarded. At the time of testing, two-fold serial dilutions were carried out to achieve the desired concentrations. Test concentrations used for DST are listed in Table 1. Test methods and media Overall, test procedures were followed according to the procedures described by Rüsch-Gerdes et al. on second-line DST.18 For DST with the MGIT 960 instrument, the MGIT with Bactec MGIT 960 DST supplement was used. Bactec 12B medium was used with Table 1
Drug concentrations used in MGIT 960
Drug Amikacin Kanamycin Capreomycin Ofloxacin Moxifloxacin Ethionamide PAS
Phase I μg/ml
Phase II μg/ml
Phase III μg/ml
0.5, 1.0, 2.0 1.25, 2.5, 5.0 0.625, 1.25, 2.5 1.0, 2.0, 4.0 1.0, 2.0, 4.0 0.625, 1.25, 2.5 2.0, 4.0, 8.0
0.5, 1.0, 2.0 1.25, 2.5, 5.0 0.625, 1.25, 2.5 1.0, 2.0, 4.0 1.0, 2.0, 4.0 0.625, 1.25, 2.5 2.0, 4.0, 8.0
0.5, 1.0, 2.0 1.25, 2.5, 5.0 1.25, 2.5, 5.0 1.0, 2.0, 4.0 1.0, 2.0, 4.0 1.25, 2.5, 5.0 2.0, 4.0, 8.0
MGIT = Mycobacterial Growth Indicator Tube; PAS = para-aminosalicylic acid.
the Bactec 460 TB instrument for comparison. The procedures for DST using the Bactec 460 method were followed as specified in the procedure manual.19 All media and reagents were supplied by the manufacturer. The standard protocol recommended by the manufacturer for first-line drugs was followed for DST by the MGIT 960 method.20 For the preparation of the test inoculum, a positive 7 ml MGIT tube was used the day after it first became positive on the Bactec MGIT 960 instrument (Day 1), up to and including the fifth day (Day 5) after instrument positivity. A Day 1 or Day 2 positive tube was used directly for the inoculation procedure for DST, while for a Day 3, Day 4 or Day 5 positive tube, 1:5 diluted (1 ml of positive broth in 4 ml of sterile saline) suspension was used for the inoculation procedures. All MGIT tubes were labelled appropriately and 0.8 ml of Bactec MGIT DST supplement was added to each tube; 100 μl of the appropriate drug solution was added to the respective labelled MGIT tube. Each drug-containing tube was inoculated with 0.5 ml of the test inoculum. The growth control (GC) tube was inoculated with 0.5 ml of 1:100 diluted organism suspension (0.1 ml of the test inoculum into 10 ml of sterile saline). All of the tubes were tightly recapped and mixed well. All inoculated drug-containing and GC tubes were placed in the DST set carrier and entered into the MGIT 960 instrument as ‘unknown drugs’ using the AST (antimicrobial susceptibility testing) entry feature. The instrument flagged the AST set ‘complete’ when the growth control reached a growth unit (GU) value of 400. At that point, the GU values of drug-containing tubes were retrieved from the instrument by printing out the DST set report, and the results were interpreted manually. If the GU of the drugcontaining tube was >100 when the GU of the growth control was 400, the results were defined as resistant. If the GU values of the drug-containing tubes were ⩽100, the results were considered susceptible. Study design The study was conducted in three phases. In Phase I, a total of 11 strains of M. tuberculosis, 10 clinical isolates susceptible to first-line drugs and other drugs based on previous testing were included. One quality control (QC) strain M. tuberculosis H37Rv (ATCC [American Type Cell Collection] 27294) was also tested. Phase I was designed to validate the basic test procedure and to determine the range of minimal inhibitory antimicrobial test concentrations (MICs). Three concentrations of each drug were tested. In Phase II, 20 strains with different degrees of resistance to the previously tested first-line drugs and some other drugs were included. As many resistant strains as possible, especially MDR strains, were selected. Drug concentrations were kept the same as in Phase I. Results were analysed to identify the concentration that yielded the best demarcation between susceptible and
Second-line DST using MGIT 960 TB
resistant strains. It was anticipated that results from Phase I and Phase II would help in understanding and establishing a concentration that would determine the breakpoint between susceptible and resistant strains. In Phase III, 73 clinical isolates expected to be MDR/ XDR were included. The optimal drug concentrations established in Phase II were tested, except that the lowest concentration was dropped and one higher concentration was added for CPM and ETH. Quality control and resolution of discrepancies The standard QC strain of M. tuberculosis H37Rv (ATCC 27294) was used in all three phases for QC. If the QC strain showed unexpected results, all tests of that batch had to be repeated. If discrepant results were obtained between the two methods, retesting was performed twice using both methods. The results of retesting were used for the data analysis.
