detailed clinical and social data were collected by case managers for all tb patients undergoing treatment in london on july 1 , 2003 .
analyses were restricted to pulmonary patients 1560 years of age ( n = 970 ) .
we used univariate analyses to compare the characteristics of crack cocaine users , other hard - drug users ( predominantly heroin users but excluding those who used only alcohol and marijuana ) , and those not known to use drugs .
a separate category was included for hard - drug users not known to use crack cocaine to have a group with comparable levels of social deprivation , addiction related problems , and difficulty in accessing health services . to test the hypothesis that smear positivity at diagnosis
was associated with crack cocaine use , we used a multivariate model with backwards elimination to exclude variables that did not make a significant contribution to the model .
variables initially included are shown in table 1 ; the final model is shown in table 2 . *
tb , tuberculosis ; ed , emergency department ; mdr , multidrug resistant ; inh , isoniazid ; dot , directly observed therapy . * or ,
odds ratio ; ci , confidence interval ; inh , isoniazid resistant ; mdr , multidrug - resistant ; ed , emergency department .
drug users to have been born in the united kingdom , of white or black caribbean ethnic origin , homeless , alcohol abusers , or have a history of imprisonment .
non  crack drug users tended to have the longest delays between diagnosis and treatment and crack users the shortest , but this tendency did not reach significance ( table 1 ) . crack cocaine users were statistically significantly more likely to seek treatment at emergency departments , to adhere poorly to treatment regimen , or default from treatment altogether .
drug users were also more likely to have isoniazid - resistant disease . among crack cocaine users
this was primarily related to a large outbreak of isoniazid - resistant tb ( 11 ) . among crack cocaine users
, diagnosis showed that 86% were smear positive compared with 36% of patients not known to use drugs ( relative risk [ rr ] 2.4 , 95% confidence interval [ ci ] 2.02.9 ) , p<0.001 ) and 59% of drug users not known to use crack cocaine ( rr 1.6 , 95% ci 1.42.0 , p<0.001 ) .
multivariate analysis showed that the risk for smear - positive disease was higher for drug users than for those not known to use drugs ( odds ratio [ or ] 1.9 , 95% ci 1.23.0 , p = 0.007 ) and highest in crack cocaine users ( or 6.6 , 95% ci 1.824.3 , p = 0.005 ) .
other significant risk factors for smear positivity were being of black caribbean ethnicity , having multidrug - resistant disease , and seeking treatment at an emergency department .
when the multivariate model was restricted to include only hard - drug users , crack cocaine users were still significantly more likely than other drug users to be smear positive ( p = 0.02 ) .
smear - positive disease is 2.4 times more likely to be diagnosed in crack cocaine users than in non  drug users , whereas hard - drug users not known to use crack cocaine are 1.6 times more likely to be diagnosed with smear - positive disease .
crack cocaine users were significantly more likely than other drug users to be smear positive on diagnosis .
the increased risk for smear - positive disease in crack cocaine users was not due to diagnostic delays .
hard - drug users who were not confirmed as crack cocaine users had the longest diagnostic delays . crack cocaine users had the shortest diagnostic delays , potentially attributable to rapidly progressive , debilitating disease .
crack cocaine users were also more likely to seek treatment at an emergency department rather than primary care services . again , the choice of healthcare service may be related to the severity of disease .
we included non  crack drug users as a comparison group because they have a similar social profile and similar access to healthcare .
therefore , we are confident that the extremely high levels of smear positivity on diagnosis in crack cocaine users are not due to a long duration of clinical illness preceding diagnosis . in the multivariate model , crack cocaine use remains strongly associated with smear - positive disease after controlling for a wide variety of other potential confounders .
other risk factors include ethnicity ( drug use was common among black carribean patients and may have been underreported ) ; treatment at an emergency department ( possibly a marker of disease severity ) ; and multidrug - resistant disease .
we are uncertain why multidrug - resistant cases were more likely to be smear - positive on diagnosis ; however , previous studies have found that cavitary disease is a risk factor for drug resistance ( 12 ) .  
the fact that smear positivity was significantly more prevalent in patients known to use crack cocaine when compared with other hard - drug users suggests that this additional risk may be attributable to a biological component .
plausible biological mechanisms to explain the increased risk of smear - positive disease include poor alveolar macrophage antimicrobial activity in crack cocaine users due to decreased inducible nitric oxide synthase activity ( 13 ) and direct effects on the lung ( 10 ) .
it is likely that a proportion of hard - drug users were incorrectly classified as not using crack cocaine due to nondisclosure .
this would reduce the apparent differences in levels of smear positivity between the groups . nevertheless
, despite relatively small numbers of known crack cocaine users , there is a significantly ( p = 0.02 ) higher proportion of smear - positive disease in these patients compared with other hard - drug users .
persons frequenting crack houses are likely to have multiple risk factors for active pulmonary tb .
prolonged sharing of closed and confined airspace , intensive coughing , and other acute pulmonary complications of crack cocaine inhalation promote transmission .
drug users are more likely than non  drug users to default treatment , to remain infectious for prolonged periods after diagnosis , and to acquire drug - resistant tb ( 15 ) .
. users may experience addiction - related problems that complicate access to healthcare and aggravate transmission , possibly aggravated by a biological driver that may increase susceptibility to infection and progression to infectious disease .
additional studies are needed to investigate the possible biological role of crack cocaine in the development of infectious forms of tb .
 we hypothesize that crack cocaine is independently associated with smear - positive tuberculosis ( tb ) . in a case  control study of tb in london , 19 ( 86% ) of 22 crack cocaine users with pulmonary tb were smear positive compared with 302 ( 36% ) of 833 non  
 drug users . 
 respiratory damage caused by crack cocaine may predispose drug users to infectivity . 
The Study
Conclusions