of diagnosis and treatment across this broad range of drug resistance, with eight speakers considering both the general topic and treatments directed at specific points along the spectrum of resistance. This session also featured an extended discussion, summarized at the end of this chapter, of the scientific validity and societal implications of using the term “untreatable” TB.
Patients with MDR and XDR TB exhibit a wide variety of resistance patterns, noted Richard E. Chaisson, Professor of Medicine, Epidemiology, and International Health, Center for TB Research, Johns Hopkins University. To illustrate, Chaisson cited recent data from China (Zhao et al., 2012). According to a national survey conducted by the China CDC, 5.7 percent of new cases and 25.6 percent of retreatment cases had MDR TB. Eleven percent of the new cases and 18 percent of the retreatment cases had what Chaisson called “simple MDR”—resistance to two drugs. At the other end of the spectrum, 8 percent had XDR TB. In the middle, 55 percent of MDR TB patients had ethambutol resistance, and 32 percent had resistance to kanamycin or ofloxacin.
Similarly, in a study of more than 1,200 patients with MDR TB from eight countries, half had resistance to all FLDs tested, and a large proportion had resistance to SLDs as well (Dalton et al., 2012). More than 40 percent had resistance to at least one SLD, 20 percent to an injectable, and 13 percent to a fluoroquinolone, and 6.7 percent had XDR TB. Thus, resistance is highly heterogeneous in this sample, and studies of other populations have found similar variety (Salvo et al., 2012).
Clinicians frequently know nothing about this heterogeneity when they make treatment decisions. As pointed out earlier in the workshop, new technologies for detecting and characterizing MDR TB work very quickly. In real life, however, other problems can delay the results of drug susceptibility tests. In laboratories studied in South Africa, for example, the availability of Hain genotyping reduced the period from sputum collection to availability of drug susceptibility results in half, but 26 days still separated the two (Hanrahan et al., 2012). Furthermore, the median time to the initiation of treatment remained 62 days because of the challenges of implementing the test in a clinical setting.
There is a large menu of drugs for the treatment of MDR TB, Chaisson pointed out (Table 9-1), some newly developed and others older and with unclear efficacy. But patients can be treated only with those drugs that are available in the settings where they are being treated. If a preferred drug
1 This section is based on the presentation by Richard E. Chaisson, Professor of Medicine, Epidemiology, and International Health, Center for TB Research, Johns Hopkins University.