RESULTS Phase I: the QC strain showed consistently susceptible results throughout all three phases of testing. The results of Phase I are summarised in Table 2. As shown, all strains tested for susceptibility against AMK, KM, Table 2
Phase I drug susceptibility testing results*
Drug and Cumulative, % concentration μg/ml MGIT 960 Bactec 460 MGIT 960 Bactec 460 Amikacin ⩽0.5 1 2 Kanamycin ⩽1.25 2.5 5.0 Capreomycin ⩽0.625 1.25 2.5 Ofloxacin ⩽1 2 4 Moxifloxacin ⩽1 2 4 Ethionamide ⩽0.625 1.25 2.5 PAS ⩽2 4 8
11 0 0
11 0 0
11 (100) 0 0
11 (100) 0 0
11 0 0
11 0 0
11 (100) 0 0
11 (100) 0 0
2 5 4
7 4 0
11 0 0
11 0 0
11 0 0
11 0 0
11 (100) 0 0
10 1 0
11 0 0
90.90
11 0 0
11 0 0
11 (100) 0 0
18.2 63.7 100
63.6 100 0
100 0 0
100 0 0
0
OFX, MXF and PAS showed high concordance of MIC between the MGIT 960 and Bactec 460 results. All 11 strains were fully susceptible to AMK, with an MIC of ⩽0.5 μg/ml, and to KM, with an MIC of ⩽1.25 μg/ml. The MICs of both OFX and MXF were ⩽1.0 μg/ml, whereas all strains showed susceptibility to PAS at an MIC of ⩽2 μg/ml. For CPM, four strains showed resistance at 1.25 μg/ml using MGIT 960, but all showed susceptibility at 1.25 μg/ml with Bactec 460. For ETH, all of the test strains showed susceptibility at 1.25 μg/ ml with MGIT 960 and 0.625 μg/ml with Bactec 460. Phase II: the results of Phase II are summarised in Table 3. The test strains showed different degrees of resistance to one or more second-line drugs. In the case of AMK, taking 1.0 μg/ml as a cut-off point, there was one resistant result with Bactec 460 and two with MGIT 960, one of which was false-resistant. With KM at 2.5 μg/ml, there were two resistant results with Bactec 460 and three with MGIT 960, with one false-resistant result using MGIT 960. For OFX, there were no resistant results with either Bactec 460 or MGIT 960 at 2.0 μg/ml. Taking 1.0 μg/ml of MXF,
Table 3 Phase II drug susceptibility testing results
Drug and concentration μg/ml
Strains for which the indicated concentration is the MIC
11 (100) 0 0 100 0 11 (100) 0 0
* Ten strains and H37Rv were tested. MIC = minimum inhibitory concentration; MGIT = Mycobacterial Growth Indicator Tube; PAS = para-aminosalicylic acid.
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Amikacin ⩽0.5 1 2 >2 Kanamycin ⩽1.25 2.5 5.0 >5 Capreomycin ⩽0.625 1.25 2.5 >2.5 Ofloxacin ⩽1 2 4 Moxifloxacin ⩽1 2 4 Ethionamide ⩽0.625 1.25 2.5 >2.5 PAS ⩽2 4 8 >8
Strains for which the indicated concentration is the MIC Bactec MGIT 960 n (%)
Bactec 460 n (%)
16 (80) 2 (10) 1 (5) 1 (5)
19 (95) 0 0 1
13 (65) 4 (20) 2 (10) 1 (5)
18 (90) 0 1 (5) 1 (5)
4 (20) 14 (70) 1 (5) 1 (5)
16 (80) 3 (15) 0 1 (5)
18 (90) 2 0
18 (90) 2 0
19 (95) 1 (5) 0
20 (100) 0 0
11 (55) 4 (20) 4 (20) 1 (5)
13 (65) 4 (20) 2 (20) 1 (5)
17 (85) 1 (5) 0 2 (10)
17 (85) 1 (5) 0 2 (10)
MIC = minimum inhibitory concentration; MGIT = Mycobacterial Growth Indicator Tube; PAS = para-aminosalicylic acid.
