the denver metro tb clinic of denver public health department provides tb control activities for metropolitan denver , colorado , which includes the counties of adams , arapahoe , denver , douglas , and jefferson .
active tb is treated by use of directly observed therapy in the clinic or in the community ; outside the clinic , outreach workers deliver the therapy using incentives and enablers ( i.e. , transport , food coupons , temporary housing , and treatment for substance abuse or mental illness ) .
temporary housing for the homeless in denver is available in large communal shelters , through residential drug and alcohol treatment programs , and in individual or family transitional housing .
our program focused on the large communal shelters and the drug and alcohol treatment programs because these settings were the likely loci of tb transmission .
the screening program included a simple symptom assessment and a tuberculin skin test for persons who had no history of a positive skin test . the symptom assessment was a brief interview to detect tb symptoms such as chronic cough , weight loss , night sweats , and hemoptysis .
we used the mantoux method of tuberculin skin testing ( 6 ) and defined a positive result as induration of > 10 mm .
patients with symptoms that suggested active tb ( regardless of tuberculin skin - test results ) and those with a previous or current positive skin test were sent to the denver metro tb clinic for further evaluation and chest radiograph . to stay in a homeless shelter or residential substance abuse treatment program for more than 3 to 5 days , patients were required to complete the screening .
all persons with active tb were treated with directly observed therapy , and persons with latent tb infection who met criteria for treatment were offered directly observed preventive treatment .
after completion of screening , homeless persons were given a card from the clinic that allowed them to remain in one of the shelters for up to 6 months .
the screening program , started on a voluntary basis in 1993 at a day shelter and a clinic for the homeless , was expanded to four large shelters and six residential drug and alcohol treatment programs by 1995 .
one experienced public health nurse and four assistant outreach workers staff the program 15 hours each week , monday to friday .
they provide tuberculin skin testing at some overnight shelters and at a day shelter and a clinic for the homeless . in our analysis of the program
, we included all persons identified as homeless or in the residential drug / alcohol treatment programs between january 1995 and december 1998 .
the denver metro tb clinic of denver public health department provides tb control activities for metropolitan denver , colorado , which includes the counties of adams , arapahoe , denver , douglas , and jefferson .
active tb is treated by use of directly observed therapy in the clinic or in the community ; outside the clinic , outreach workers deliver the therapy using incentives and enablers ( i.e. , transport , food coupons , temporary housing , and treatment for substance abuse or mental illness ) .
temporary housing for the homeless in denver is available in large communal shelters , through residential drug and alcohol treatment programs , and in individual or family transitional housing .
our program focused on the large communal shelters and the drug and alcohol treatment programs because these settings were the likely loci of tb transmission .
the screening program included a simple symptom assessment and a tuberculin skin test for persons who had no history of a positive skin test . the symptom assessment was a brief interview to detect tb symptoms such as chronic cough , weight loss , night sweats , and hemoptysis .
we used the mantoux method of tuberculin skin testing ( 6 ) and defined a positive result as induration of > 10 mm .
patients with symptoms that suggested active tb ( regardless of tuberculin skin - test results ) and those with a previous or current positive skin test were sent to the denver metro tb clinic for further evaluation and chest radiograph . to stay in a homeless shelter or residential substance abuse treatment program for more than 3 to 5 days , patients were required to complete the screening .
all persons with active tb were treated with directly observed therapy , and persons with latent tb infection who met criteria for treatment were offered directly observed preventive treatment .
after completion of screening , homeless persons were given a card from the clinic that allowed them to remain in one of the shelters for up to 6 months .
the screening program , started on a voluntary basis in 1993 at a day shelter and a clinic for the homeless , was expanded to four large shelters and six residential drug and alcohol treatment programs by 1995 .
one experienced public health nurse and four assistant outreach workers staff the program 15 hours each week , monday to friday .
