Reproducibility of ethambutol (EMB) susceptibility test results forMycobacterium tuberculosis has always been difficult for a variety of reasons, including the narrow range between the critical breakpoint for EMB resistance and the MIC for susceptible strains, borderline results obtained with the BACTEC 460TB method, the presence of microcolonies determined using the agar proportion (AP) method, and a lack of agreement between these two testing methods. To assess the frequency of these problems, M. tuberculosis drug susceptibility data were collected in a multicenter study involving four laboratories. Resistant, borderline, and susceptible isolates were shared among the laboratories to measure interlaboratory test agreement. Half of isolates determined by BACTEC 460TB to be resistant were determined to be susceptible by the AP method. Isolates determined to be resistant to EMB by both BACTEC 460TB and AP methods were almost always resistant to isoniazid. Results from isolates tested by the BACTEC 460TB method with an EMB concentration of 3.75 g/ml in addition to the standard 2.5 g/ml did not show improved agreement by the AP method. While these results do not indicate that the AP method is more accurate than the BACTEC 460TB method, laboratories should not report EMB monoresistance based on BACTEC 460TB results alone. Monoresistance to EMB should only be reported following confirmation by the AP method. Microcolonies could not be confirmed as resistant by the BACTEC 460TB method or by repeat testing with the AP method and do not appear to be indicative of resistance.Radiometric detection of bacterial growth (BACTEC 460TB system; Becton Dickinson and Company, Sparks, Md.) is the most commonly used method in the United States for determining resistance to the primary drugs used to treat Mycobacterium tuberculosis disease (15). This technique was designed to provide rapid susceptibility test results for streptomycin (SM), isoniazid (INH), rifampin (RIF), and ethambutol (EMB) that are equivalent to those obtained by the reference agar proportion (AP) method using Middlebrook agar (7,9,10,12,13).Testing of M. tuberculosis for susceptibility to EMB can be problematic by both the radiometric and AP methods. This may be due to the bacteriostatic nature of EMB, the reduced activity of the drug in a culture medium, or the narrow range between the MICs of susceptible and resistant isolates of M. tuberculosis (4, 6). While the radiometric method has been modified over the years, whether it accurately determines susceptibility to EMB remains in question (5,9,14,16,17). Decisions are unclear on the interpretation and reporting of small colonies of mycobacteria (microcolonies) as resistant mutants on the EMB drug quadrant by AP testing (11). To characterize the extent of these problems with EMB susceptibility testing and provide further information for assistance with test interpretation, we collected and analyzed data from four public health laboratories in a multicenter study. MATERIALS AND METHODSStudy design. Mycobacteriol...

Laboratories and laboratory networks are a fundamental component of tuberculosis (TB) control, providing testing for diagnosis, surveillance and treatment monitoring at every level of the health-care system. New initiatives and resources to strengthen laboratory capacity and implement rapid and new diagnostic tests for TB will require recognition that laboratories are systems that require quality standards, appropriate human resources, and attention to safety in addition to supplies and equipment. To prepare the laboratory networks for new diagnostics and expanded capacity, we need to focus efforts on strengthening quality management systems (QMS) through additional resources for external quality assessment programmes for microscopy, culture, drug susceptibility testing (DST) and molecular diagnostics. QMS should also promote development of accreditation programmes to ensure adherence to standards to improve both the quality and credibility of the laboratory system within TB programmes. Corresponding attention must be given to addressing human resources at every level of the laboratory, with special consideration being given to new programmes for laboratory management and leadership skills. Strengthening laboratory networks will also involve setting up partnerships between TB programmes and those seeking to control other diseases in order to pool resources and to promote advocacy for quality standards, to develop strategies to integrate laboratories' functions and to extend control programme activities to the private sector. Improving the laboratory system will assure that increased resources, in the form of supplies, equipment and facilities, will be invested in networks that are capable of providing effective testing to meet the goals of the Global Plan to Stop TB.

Under the direction of the US Global AIDS Coordinator's Office, Department of Health and Human Services, the CDC Global AIDS programme helps resource-constrained countries to address the global HIV/AIDS pandemic. Activities include laboratory capacity and laboratory infrastructure development in 25 resource-constrained countries. Medical practitioners and public health programme leaders in industrialized countries rely on the use of quality laboratory data for evidence-based medical decision-making to determine policy for the implementation of disease control measures, to monitor disease to determine the impact of control programmes, and to support surveillance activities. In these countries, laboratory data to support decision-making processes have a level of quality attributable to laws, regulations and guidelines developed over many years. However, resource-constrained countries have not had similar experiences. Few countries have developed laws, regulations or guidelines, nor is there a data-use culture (e.g. evidence-based medicine) for those in the decision-making environment in resource-constrained countries. The strengthening of laboratory capability and capacity in resource-constrained countries is an important goal to improve accurate and reliable data for the diagnosis, treatment and monitoring of disease.A process for the implementation of a quality systems approach for a laboratory is presented: (i) acknowledgement of the need to improve the laboratory programme in the country at the Ministry of Health and at all decision-making levels within the provinces/states of the country; (ii) assessment of capabilities, capacities, infrastructure, and training needs; (iii) implementation of a national meeting of laboratorians; (iv) designation of a national Quality Assurance Office and leadership within that office; (v) the development and provision of technical training.

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