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Table 6 Final critical concentrations established for Bactec MGIT 960 compared with Bactec 460
Table 4 Phase II discordant results between Bactec 460 and Bactec MGIT 960 using the best cut-off point for the critical concentrations Drug and concentration Bactec 460/MGIT 960 μg/ml
Critical concentrations, μg/ml Drug
Amikacin 1.0/1.0 Kanamycin 2.5/2.5 Capreomycin 1.25/2.5 Ofloxacin 2.0/2.0 Moxifloxacin 1.0/1.0 Ethionamide 2.5/2.5 PAS 4.0/4.0
S/R
R/S
1 1 0 0 1 0 0
0 0 0 0 0 0 0
Amikacin Kanamycin Capreomycin Ofloxacin Moxifloxacin Ethionamide PAS
Bactec MGIT 960
Bactec 460
1.0 2.5 2.5 2.0 1.0 5.0 4.0
1.0 2.5 1.25 2.0 1.0 2.5 4.0
MGIT = Mycobacterial Growth Indicator Tube; PAS = para-aminosalicylic acid.
MGIT = Mycobacterial Growth Indicator Tube; S = susceptible; R = resistant; PAS = para-aminosalicylic acid.
there was one false-resistant result using MGIT 960. However, at 2.0 μg/ml, all were susceptible with both methods. For ETH, there were three resistant results with Bactec 460 and five resistant results with MGIT 960 at 1.25 μg/ml. The most concordant results between Bactec 460 and MGIT 960 for ETH were seen at an MIC of 2.5 μg/ml with both systems. For PAS, an MIC of 4.0 μg/ml clearly differentiated between susceptible and resistant strains. Table 5 Phase III drug susceptibility test results comparing different concentrations in each test system Drug and concentration Bactec 460/MGIT 960 μg/ml Amikacin 0.5/0.5 1.0/1.0* 2.0/2.0 Kanamycin 1.25/1.25 1.25/2.5 2.5/2.5* 2.5/5.0 5.0/5.0 Capreomycin 1.25/1.25 1.25/2.5* 2.5/2.5 2.5/5.0 5.0/5.0 Ofloxacin 1.0/1.0 2.0/2.0* Moxifloxacin 1.0/1.0* 1.0/2.0 2.0/2.0 Ethionamide 1.25/1.25 1.25/2.5 2.5/2.5 2.5/5.0* 5.0/5.0 PAS 2.0/2.0 4.0/4.0* 8.0/8.0
S/S
R/R
R/S
S/R
39 44 45
32 28 27
0 0 0
2 1 1
35 38 42 42 43
35 31 31 30 30
0 4 0 1 0
3 0 0 0 0
32 45 47 56 66
27 25 17 7 2
1 2 0 10 0
13 1 9 0 5
57 70
11 3
2 0
3 0
71 72 72
2 0 1
0 1 0
0 0 0
34 38 39 45 47
32 31 28 26 22
0 1 0 2 0
7 3 6 0 4
46 53 56
27 20 17
0 0 0
0 0 0
* Breakpoints considered for interpretation of results. MGIT = Mycobacterial Growth Indicator Tube; S = susceptible; R = resistant; PAS = para-aminosalicylic acid.
An analysis of the optimal critical concentration that yielded the most concordant results between Bactec 460 and MGIT 960 is shown in Table 4. According to the results of this study, the MIC value for CPM was higher in MGIT 960 than in Bactec 460. Therefore, a cut-off point for the critical concentration was established for CPM that was one concentration higher than that defined for Bactec 460. Phase III: clinical isolates of M. tuberculosis that were expected to be resistant were tested in this phase. The results of Phase III are listed in Table 5. For comparison, the critical concentration for Bactec 460 was kept the same as in Phase II. As observed in Phase II, breakpoint concentrations for AMK, KM, OFX, MXF and PAS were the same using the two methods, while for CPM and ETH, the most concordant results were obtained with one higher concentration in MGIT 960 (Table 6). Overall, 73 strains were tested in this phase, resulting in a total of 511 tests (Table 5). As indicated by the DST results, a large number of resistant strains were included in this testing: AMK (n = 28), KM (n = 31), CPM (n = 25), OFX (n = 3), MXF (n = 2), ETH (n = 26) and PAS (n = 20). Comparing the results for Bactec 460, which was taken as a gold standard, with those of Bactec MGIT 960 at the established critical concentrations, only four false-susceptible results (CPM and ETH) and two false-resistant results (AMK and CPM) were observed with MGIT 960.
