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Journal of Clinical Microbiology, October 2008, p. 3526-3529, Vol. 46, No. 10
0095-1137/08/$08.00+0     doi:10.1128/JCM.02083-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Inter- and Intra-Assay Reproducibility of Microplate Alamar Blue Assay Results for Isoniazid, Rifampicin, Ethambutol, Streptomycin, Ciprofloxacin, and Capreomycin Drug Susceptibility Testing of Mycobacterium tuberculosis ^,

Brian Leonard,¹ Jorge Coronel,¹ Mark Siedner,¹ Louis Grandjean,¹ Luz Caviedes,¹ Pilar Navarro,¹ Robert H. Gilman,^1,2,3,4 and David A. J. Moore^1,2,3,4*

Laboratorio de Investigación de Enfermedades Infecciosas, Facultad de Ciencias y Filosofía, Universidad Peruana Cayetano Heredia, Av. Honorio Delgado No. 430, San Martín de Porras, Lima 31, Perú,¹ Department of International Health, Johns Hopkins Bloomberg School of Public Health, 615 North Wolfe St., Room 5515, Baltimore, Maryland 21205,² Asociación Benéfica PRISMA, Carlos Gonzales No. 251, San Miguel, Lima 32, Perú,³ Department of Infectious Diseases and Immunity and Wellcome Centre for Clinical Tropical Medicine, Faculty of Medicine (Hammersmith Campus), Imperial College London, DuCane Road, London W12 0NN, United Kingdom⁴

Received 27 October 2007/ Returned for modification 4 February 2008/ Accepted 8 July 2008

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The intersample and intrasample variability of the results obtained with the microplate Alamar blue assay for the indirect drug susceptibility testing of Mycobacterium tuberculosis was investigated. Between 1.2 and 8.5% of paired MICs differed by more than one twofold dilution, resulting in discordant susceptible-resistant designations at frequencies between 0.6% (rifampin) and 18.9% (ethambutol).

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The widespread need for rapid, accurate tuberculosis (TB) drug susceptibility testing (DST) is highlighted by the steady emergence of multidrug-resistant and extensively drug-resistant TB (1, 5). As more laboratories utilize low-cost DST methods (10), it is essential to determine if the methodologies currently in use are robust and provide accurate, reproducible results.

The inexpensive microplate Alamar blue assay (MABA) is an indirect colorimetric DST method for determining the MICs of TB drugs for strains of Mycobacterium tuberculosis (10). In this assay, the redox indicator Alamar blue turns from blue to pink in the presence of mycobacterial growth. When compared with "gold standards" such as BACTEC 460 (3, 4) and the proportion method (12), the MABA has been shown to give results with high levels of agreement for rifampin (RIF) and isoniazid (INH) and lower levels of concordance for ethambutol (EMB) and streptomycin (SM). In view of the lack of published data on the reproducibility of MABA results, and in recognition that variability in MIC results is clinically important only for MICs near the critical concentration of the drug, this analysis addresses the following questions: for each of six antimycobacterial drugs, what is the MIC drift (how much do MICs vary) upon repeated microplate Alamar blue testing and how important is this variability in defining strains as susceptible or resistant?

Strains were first harvested from sputum cultures positive for M. tuberculosis by the microscopic observation drug susceptibility assay (MODS), which uses Middlebrook 7H9 liquid medium (11), by Lowenstein-Jensen (LJ) culture, or by both methods (Fig. 1). These strains were then each subjected to indirect DST by the MABA (3, 4, 6, 12, 13). We analyzed the internal and external reproducibility of MIC results and susceptible-resistant designations by (i) reviewing the consistency of assay results for strains from the same sample cultured on different media and (ii) comparing results for strains from consecutive, but distinct, sputum samples from a single patient, cultured on the same medium. The parent study for this analysis was an evaluation of novel diagnostics for TB and multidrug-resistant TB described in detail previously (11). Patients contributed one or two samples within the same week, and the samples were cultured in parallel in MODS and LJ media. In the parent study, MABA results were used for discrepant analyses when reference test results were discordant, and thus, the MABA was performed on all strains from both MODS and LJ cultures. An analysis of these MABA results is reported here.

