MDR-TB usually develops during treatment of fully sensitive TB when the course of antibiotics is interrupted and concentrations of active drugs in the body are insufficient to kill 100% of bacteria. This can happen for a number of reasons: Patients may feel better and halt their antibiotic course, drug supplies may run out or become scarce, or patients may forget to take their medication from time to time. Although mutations that cause resistance to certain drugs may also cause lower fitness in the mutated M. tuberculosis
strains, selected mutation sites may allow MDR M. tuberculosis
strains to spread from person to person as readily as drug-sensitive TB and in the same manner 
The results of our study show a high level drug resistance among the admitted patients of the tertiary reference hospital, the National Institute of Diseases of the Chest and Hospital situated in Dhaka city. The results are comparable with the results of the study involving randomly selected patients having clinical and/or radiological features of tuberculosis attending the same hospital 
. Among 363 cases, resistance rates for INH, RMP, EMB and STM were 76.3%, 71.6%, 27.6% and 55.7% respectively with a total of 221(60.9%) cases detected as MDR-TB 
. Our study also shows very high level of MDR cases (73.5%) among the referred cases attending this tertiary institution. Global report on multidrug and extensively drug-resistant TB surveillance and response by WHO in 2010 highlighted the results of a population based case study by the Damian Foundation, Bangladesh, which showed that 28% of the 599 previously treated cases notified in 2008 had confirmed MDR-TB, with particularly high risk of MDR-TB among cases failing treatment 
. Among cases that failed an initial treatment regimen, 58% had MDR-TB. Among those that failed a Category II retreatment regimen, 91% had MDR-TB 
. This indicates the higher degree of drug resistance among patients who received anti-TB treatment previously or those having chronic TB. In previously treated patients, the chances of resistance to any drug is over 4-fold higher, and of MDR-TB over 10-fold higher, in comparison to untreated patients 
. The number of previously treated cases in the country correlate with the overall prevalence of drug resistance, as drug resistance is more commonly found in cases with previous history of anti-TB medication. Cases with previous treatment ranged from 4.4% to 26.9% of all registered patients under DOTS programmes in countries with a high burden of TB 
. In case of China and India, the two largest high-TB burden countries, 20% of sputum smear-positive cases had prior history of treatment 
. Retrospective chart review based on positive cultures isolated in a high volume mycobacteriology laboratory in Christian Medical College Vellore, India between 2002 and 2007 examined 47 XDR, 30 MDR and 117 susceptible controls. Drug resistant cases were less likely to be extrapulmonary, and mostly had received previous treatment regimens 
In this study almost half of the study subjects were from the Dhaka division, the number of MDR cases was higher in this location than other areas of Bangladesh. The results of our study showed that males were more commonly infected with TB (75%), including MDR-TB (75%) than females. This trend is also observed in some other studies 
. In Western Europe MDR-TB cases were more common in males while, there was no association of MDR-TB with the male gender in Eastern Europe 
. One of the underlying causes for high rate of MDR-TB among males is believed to be the reduced adherence of males to treatment compared to females. Another important finding of our study was the higher incidence of MDR-TB among the patients below the age of 45 years (76%). This finding has also been observed in other studies 
. This is possibly due to the exposure of elderly persons to the organisms in the past, when the circulating bacilli were susceptible, and during the process of treatment, resistance was acquired while young patients are more likely to have acquired the bacilli more recently when they were more likely to be resistant. However, to confirm the findings of this study it is important to perform a molecular epidemiological study with a larger sample size.
In this study the strains were also characterized by deletion analysis and spoligotyping. Deletion analysis using RD9 revealed that all the samples investigated were M. tuberculosis
. Screening for TbD1, the ‘M
specific deletion region 1′ was done for all the samples. The TbD1 analysis employed in this study showed that the majority of the isolates (69%) belonged to the TbD1- modern strains, whereas this region was found intact among 59 (31%) samples and these strains were named ancestral type M. tuberculosis
strains because they belong to a lineage of strains that divided from all other M. tuberculosis
strains before the deletion of TbD1- occurred 
. The percentage of modern strain was higher in this study than the study conducted in rural area of Bangladesh 
. Multi drug resistance was significantly higher among the modern type strains than the ‘ancestral’ strains (data not shown).
In our study the most predominant spoligotype of strains was the Beijing genotype, it represented about 19% (36 strains) of all isolates. The majority of the Beijing family members originated from the province of Beijing in China, and strains of this family are highly prevalent in many Asian countries 
. Rate of infection with Beijing family strains are higher in Asia than those in the more distant countries, suggesting that the Beijing family may have radiated from the Beijing area to other regions. Almost all of the Beijing isolates were from the patients who were previously treated with antitubercular drugs for certain periods. Previous data showed that Beijing strains are often associated with drug resistant TB 
. Majority of the Beijing strains (75%) were found to be MDR in this study. Previous study showed that 31% strains were of Beijing type which is higher than what has been found in this study 
. In addition to the prevalent Beijing family strains, the second most frequently occurring strains showed spoligotype characteristic of the “T” family. Among the T family, T1 was most predominant (70%) in which spacer 33 to 37 along with spacer 4, 5 were absent. Among the T family 21 isolates were MDR. Although the T family is one of the most prevalent types, it remains an ill-defined family of M. tuberculosis
that is found worldwide 
. It has been suggested from our study findings that strains of this family are also prevalent in Bangladesh. Our results suggest that emergence of drug resistant M. tuberculosis
strains belonging to the ‘Beijing’ and the ‘T’-types remains a serious threat to the local TB control program.
At the time of planning of this study we didn’t have an accurate idea of drug resistance nationally. As the NIDCH had patients from all over the country, the intention was to get deeper insights into the drug sensitivity profile and the strain pattern in a cohort of patients admitted to this tertiary referral hospital. In our NTP treatment strategy, patients with PTB are usually treated in outpatient department (DOTS corner) unless the patient is seriously ill or associated with other complications or non responsive to the applied chemotherapy. According to the national TB control guideline suspected MDR-TB cases from all over the country are usually referred to NIDCH. Thus, the patients visiting this health facility are representative of the country as a whole. However, it should be mentioned that among the enrolled patients, re-treatment cases and chronic TB cases were frequent, which might have contributed towards the relatively high number of patients with drug resistant TB identified, that therefore may not reflect the community status.
In summary, the drug-resistance rate of PTB, especially MDR-TB, was higher in patients with previously incomplete anti-tuberculosis treatment at a tertiary referral hospital in Bangladesh. A high level of drug resistance among the re-treatment TB patients poses a threat of transmission of resistant strains to susceptible persons in the community. For these reasons molecular characterization and determination of individual drug resistance of patient isolates is of importance. Proper counseling of patients and attention towards the completion of the anti-TB treatment are needed. In TB prevalent areas, more studies on anti-TB drug resistance preferably population based continuous surveillance should be carried out.