we would need 784 cases ( i.e. , 1.960.050.5(1  0.5)/(0.07/2 ) = 784 ) to achieve 95% precision and a margin of error of 7% , and assume no prior knowledge of prevalence of drug resistance , to measure prevalence of any form of drug resistance across the entire province ( i.e. , not enabling stratification ) in each year . assuming that > 10% of samples would be lost , we sought to obtain 900 cases . with no prior information for prevalence of drug resistance at different sites , we randomly selected 30 tb treatment centers in 30 counties ( among 90 centers in zhejiang province ) and anticipated that each center would recruit > 30 sputum smear  positive patients . three surveys were conducted at the same 30 sites to obtain the same sample size in each year ( 5 ) . sputum was collected from persons with suspected tb who came to clinics for a diagnosis . three sputum samples were obtained ( morning , midday , and evening ) , and patients with > 2 samples with positive sputum smear results were enrolled in the study . drug sensitivity tests were performed in provincial reference laboratories by using the percentage method , and results were compared with results for standard drug - resistant strains ( 6 ) . quality of provincial reference laboratories was ensured by the republic of korea supranational reference laboratory ( seoul , south korea ) during 3 surveys , and was evaluated annually by the national reference laboratory in china . recruitment of consecutive case - patients continued until > 30 ( often more ) were enrolled at each site . new cases , re - treatment cases , and cases of mdr tb were defined as described by the world health organization ( 5 ) . prevalence of resistance to isoniazid and rifampin or mdr tb was defined as the number of resistant cases in patients who were given a diagnosis of tb in clinics and tested for drug resistance . although surveys were not designed a priori to evaluate time trends for prevalence of drug resistance , we investigated trends by using repeated measures analysis of variance and making appropriately cautious conclusions . logistic regression models were used to investigate factors associated with single drug resistance and mdr tb . totals of 1,013 , 984 , and 938 mdr tb case - patients were recruited from routinely diagnosed new and re - treatment case - patients in 1999 , 2004 , and 2008 , respectively ( table 1 ) . in the 3 surveys , 71% , 74% , and 69% of cases were in men , and 17% , 16% , and 10% were re - treatment cases . * in the 3 surveys , average prevalence of new cases resistant to isoniazid and rifampin and having mdr tb was 10.5% ( 95% ci 8.4%12.5% ) , 5.1% ( 95% ci 3.6%6.6% ) , and 3.3% ( 95% ci 2.1%4.5% ) , respectively . equivalent percentages among re - treatment cases were 38.5% ( 95% ci , 28.8%48.2% ) , 33.3% ( 95% ci 23.9%42.7% ) , and 29.2% ( 95% ci 20.1%38.3% ) , respectively . compared with new cases , re - treatment cases were more likely to be resistant to isoniazid ( odds ratio [ or ] 1.8 , 95% ci 1.22.7 ) and rifampin ( or 6.3 , 95% ci 4.29.5 ) or to have mdr tb ( or 9.0 , 95% ci 6.412.7 ) ( table 2 ) . resistance to isoniazid was strongly associated with resistance to rifampin and vice versa ( models 1 and 2 ; or 19.9 , 95% ci 1331 ) ( table 2 ) . * reference groups are > 65 years for age , female for sex , and new for re - treatment . or , odds ratio ; mdr tb , multidrug - resistant tuberculosis . prevalence of resistance to isoniazid and mdr tb tended to be lower in case - patients 1564 years of age than in those > 65 years of age ( models 1 and 3 ) , but this effect was not shown for rifampin ( model 2 ) ( table 2 ) . there was no significant difference in prevalence of resistance between male and female case - patients across all surveys ( models 13 ) ( table 2 ) . prevalence of isoniazid and rifampin resistance and mdr tb changed little across the 3 surveys among new and re - treatment cases ( table 1 ) . time trends for isoniazid prevalence ( increase ) and mdr tb ( decrease ) among new cases were marginally significant ( f  =  3.33 , p<0.05 , and f  =  1.13 , p<0.05 ) but in opposite directions . there were no significant trends in resistance among re - treatment cases or among men or women . approximately 25% of the world s mdr tb cases are in china , and it is vital to know whether this number is increasing , decreasing , or stable . there are few data with which to judge trends in drug resistance in china , although a few regions , including shanghai municipality ( 7 ) , shenzhen province ( z. jia , y. yong , unpub . data ) , and zhejiang province ( this study ) , have conducted cross - sectional surveys . the principal finding of our study is that although drug - resistant tb needs careful management in zhejiang province ( 6% of all tb cases in 2008 were mdr tb and resistance to second - line drugs has also been found in the province ; x. wang , unpub . data ) , prevalence of isoniazid and rifampin resistance and mdr tb monitored at the same 30 sites changed little during 19992008 . although surveys were not designed to detect time trends in drug resistance , prevalence of mdr tb decreased from 8.6% in 1999 to 6.0% in 2008 . this decrease in zhejiang was consistent with changes observed during 20002010 in 2 national tb prevalence surveys ( 3,4 ) . our results contrast with those that mdr tb prevalence increased in nearby shanghai during 20002006 . shen et al . ( 7 ) reported that introduction of directly observed treatment , short course , and other improved management practices contained spread of drug resistance in shanghai after 2004 , and introduction of similar practices in zhejiang may also have stopped the increase in mdr tb after 2002 . however , the role of improved tb control practices can not be shown from these data . nevertheless , possible differences among different sites underline the need for monitoring resistance trends locally and nationally in china . it is also necessary to monitor treatment outcomes , which will be linked to development of drug resistance . in this context , the percentage of patients who sought re - treatment was lower in 2008 than in previous years , which suggested that case management had improved . the greatest risk factor for resistance to either isoniazid or rifampin in this study was resistance to the other drug , a finding that indicates the high risk for acquiring mdr tb after treatment failure . in this context , and consistent with previous studies ( 8,9 ) , prevalence of mdr tb was higher among re - treatment cases than new cases . these results also underscore the need for following good management practices as described by the world health organization ( 10 ) .
to evaluate levels and trends in drug - resistant tuberculosis ( tb ) in zhejiang province , china , we conducted 1 survey in each of 3 years ( 1999 , 2004 , and 2008 ) . 
 we found that < 5% of new cases were multidrug - resistant tb . 
 the prevalence of multidrug - resistant tb has not increased in new or re - treated cases in this province .