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FIG. 1. Derivation of strains tested by the MABA (designated MABA_MODS-1, MABA_LJ-1, MABA_MODS-2, and MABA_LJ-2). The top row corresponds to the subject, the second row corresponds to sputum samples, the third row corresponds to cultures, and the bottom row corresponds to the strains derived from each culture. (A) Intrasample analysis. (B) Intersample analysis of paired MODS cultures. (C) Intersample analysis of paired LJ cultures.

The MABA was performed as described previously (see appendix Fig. S1 in the supplemental material); briefly, 200-µl volumes of sterile deionized water were added to outer-perimeter wells of sterile 96-well plates. Wells in columns 3 to 11 of rows B to G received 100 µl of Middlebrook 7H9-oleic acid-albumin-dextrose-catalase broth. Aliquots of 100 µl of the highest-concentration drug solutions were added to each well in columns 2 and 3; after mixing, a 100-µl sample was transferred from each column 3 well to the corresponding well in column 4, and the contents of the column 4 wells were then thoroughly mixed. Identical serial 1:2 dilutions were continued until desired concentrations were reached; 100 µl of excess medium was discarded from each column 10 well. Final drug concentration ranges were as follows: INH, 0.125 to 32.0 µg/ml; RIF, 0.063 to 16 µg/ml; SM, 0.125 to 32.0 µg/ml; EMB, 0.5 to 128 µg/ml; capreomycin (CAP), 0.031 to 8 µg/ml; and ciprofloxacin (CIP), 0.063 to 16 µg/ml. M. tuberculosis suspensions at a McFarland standard of 1 were prepared from LJ or MODS cultures and diluted 1:25 in Middlebrook 7H9-oleic acid-albumin-dextrose-catalase to produce samples of 100 µl, which were added to wells in columns 2 to 11 of rows B to G, yielding a final well volume of 200 µl. Column 11 wells served as drug-free (inoculum-only) controls. Plates were sealed in individual Ziplock bags and incubated at 37°C for 5 days, after which the first control well (B11) was examined under an inverted light microscope for evidence of growth; if growth was observed, a freshly prepared 50-µl 1:1 mixture of Alamar blue (Trek Diagnostic Systems, OH) and 10% Tween 80 was added to this well. Plates were reincubated for 24 h, and if well B11 turned pink, the reagent mixture was added to all wells; if the well remained blue, the next control well (C11) was examined for growth and the reagent mixture was added if growth was observed. After the first day that a control well turned pink and reagent was added to all wells, the microplate was resealed and incubated for an additional 24 h at 37°C, after which all well colors were recorded. Blue was interpreted to indicate no growth, and pink was interpreted to indicate growth. The MIC was defined as the lowest drug concentration which prevented a blue-to-pink color change.

Two matched-pair analyses were performed using STATA 9.0 (Stata Corporation, College Station, TX). First, the general reproducibility of MABA results was examined by comparing the results for two samples provided by the same patient. Second, the intra-assay reproducibility of MABA results was evaluated for single samples that were aliquoted and cultured simultaneously in both LJ and MODS media. The percent agreement and the kappa statistics (to determine agreement beyond chance) were calculated for paired MABA results. CAP currently has no defined MABA breakpoint for susceptibility, so the resistant-susceptible designation analysis was not performed. DST thresholds employed for the other drugs (2, 8, 10) were as follows: INH susceptibility, MIC of 0.25 µg/ml, and INH resistance, MIC of 0.5 µg/ml; RIF susceptibility, MIC of 1.0 µg/ml, and RIF resistance, MIC of 2.0 µg/ml; EMB susceptibility, MIC of 2.5 µg/ml, and EMB resistance, MIC of 4.0 µg/ml; SM susceptibility, MIC of 1.0 µg/ml, and SM resistance, MIC of 2 µg/ml; and CIP susceptibility, MIC of 1.0 µg/ml, and CIP resistance, MIC of 2.0 µg/ml.

Three-hundred and thirty samples yielded positive cultures in both MODS and LJ media; the levels of concordance of paired MICs of each drug and the resulting susceptible-resistant designations are shown in Table 1. For 128 patients, paired LJ culture-positive samples were available, and for 153 patients, paired MODS culture-positive samples were available for comparison; the concordance of paired MICs of each drug and the measures of discordance regarding susceptibility designations in the intersample analysis are shown in Table 2. The cross-tabulated raw data, with corresponding scatterplots, are available in the appendix in the supplemental material.

