the development of t - cell - based interferon - gamma ( ifn- ) assays ( igras ) for mycobacterium tuberculosis ( mtb)-specific antigens represents a significant advance that is expected to replace the tuberculin skin test ( tst ) in screening for active tb disease or latent tuberculosis infection ( ltbi ) [ 13 ] . these igras have demonstrated excellent specificity and shown a closer correlation with the exposure gradient than the tst [ 46 ] . however , it is generally assumed that these tests , like tst , can not discriminate between active tb disease and ltbi . the utility of these assays for monitoring disease activity and treatment response , or as predictors of the progression of ltbi to active tb disease , is currently the subject of ongoing research . two commercial igras have recently become available , quantiferon - tb gold in - tube(cellestis ltd . , victoria , australia ) with elisa used to measure ifn- concentrations in supernatants , and the t-spot.tb test ( oxford immunotec , oxford , uk ) with an enzyme - linked immunospot ( elispot ) assay used to detect individual t cells producing ifn- [ 1 , 7 ] . quantiferon tb-2 g ( qft-2 g ) is commonly used for the diagnosis of active tb disease or ltbi , but t-spot.tb or qft gold in - tube has not yet been approved in japan . we therefore compared quantitative t - cell responses to mtb - specific antigens between cohorts of contacts with ltbi and patients with active tb disease using the commercial qft-2 g test . one hundred eighty patients confirmed to be positive for mtb on culture of sputum , bronchoalveolar lavage fluid , peritoneal fluid , or lymph node tissue samples and a cohort of 50 screening contacts with ltbi ( positive qft-2 g response with or without a positive tst result ) drawn from the community were prospectively enrolled starting in january 2005 . screening contacts with ltbi all had exposure histories to their index case 812  weeks earlier and all index cases were smear- and culture - positive for mtb using sputum specimens . however , none of these subjects with ltbi showed any clinical symptoms ( fever , cough , or sputum ) , abnormal inflammatory responses , or abnormal radiological findings suggesting pulmonary tb . all patients with active tb disease showed negative findings on enzyme - linked immunosorbent assay ( elisa ) testing for hiv . regarding subjects screened for contact with the index case , hiv testing was not performed , but subjects with a history of hiv infection were excluded from data analyses . demographic data from patients with active tb disease and screening contacts with ltbi were collected , including any history of previous tb or antituberculous treatment , other underlying diseases , and administration of immunosuppressive drugs within 3  months before enrolment in this study . a heparinized blood sample was collected by venipuncture from each patient for whole - blood ifn- assay . 0.1  ml of tuberculin ppd [ nippon bcg , equivalent to three tuberculin units ( tu ) of purified protein derivative solution ( ppd - s ) ] was injected intradermally into the volar aspect of the forearm , and the transverse induration diameter was measured 48  h later . the results of the test were interpreted by hospital staff based on the patient s degree of risk , according to current guidelines . a lower cutoff of 5  mm for a positive test was used for individuals . the qft-2 g test was performed according to the manufacturer s recommendations , and the test result was judged according to cdc guidelines . the test result was considered positive if the ifn- level in the sample well after stimulation with esat-6 and/or cfp-10 was 0.35  iu / ml , irrespective of the result for the positive control well . the test was considered negative if the ifn- level was < 0.35  iu / ml and the ifn- level of the positive control well was 0.35  iu / ml . the test result was considered indeterminate if the ifn- level was < 0.35  iu / ml in both antigen wells and < 0.5  iu / ml in the positive control well . the qft-2 g test was performed for all patients before initiating the administration of antituberculous drugs . redmond , wa ) and then transferred to santa software , version 7.0 ( santa , vienna , austria ) , for statistical analysis . pearson s  test was used to compare data from the two groups and mcnemar s test was used to compare paired data . the mann one hundred eighty patients confirmed to be positive for mtb on culture of sputum , bronchoalveolar lavage fluid , peritoneal fluid , or lymph node tissue samples and a cohort of 50 screening contacts with ltbi ( positive qft-2 g response with or without a positive tst result ) drawn from the community were prospectively enrolled starting in january 2005 . screening contacts with ltbi all had exposure histories to their index case 812  weeks earlier and all index cases were smear- and culture - positive for mtb using sputum specimens . however , none of these subjects with ltbi showed any clinical symptoms ( fever , cough , or sputum ) , abnormal inflammatory responses , or abnormal radiological findings suggesting pulmonary tb . all patients with active tb disease showed negative findings on enzyme - linked immunosorbent assay ( elisa ) testing for hiv . regarding subjects screened for contact with the index case , hiv testing was not performed , but subjects with a history of hiv infection were excluded from data analyses . demographic data from patients with active tb disease