DISCUSSION The aim of our study was to establish the critical test concentrations of second-line and more recent antituberculosis drugs in our laboratory setting. The test concentration that gave the least number of discrepant results on testing a large number of susceptible and resistant cultures was considered the critical concentration. The Bactec 460 test results were considered the gold standard for comparison purposes, as it is also a broth-based system. The critical test concentrations for most of the drugs have already been established using this system.7 Only a few studies in the literature have reported the MICs and test concentrations of second-line drugs
Second-line DST using MGIT 960 TB
in liquid media, especially Bactec MGIT.17,18,21,22 Three major studies have been published earlier, one for the radiometric Bactec 460 TB System,23 another a multicentre evaluation comparing the Bactec MGIT 960 system with the Bactec 460 radiometric method18 and a third comparing MGIT 960 with the resistance ratio method.17 In the Bactec 460 study conducted by Pfyffer et al., the range of ETH concentration for Bactec 460 was 1.25–2.5 μg/ml, with 1.25 μg/ml recommended as the final test concentration.23 However, data from the present study suggest that 2.5 μg/ml is more appropriate than 1.25 μg/ml in the Bactec 460 DST, as it differentiates resistance more distinctly. In Pfyffer et al., the recommended range of KM concentration for Bactec 460 was 2.5–5.0 μg/ml, with 5.0 μg/ ml as the final test concentration;23 data from the present study suggest, however, that 2.5 μg/ml would be more appropriate than 5.0 μg/ml in the Bactec 460 DST. The first phase of the study helped us to establish an MIC of pan-susceptible strains for each drug; Phase II helped in establishing the optimal concentrations that will give breakpoints for susceptible and resistant strains; in the last phase, we tested clinical isolates to adjust and validate the critical concentrations established in Phase II. Initial results indicated that there could be different MICs and breakpoints in the case of CPM and ETH. CPM 1.25 μg/ml in Bactec 460 gave the best results, compared to 2.5 μg/ml in MGIT 960. In the case of ETH, the MIC value obtained was 1.25 μg/ml for both systems in Phase I testing, but Phase II data indicated a higher MIC value, of 2.5 μg/ml. As the results were not clear in Phase II, it was decided to try a higher test concentration in Phase III. We therefore dropped 0.625 μg/ml in Phase III and added 5.0 μg/ml. Phase II data clearly indicated the working critical concentrations of the test drugs that would yield results equivalent to those for Bactec 460. AMK at 1.0 μg/ml, KM at 2.5 μg/ml and MXF at 1.0 μg/ml yielded satisfactory results for both methods, with one false-resistant result for each using MGIT 960. For CPM, 1.25 μg/ml in Bactec 460 gave the best result against 2.5 μg/ml in MGIT 960. ETH at 2.5 μg/ml in Bactec 460 and 2.5 in MGIT yielded concordant results using both methods. Critical concentrations were found to be the same in the case of OFX (2.0 μg/ml) and PAS (4.0 μg/ml). Phase III represents the work in the field, as clinical isolates were tested with a high number of resistant strains. Applying the critical concentrations established in Phase II, most of the test drugs yielded concordant results for both methods. Phase III data clearly indicated that the critical test concentrations in MGIT 960 should be higher than for Bactec 460 in the case of CPM and ETH. This could be explained by the fact that the composition of 12B and MGIT media is different, MGIT being a richer medium. Applying the
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final established critical concentration criteria, with a total of 511 data points, there were four falsesusceptible results (0.8% very major errors) and two false-resistant results (0.3% major errors). A remarkable concordance is thus observed between the MGIT 960 and Bactec 460 results. Among the fluoroquinolones, MXF has crossresistance with OFX. The MICs for newer analogues are generally lower than for older drugs.24 In this study, the test concentration was found to be lower for MXF than for OFX. There were five resistant strains for OFX (two from Phase II and three from Phase III), but only two (Phase III) for MXF. In the randomly selected Phase III group, there were few fluoroquinolone-resistant isolates. The critical concentrations for these drugs are preliminary, and further study is required for validation. The prevalence of MDR-TB was found to be as high as 65% in our institute. The high rate of drug resistance could be attributed to the fact that ours is a tertiary referral centre and most of the patients are chronic cases who are more likely to be unresponsive to treatment.25 In institutions dealing with referral cases with a high prevalence of drug resistance, the availability of a liquid media system is extremely important. The early availability of DST results, not only for first-line but also for second-line drugs, would be specifically beneficial to patients harbouring resistant organisms, enabling them to receive effective treatment with appropriate regimens. Time to report results also has a significant impact on patient management. According to US Centers for Disease Control and Prevention recommendations, it is important to report DST results within an average of 2 weeks and complete isolation, differentiation of M. tuberculosis and DST of the isolated culture within on average 4 weeks.26 The average reporting time for the DST results ranged from 5 to 8 days for the Bactec MGIT 960 system compared to 6–10 days for the Bactec 460 TB system and 2–4 weeks using conventional Löwenstein-Jensen medium in our studies. Our study helped to establish a rapid and reliable DST method by using the Bactec MGIT 960 test procedures and the critical test concentrations for the second-line anti-tuberculosis drugs that are being used to treat drug-resistant cases in our area. It is important that other major institutions dealing with drugresistant TB cases also assess the use of rapid brothbased systems. It is also important to use an established procedure for second-line DST, with strict quality control. Further countrywide testing of a large number of resistant isolates would validate the test concentrations of second-line anti-tuberculosis drugs established here. Acknowledgement The authors thank Becton Dickinson Diagnostic Systems for financially supporting this study.