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TABLE 1. Concordance of MIC measurements and susceptible-resistant assignments in the intrasample analysisa

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TABLE 2. Concordance of MIC measurements and susceptible-resistant assignments in the intersample analysisa

This analysis demonstrates that, regardless of the drug tested, MABA result reproducibility is not affected by whether the tested isolate is derived from LJ or MODS (Middlebrook 7H9) cultures, and intra- and intersample comparisons showed similar degrees of variability. The MIC drift varied according to the drug under consideration, with higher levels of variability noted for EMB, SM, CIP, and CAP than for INH and RIF. This variability translated into important discrepant susceptible-resistant assignments for all drugs except INH and RIF. Even paired MICs of the drugs with which the MABA performed best (INH and RIF) showed only moderate concordance. However, for these two drugs, this discrepancy had little impact on susceptible-resistant assignments because the MIC drift was generally over concentrations distant from the breakpoint.

Two limitations of this analysis were that sensitivity results for CIP were skewed due to the low number of resistant strains and that the lack of a defined MABA breakpoint for CAP precluded the assessment of the importance of CAP MIC drift.

This analysis shows that while the paired-MIC correlation was moderate at best (kappa > 0.5) (7), the majority of pairs differed by one doubling dilution or less, as a result of which the susceptible-resistant assignments were generally robust except for EMB, an agent for which DST is recognized to be challenging (9). The MABA generates more information than is generally needed by clinicians, who need to know about susceptibility or resistance for effective patient care and who rarely require specific MICs. We recommend that MABA results be reported as susceptible or resistant and that, if the measured MIC is within one dilution of the breakpoint, the assay be repeated using the same isolate.

 
This study and D.A.J.M. were supported by the Wellcome Trust (grant no. 064672).

Sincere thanks are due to the numerous medical and laboratory staff members at community clinic and hospital study sites who ensured smooth running of the parent study protocol, particularly Yuri García, Adolfo Orellana Marin, and Raul Miranda Arrostigue (CS Carlos Cueto Fernandini); Guillermo Vera Mallqui (CBS Los Olivos); Luis Rivera Pérez (CS Infantas); Walter Ramos Maguiña (CS Villa Norte); Luz Vásquez Chávez (CS Primavera); Félix Pari Loayza (CEMI Juan Pablo II); Ruth Flores Escobar (PS Los Olivos de Pro); Jesús Castillo Diaz (CS-CLAS Laura Caller Iberico); Milciades Reátegui Sanchez (CS-CLAS San Martín de Porres); and Alicia Vigo Alegria (PS Enrique Milla Ochoa), and to laboratory, support, and field staff members at Universidad Peruana Cayetano Heredia and Asociación Benéfica PRISMA, particularly Paula Maguiña, Fanny Garcia, Eleana Sanchez, Yrma Chuquiruna, Rosmery Gutierrez, Sonia Lopez, Christian Solis, Indira Villaverde, and Pilar Navarro.

 
* Corresponding author. Mailing address: Laboratorio de Investigación de Enfermedades Infecciosas, Facultad de Ciencias y Filosofía, Universidad Peruana Cayetano Heredia, Av. Honorio Delgado No. 430, San Martín de Porras, Lima 31, Perú. Phone: 511 382 3398. Fax: 511 464 0781. E-mail: dajmoore{at}imperial.ac.uk

Published ahead of print on 13 August 2008.

Supplemental material for this article may be found at http://jcm.asm.org/.

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Journal of Clinical Microbiology, October 2008, p. 3526-3529, Vol. 46, No. 10
0095-1137/08/$08.00+0     doi:10.1128/JCM.02083-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

This Article Abstract Full Text (PDF) Supplemental material Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Reprints and Permissions Copyright Information Books from ASM Press MicrobeWorld Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Leonard, B. Articles by Moore, D. A. J. Search for Related Content PubMed PubMed Citation Articles by Leonard, B. Articles by Moore, D. A. J.