they provide tuberculin skin testing at some overnight shelters and at a day shelter and a clinic for the homeless . in our analysis of the program
, we included all persons identified as homeless or in the residential drug / alcohol treatment programs between january 1995 and december 1998 .
persons with active tb were considered homeless if they lived in one of the shelters or residential substance - treatment centers or were reported as homeless in clinic records between 1988 and 1998 .
data were available for 1995 to 1998 on whether tb diagnoses were detected by the screening program or by staff at a medical facility .
we obtained the numbers of homeless persons from studies done in 1995 and 1998 by the colorado department of human services ( 7 ) .
these studies enumerated the persons living in shelters and residential substance treatment centers and also estimated the number of persons living in other temporary housing and on the streets .
data for homeless populations in 1996 and 1997 were not available , so we estimated this number , assuming a linear increase in the total .
we analyzed tb incidence for the 11-year period , from 1988 ( the year that isolates were first available for dna fingerprinting ) through 1998 , compiling data for 7 years before and 4 years after the screening program was implemented for the homeless in 1995 .
initial dna fingerprinting used the is6110 technique , and secondary fingerprinting used ptbn12 for isolates having five or fewer hybridizing bands ( 4,8 ) . to estimate recent tb transmission in the homeless population , we calculated the proportion of cases clustered within a 2-year period of a preceding case with the identical dna fingerprint ( 9 ) .
we compared the proportion of cases caused by recent transmission during the 7-year period before the program ( 19881994 ) with the proportion caused during the first 4 years of the program ( 19951998 ) to assess the effect of the screening program on tb transmission .
analyses were done by using epi info , version 6.0 ( centers for disease control and prevention , atlanta , ga ) .
the proportion clustered in the two time periods was compared by using the fisher exact test .
the estimated number of homeless persons in denver , colorado , increased from 3,330 in 1995 to 5,792 in 1998 .
this increase was largely attributed to the combination of population growth in the metropolitan area and rising housing costs ( 7 ) .
the four large communal shelters could accommodate < 1,176 persons and the transitional housing and treatment programs < 980 persons .
the number of homeless persons who had tuberculin skin tests increased from 893 in 1995 to 3,897 by 1998 ( table 1 ) . the screening ratio ( number of completed skin tests divided by the estimated population ) increased from 26.7 to 67.3 per 100 persons during this period .
the proportion of persons with positive tuberculin skin tests decreased during the study period ( 17% in 1995 , 12% in 1998 , p<0.01 ) ( table 1 ) .
active tb was diagnosed in 94 homeless persons during the 11-year period from 1988 to 1998 . of those ,
the number of confirmed cases increased to a peak of 17 cases in 1995 and then decreased to 7 during each of the next 3 years ( table 2 ) .
when the increase in the number of homeless persons during this period is considered , the estimated incidence of active tb decreased from 510 to 121 per 100,000 persons from 1995 to 1998 .
numbers in brackets refer to the 14 case isolates from nonhomeless patients that share dna fingerprinting patterns with the homeless patients . 
number of case isolates with dna fingerprinting results divided by total culture - positive cases ( % tested ) .
the screening program was more successful in early identification of tb cases than in treatment of latent tb infection , which had low acceptance and completion rates ( table 1 ) that did not change substantially from 1995 to 1998 . over the 4-year period ,
1,284 positive tuberculin skin tests were recorded , but only 272 homeless persons initiated isoniazid treatment ; of those , 77 ( 28% ) completed therapy .
five ( 29% ) of the 17 confirmed cases of tb diagnosed in 1995 were detected through the screening program .
of seven tb cases diagnosed during each of the 3 subsequent years ( 19961998 ) , the screening detected three , five and four cases , respectively , for a mean of 57% .
dna fingerprinting results were available for 76 ( 87% ) of the 87 culture - positive cases ( table 2 ) .
nine clusters , in which identical patterns occurred at any time during the study period , consisting of 212 patients included 34 ( 45% ) of the 76 patients .