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RÉSUMÉ
Etablir la concentration minimale inhibitrice (CMI) des sept médicaments de deuxième ligne par le système Bactec Mycobacterial Growth Indicator Tube (MGIT) 960 et comparer son efficacité avec celle du Bactec système 460 TB.
OBJECTIF :
Cette étude a été menée en trois phases. Dans la Phase I, on a testé des souches complètement sensibles pour établir la CMI ; dans la Phase II, on a testé le plus souvent des souches résistantes pour déterminer une concentration critique pour ce test ; et dans la Phase III, des
SCHÉMA :
Second-line DST using MGIT 960 TB
isolats cliniques réels ont été testés pour valider les concentrations critiques optimales déterminées dans les Phases I et II. R É S U LTAT S : Nous avons établi les concentrations critiques pour sept médicaments de deuxième ligne par le système Bactec MGIT 960 en recommandant les données suivantes : amikacine 1,0 μg/ml ; capréomycine 2,5 μg/ ml ; kanamycine 2,5 μg/ml ; ofloxacine 2,0 μg/ml ; moxifloxacine 1,0 μg/ml : éthionamide 5,0 μg/ml ; acide paraaminosalicylique 4,0 μg/ml.
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Les résultats de notre étude démontrent que le système Bactec MGIT 960 est une méthode précise et fiable pour les tests rapides de sensibilité (DST) de Mycobacterium tuberculosis à l’égard des médicaments de deuxième ligne. Dans cette étude, la résistance aux fluoroquinolones a été moins représentée et des DST complémentaires sont indispensables pour ce groupe.
CONCLUSION :
RESUMEN O B J E T I V O : Definir las concentraciones inhibitorias mínimas (CIM) para siete medicamentos antituberculosos de segunda línea con el sistema Bactec MGIT 960 TB y comparar su eficacia con la del sistema Bactec 460 TB. M É T O D O S : El presente estudio se llevó a cabo en tres etapas. En la primera etapa, se evaluaron cepas sensibles a todos los medicamentos, a fin de definir las CIM ; en la segunda etapa, se evaluaron casi exclusivamente cepas resistentes, a fin de determinar la concentración discriminatoria óptima de la prueba ; y en la tercera etapa, se evaluaron aislados clínicos verdaderos, con el propósito de validar las concentraciones críticas óptimas definidas en las dos primeras etapas. R E S U LTA D O S : Se establecieron las concentraciones crí-
ticas para siete medicamentos de segunda línea en el sistema Bactec MGIT 960 con las siguientes recomendaciones : amikacina 1,0 μg/ml ; capreomicina 2,5 μg/ml ; kanamicina 2,5 μg/ml ; ofloxacino 2,0 μg/ml ; moxifloxacina 1,0 μg/ml ; etionamida 5,0 μg/ml y ácido paraaminosalicílico 4,0 μg/ml. C O N C L U S I Ó N : Los resultados indican que el sistema Bactec MGIT 960 es un método preciso y fiable de evaluación rápida de la sensibilidad (DST) de Mycobacterium tuberculosis a los medicamentos de segunda línea. En el presente estudio, las cepas resistentes a fluoroquinolona fueron escasas, por lo cual se precisan nuevas DST para este grupo.
