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Drug-resistant Tuberculosis

6. Why is it difficult to gather information on drug-resistant tuberculosis?

    Traditional skin tests do not detect drug resistance.
    Traditional skin tests do not detect drug resistance.
    Source: Greg Knobloch

    In order to gather information on drug-resistant tuberculosis it is necessary organize surveys and repeat them. This requires sufficient laboratories capable of performing drug resistance tests, staff to interview and classify the patients, and a transport network to send samples for analysis to different laboratories inside and outside the country. These facilities are not available in all countries, particularly those where relatively large proportions of the population are affected. It would also be desirable to have large samples of patients classified in sub-categories according to their previous treatment history, to test patients for HIV and to perform second-line drug resistance tests. All these come at great additional expense and workload. That is why surveys tend to be repeated infrequently.

    In order to limit the number of samples, current survey methods are based on smear positive cases. Since many HIV co-infected TB cases are smear negative, this approach may underestimate drug resistance among those infected with both HIV and TB. In addition, surveys tend to focus on new cases and to exclude prevalent cases and patients in the private sector. Including these cases would substantially increase the cost and the workload of the surveys, and the improvement in results may not be worth it.

    It must also be borne in mind that the current survey methods do not allow to reliably differentiate between primary and acquired resistance.

    In order to gather more data and determine trends in countries with a high TB burden, surveys must be simplified. One option could be the use of new, rapid methods to test drug resistance, such as the one being evaluated in a study in Tanzania. Although these rapid tests would only be useful for measuring resistance to one or two anti-TB drugs, the testing would not require specialized laboratories or the transport of infectious samples. Where resources are limited, the priorities are to detect multidrug-resistant tuberculosis (MDR-TB) and extensively drug-resistant tuberculosis (XDR-TB). Samples could be tested first for resistance to first-line drugs and the ones that give positive results could be tested subsequently for resistance to second-line antibiotics. Alternatively, all samples could be tested for extensive drug-resistance from the outset.

    To assess trends in drug resistance over time, patients could be classified by treatment history on a routine basis and a certain number of samples could be tested each month. The process would be even simpler and more effective if the tests could be done at the time of consultation. All patients diagnosed with MDR-TB would be further screened for XDR-TB and enrolled on treatment.

    It is important to realize that different information is gathered for different purposes: to know the extent of the disease and its evolution over time, to monitor and improve the tuberculosis programmes or to answer research questions. Many countries are conducting several surveys at the same time to address each of these questions.

    This text is a summary of: WHO,  Anti-Tuberculosis Drug Resistance in the World, Fourth Global Report (2008), Chapter 4: Discussion, Survey methods (p.79-81) See also Chapter 2: Methods &Executive summary: background and methods


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