clusters in which cases occurred within 2 years of each other were present in six dna fingerprinting patterns and accounted for 27 ( 36% ) . in three other clusters , occurrences of tb in patients were separated by > 2 years .
dna fingerprinting patterns unique in the homeless population were found in the remaining 42 patients isolates , 9 of which had patterns also found in nonhomeless tb patients in the community . of these
, we detected one matching set of isolates for each of eight patterns in nonhomeless patients ; one case in a homeless patient was followed 3 years later with three cases in nonhomeless persons who had isolates of the identical dna fingerprint type ( data not shown ) . among the homeless patients , persons born outside the united states constituted 7 ( 17% ) of the 42 cases with unique isolates and 3 ( 9% ) of the 34 cases with clustered isolates
the estimated proportion of tb cases resulting from recent transmission within the homeless population , defined as cases that were clustered within 2 years , decreased from 49% ( 23/47 ) in the 7-year period ( 19881994 ) before the screening program was implemented to 14% ( 4/29 ) in the 4-year period ( 19951998 ) after the program was implemented ( p=0.03 ) .
dna fingerprinting results were available for 272 ( 55% ) of 491 of culture - positive cases among persons who were not homeless .
the proportion of nonhomeless case isolates with dna fingerprinting results increased from 47% ( 167/353 ) to 53% ( 112/21 ) for the years 19881994 and 19951998 , respectively .
case isolates in the nonhomeless population showed 233 dna fingerprinting patterns with 24 clusters of two to seven patients . in four of the clusters among homeless persons , cases in homeless persons
were followed by two to seven cases with the same dna fingerprinting pattern in nonhomeless persons ( table 2 ) . the largest cluster ( i ) originated with a case diagnosed in a homeless person in denver who had stopped treatment in another state in 1990 ;
the same strain was detected in 11 additional cases in homeless persons over a 6-year period and was also detected in 7 cases in nonhomeless persons , the first 2 of which were nosocomially acquired in 1992 .
the estimated number of homeless persons in denver , colorado , increased from 3,330 in 1995 to 5,792 in 1998 .
this increase was largely attributed to the combination of population growth in the metropolitan area and rising housing costs ( 7 ) .
the four large communal shelters could accommodate < 1,176 persons and the transitional housing and treatment programs < 980 persons .
the number of homeless persons who had tuberculin skin tests increased from 893 in 1995 to 3,897 by 1998 ( table 1 ) . the screening ratio ( number of completed skin tests divided by the estimated population ) increased from 26.7 to 67.3 per 100 persons during this period .
the proportion of persons with positive tuberculin skin tests decreased during the study period ( 17% in 1995 , 12% in 1998 , p<0.01 ) ( table 1 ) .
active tb was diagnosed in 94 homeless persons during the 11-year period from 1988 to 1998 . of those ,
the number of confirmed cases increased to a peak of 17 cases in 1995 and then decreased to 7 during each of the next 3 years ( table 2 ) .
when the increase in the number of homeless persons during this period is considered , the estimated incidence of active tb decreased from 510 to 121 per 100,000 persons from 1995 to 1998 .
numbers in brackets refer to the 14 case isolates from nonhomeless patients that share dna fingerprinting patterns with the homeless patients . 
number of case isolates with dna fingerprinting results divided by total culture - positive cases ( % tested ) .
the screening program was more successful in early identification of tb cases than in treatment of latent tb infection , which had low acceptance and completion rates ( table 1 ) that did not change substantially from 1995 to 1998 . over the 4-year period ,
1,284 positive tuberculin skin tests were recorded , but only 272 homeless persons initiated isoniazid treatment ; of those , 77 ( 28% ) completed therapy .
five ( 29% ) of the 17 confirmed cases of tb diagnosed in 1995 were detected through the screening program .
of seven tb cases diagnosed during each of the 3 subsequent years ( 19961998 ) , the screening detected three , five and four cases , respectively , for a mean of 57% .
