the national tb prevalence survey in eritrea was conducted from february through october 2005 ( 6 ) . in 40 selected villages ,
a census ( which included information about sex and age ) was taken of 875 persons in each village .
all persons > 15 years of age were asked to provide a morning and a spot sputum sample .
persons who had 2 positive sputum samples were informed about the test results and referred for treatment .
those who had 1 positive sputum sample were referred to a nearby healthcare facility for further smear examination .
if results of smear examination were negative , thoracic radiographs were taken and evaluated by 2 experienced radiologists . the case definition for a sputum smear  positive case was at least 2 sputum specimens positive for acid - fast bacilli by ziehl - neelsen staining and microscopy or at least 1 sputum specimen positive for acid - fast bacilli and radiographic abnormalities consistent with active pulmonary tb ( classification of the national tuberculosis control program in eritrea ) .
using the prevalence estimate obtained from the survey and 2 different models , we calculated the cdr for 2004 . in model 1 ,
described by styblo , cdr = ( notification rate / prevalence rate ) / ( 0.5 + 0.83  [ notification rate / prevalence rate ] ) ( 7,8 ) . in model 2 , described by dye et al .
, cdr = ( notification rate / prevalence rate ) / ( [ notification rate / prevalence rate ] + 0.5 ) ( 9,10 ) .
we then compared the calculated cdr with the cdr estimated by the world health organization ( who ) to evaluate whether comparable conclusions about tb case detection would be obtained .
a total of 38,047 persons were included in the prevalence survey . of those > 15 years of age , 18,152 ( 94.6% ) provided at least 1 sputum sample ( figure ) .
the prevalence of new smear - positive tb was estimated at 90/100,000 ( 95% confidence interval [ ci ] 35145/100,000 ) in persons > 15 years of age . in 2005 , 44.7% of
the eritrean population was < 15 years of age ( 11 ) , which resulted in an overall new smear - positive tb prevalence of 50/100,000 ( 95% ci 1980/100,000 ) under the assumption of no cases in persons < 15 years of age .
summary of tuberculosis prevalence survey in eritrea , 2005 . in 2004 , 17/100,000 new smear - positive cases were reported ( 2 ) .
for eritrea , the cdr provided by who is considerably lower than that calculated from the results of the national tb prevalence survey . both estimates indicate that eritrea has not reached the 70% target for case detection . however , the who estimate suggests that the program needs to improve case detection by a factor of 5 , whereas the survey estimate suggests that case detection needs to be improved by a factor of 1.6 .
two explanations may account for the large difference : 1 ) the cdr derived from the tb prevalence survey is too high because of an underestimation of the prevalence of smear - positive tb , or 2 ) the cdr estimate published by who is too low because of an overestimation of the incidence of smear - positive tb . in the national tb prevalence survey , measures were taken to ensure high quality of the results ; e.g. , training of data collectors , repeat census taking , reexamination of all slides found positive on fluorescence microscopy , and reexamination of a 5% random sample of the negative slides .
persons who had smear - positive tb may have been missed because they did not provide a specimen ; however , because only 5% of eligible persons did not provide a specimen , this can explain only a slight underestimation . furthermore , recorded reasons for not providing a specimen seem to be unrelated to a higher chance of having tb .
the quality of the provided specimens may have been suboptimal because instructing and motivating persons to provide a sputum sample is challenging . for diagnosis of tb , microscopic examination of saliva
is less sensitive than examination of sputum ; however , in 50% of saliva samples from patients with a positive sputum sample , bacilli can be demonstrated ( 12,13 ) . for 27,647 samples that appeared to be saliva , smear 
assuming that only 50% were detected , a maximum of 12 smear - positive tb patients may have been undetected .
taking this into account results in a prevalence of 87/100,000 . using this estimate , model 1 provides a cdr of 30% and model 2 a cdr of 28%
the possibility that persons who provided a saliva sample were not able to produce a sputum sample because they did not have pathologic pulmonary changes should also be taken into consideration .
estimation of the incidence of smear - positive tb in eritrea is complicated by the fact that no data from tuberculin or prevalence surveys were available .
the only data available for eritrea were reporting data , which experts assessed as being of low quality ( 14 ) .
use of this limited information will result in an uncertain incidence estimate , which may result in an unreliable cdr . for most countries in africa ,
little information is available for estimating the prevalence of disease and progress towards the millennium development goals ( http://unstats.un.org/unsd/mi/mi_goals.asp , accessed 2006 aug 30 ) . on the basis of case reporting ,
tb was rightly declared an emergency by african health ministers at the who africa regional committee in maputo in 2005 ( 15 ) . to be able to fight this emergency , more reliable information about the prevalence of tb in africa is needed .
furthermore , for global tb control , reliable information about the tb epidemic in africa is needed because 28% of the incident smear - positive cases occurred in the who african region in 2004 ( 2 ) . in conclusion
, the example of eritrea shows that a large gap may exist between available estimates of tb prevalence and actual tb prevalence in africa .
national tb prevalence surveys in africa would help provide better information on tb prevalence and case detection .
 we used results from a national tuberculosis prevalence survey in eritrea to calculate case detection rate ( cdr ) and compared it with the published cdr . 
 the cdr obtained from the survey was 40% , whereas the cdr published by the world health organization was 3 lower ( 14% ) . 
The Study
Conclusions