dna fingerprinting results were available for 76 ( 87% ) of the 87 culture - positive cases ( table 2 ) .
nine clusters , in which identical patterns occurred at any time during the study period , consisting of 212 patients included 34 ( 45% ) of the 76 patients .
clusters in which cases occurred within 2 years of each other were present in six dna fingerprinting patterns and accounted for 27 ( 36% ) . in three other clusters , occurrences of tb in patients were separated by > 2 years .
dna fingerprinting patterns unique in the homeless population were found in the remaining 42 patients isolates , 9 of which had patterns also found in nonhomeless tb patients in the community . of these , we detected one matching set of isolates for each of eight patterns in nonhomeless patients ; one case in a homeless patient was followed 3 years later with three cases in nonhomeless persons who had isolates of the identical dna fingerprint type ( data not shown ) . among the homeless patients , persons born outside the united states constituted 7 ( 17% ) of the 42 cases with unique isolates and 3 ( 9% ) of the 34 cases with clustered isolates .
the estimated proportion of tb cases resulting from recent transmission within the homeless population , defined as cases that were clustered within 2 years , decreased from 49% ( 23/47 ) in the 7-year period ( 19881994 ) before the screening program was implemented to 14% ( 4/29 ) in the 4-year period ( 19951998 ) after the program was implemented ( p=0.03 ) .
dna fingerprinting results were available for 272 ( 55% ) of 491 of culture - positive cases among persons who were not homeless .
the proportion of nonhomeless case isolates with dna fingerprinting results increased from 47% ( 167/353 ) to 53% ( 112/21 ) for the years 19881994 and 19951998 , respectively .
case isolates in the nonhomeless population showed 233 dna fingerprinting patterns with 24 clusters of two to seven patients . in four of the clusters among homeless persons , cases in homeless persons
were followed by two to seven cases with the same dna fingerprinting pattern in nonhomeless persons ( table 2 ) . the largest cluster ( i ) originated with a case diagnosed in a homeless person in denver who had stopped treatment in another state in 1990 ;
the same strain was detected in 11 additional cases in homeless persons over a 6-year period and was also detected in 7 cases in nonhomeless persons , the first 2 of which were nosocomially acquired in 1992 .
the institution of a tb - screening program for the homeless population in congregate settings in denver in 1995 was associated with a substantial decrease in the incidence of active tb in this high - risk population .
the mechanisms by which the program could have caused this reduction in tb incidence include treating active cases earlier and treating persons with latent tb infection , thus preventing future active tb cases .
treating persons with latent tb infection was unlikely to have played a major role , however , since we found low rates of starting and completing treatment for this type of infection , a finding noted in previous reports ( 1,10 ) .
the screening program more likely reduced tb transmission by earlier diagnosis and treatment of active tb cases ( the top priority activity in tb control ) .
the program was effective in finding tb patients , detecting over half the active tb cases reported in homeless persons from 1996 to 1998 .
in contrast to the low acceptance and completion of latent tb infection treatment , high rates of treatment completion for active tb cases can be achieved with directly observed therapy supported by public health regulations ( 11,12 ) .
the results of the dna fingerprint analysis are consistent with the conclusion that early detection of tb through the screening program leads to a reduction in transmission . new , unique strains of m. tuberculosis have continued to appear among the denver homeless population , but cases from recent transmission ( clustered in the homeless population within 2 years ) decreased .
the estimated number of active tb cases decreased despite an increasing population of homeless persons , leading to a drop in the annual incidence rate from 510 to 121 per 100,000 persons .
other potential explanations for the reduction in tb deaths in the homeless population include failure to detect subsequent cases and errors in population estimates .
the mandatory tb screening program for residents in shelters and residential substance treatment programs may have led some homeless persons to live in other settings or to leave denver ; the latter reason led to the diagnosis of subsequent cases in other jurisdictions .
however , the denver metro tb clinic receives reports of cases diagnosed in metropolitan denver and conveys them to the tb control program at the colorado department of public health and environment ; cases were not likely missed unless diagnosed outside colorado . obtaining an accurate count of homeless persons
is difficult so we based our denominator in this study upon estimates obtained by the colorado department of housing ( 7 ) .
even if the population at risk had remained the same , despite indications otherwise , the incidence rate would still have decreased from 510 to 210 ( rather than 121 ) per 100,000 persons per year from 1995 to 1998 .
we detected substantial tb transmission in the homeless population for only two of nine homeless clusters , raising the possibility that the increase in cases was the result of one or two outbreaks running their natural course .
further studies with larger numbers of cases are needed to address this issue ; however , our data support the concept that early case detection results in lower rates of transmission in homeless populations .
the possibility exists that clusters that occurred over several years were strains that lingered in the community but were not detected in our study ; we obtained dna fingerprints for only 55% of the cases in the community among nonhomeless persons and can not rule out that possibility . for clustered cases occurring many years apart
, we may have missed links in transmission because of patients who were not tested in the community or who moved out of the area without diagnosis .
as we increase our understanding of how to interpret the results of dna fingerprinting , we may find that clusters that are far apart represent a remote transmission that resulted in latent tb infection with later reactivation .
the secondary cases would then share the same pattern as the original ( source ) case , but the connection between them would be more difficult to identify since the contact in these cases occurred many years earlier . of the 42 homeless cases with unique patterns , seven ( 17% ) were diagnosed in foreign - born persons .
of those , one patient arrived in united states < 2 years before tb diagnosis , three 25 years before diagnosis , two 510 years before diagnosis , and one for an unknown period of time .
few of the unique patterns found among homeless patients represent cases imported from foreign countries .
various screening programs have been proposed to decrease tb transmission among homeless persons ( 13 ) .
chest radiographic screening provides a rapid assessment for potentially transmissible disease , but this method would have been too expensive for us to implement .
sputum examination by smear has been used ( 14 ) but does not detect smear - negative cases and is difficult to implement for large numbers of persons .
tuberculin skin testing does not provide an immediate assessment of infectious tb and gives negative test results for up to 25% of persons with active pulmonary tb .
furthermore , skin testing requires a second visit 23 days later for a reading . despite reaching only two thirds of the homeless population in 1998 ,
the true incidence of skin test positivity is likely higher than that shown in table 1 because those already known to be positive were not tested .
the symptom survey was an important component of our program . in spite of the challenges of skin testing , our screening program proved to be feasible for a large population staying at a number of shelters and residential programs .
our results suggest that congregate settings , the focus of our program , are the most important loci of tb transmission among homeless persons .
the low completion rate of treatment for latent tb infection provided further evidence that the primary effect of the program was to increase early detection of active cases of tb , not to identify and treat latent tb infection . despite the decrease in tb incidence following the screening program , the tb rate remained high ( 121 per 100,000 population ) compared to the overall rate for the metropolitan area ( 8.2 per 100,000 population ) .
further progress in decreasing tb among homeless persons will require improvements in the acceptance and completion of latent infection treatment ( 6 ) .
 we describe the implementation of a mandatory tuberculosis ( tb ) screening program that uses symptom screening and tuberculin skin testing in homeless shelters . 
 we used the results of dna fingerprinting of mycobacterium tuberculosis isolates to evaluate the effect of the program on tb incidence and transmission . after the program was implemented , the proportion of cases among homeless persons detected by screening activities increased , and the estimated tb incidence decreased from 510 to 121 cases per 100,000 population per year . 
 recent transmission , defined by dna fingerprinting analysis as clustered patterns occurring within 2 years , decreased from 49% to 14% ( p=0.03 ) . 
 our results suggest that the shelter - based screening program decreased the incidence of tb by decreasing its transmission among the homeless . 
Methods
Description of Screening Program
Data Analysis
Results
Screening Program Evaluation
DNA Fingerprinting Analysis
Discussion