and screening contacts with ltbi were collected , including any history of previous tb or antituberculous treatment , other underlying diseases , and administration of immunosuppressive drugs within 3  months before enrolment in this study . a heparinized blood sample was collected by venipuncture from each patient for whole - blood ifn- assay . , 0.1  ml of tuberculin ppd [ nippon bcg , equivalent to three tuberculin units ( tu ) of purified protein derivative solution ( ppd - s ) ] was injected intradermally into the volar aspect of the forearm , and the transverse induration diameter was measured 48  h later . the results of the test were interpreted by hospital staff based on the patient s degree of risk , according to current guidelines . a lower cutoff of 5  mm for a positive test was used for individuals . the qft-2 g test was performed according to the manufacturer s recommendations , and the test result was judged according to cdc guidelines . the test result was considered positive if the ifn- level in the sample well after stimulation with esat-6 and/or cfp-10 was 0.35  iu / ml , irrespective of the result for the positive control well . the test was considered negative if the ifn- level was < 0.35  iu / ml and the ifn- level of the positive control well was 0.35  iu / ml . the test result was considered indeterminate if the ifn- level was < 0.35  iu / ml in both antigen wells and < 0.5  iu / ml in the positive control well . the qft-2 g test was performed for all patients before initiating the administration of antituberculous drugs . redmond , wa ) and then transferred to santa software , version 7.0 ( santa , vienna , austria ) , for statistical analysis . pearson s  test was used to compare data from the two groups and mcnemar s test was used to compare paired data . the mann one hundred thirty subjects screened after contact with index cases were included in this study protocol . they consisted of 70 males and 60 females , and the mean age was 41.8    9.8  years ( range  =  1876 ) . the comparison of tst and qft-2 g test results for all contacts screened is shown in table  1 . the tst induration diameter was larger than 5  mm in 78 ( 60% ) subjects , and 50 ( 38% ) showed positive qft-2 g test results . while both tst and qft-2 g tests were positive in 34 screening contacts ( 26% ) , only the qft-2 g test was positive in 16 ( 12% ) and only the tst was positive in 44 ( 34% ) . of all screening contacts , 50 subjects ( 38% ) fulfilled the inclusion criteria for ltbi in this study.table  1comparison of tst and qft-2 g test results for all contacts screened ( 130 cases)qft-2 g testtstpositivenegativetotalpositive34 ( 26%)16 ( 12%)50 ( 38%)negative44 ( 34%)31 ( 24%)75 ( 58%)indeterminate05 ( 4%)5 ( 4%)total78 ( 60%)52 ( 40%)130 ( 100%)tst tuberculin skin test , qft-2 g quantiferon tb-2 g comparison of tst and qft-2 g test results for all contacts screened ( 130 cases ) tst tuberculin skin test , qft-2 g quantiferon tb-2 g a comparison of tst and qft-2 g test results for patients with active tb disease is shown in table  2 . the positive response rate on tst did not significantly differ between all subjects screened after contact ( 60% ) and those with active tb disease ( 63% ) . the positive response rate to the qft-2 g test was significantly higher in patients with active tb disease ( 83% ) than in all screening contacts ( 38% ) . the positive response rate to the qft-2 g test ( 83% ) was significantly higher than to the tst ( 63% ) in patients with active tb disease . the positive response rate to the qft-2 g test ( 38% ) was significantly lower than to the tst ( 60% ) in all screening contacts.table  2comparison of tst and qft-2 g test results for patients with active tb disease ( 180 cases)qft-2 g testtstpositivenegativetotalpositive114 ( 63%)36 ( 20%)negative011 ( 6%)indeterminate019 ( 11%)total114 ( 63%)66 ( 37%)tst tuberculin skin test , qft-2 g quantiferon tb-2 g comparison of tst and qft-2 g test results for patients with active tb disease ( 180 cases ) tst tuberculin skin test , qft-2 g quantiferon tb-2 g in 180 patients with active tb disease , acid - fast smear results of samples were positive in 93 patients ( 52% ) and negative in the remaining 87 patients . the clinical characteristics of the 50 screening contacts with ltbi and the 180 patients with active tb disease are shown in table  3 . there were no significant differences in the clinical characteristics between ltbi and active tb disease patients except for age , sex , and underlying disease including respiratory disease.table  3clinical characteristics of 50 screening contacts with latent tuberculosis infection ( ltbi ) and 180 patients with active tuberculosis ( tb ) diseaseclinical characteristicsltbi ( n  =  50)active tb disease ( n  =  180)p valueage ( years ) ( mean    sd)41.6    9.563.8    10.60.010sex ( male : female)24:26140:400.022smoking history30 ( 60%)120 ( 67%)0.768alcohol abuse history6 ( 12%)32 ( 18%)0.356underlying disease18 ( 36%)133 ( 74%)0.035  respiratory disease5 ( 12%)61 ( 34%)0.038  healed pulmonary tb0 ( 0%)20 ( 11%)0.008  copd2 ( 4%)13 ( 7%)0.320  others4 ( 8%)28 ( 16%)0.101  nonrespiratory disease13 ( 26%)72 ( 40%)0.070  gastrointestinal disease4 ( 8%)18 ( 10%)0.755  diabetes mellitus3 ( 6%)15 ( 8%)0.842  cancer1 ( 2%)14 ( 8%)0.084  cardiovascular disease3 ( 6%)13 ( 7%)0.893  collagen vascular disease1 ( 2%)7 ( 4%)0.396  chronic renal failure0 ( 0%)5 ( 3%)0.108immunosuppressive treatment1 ( 2%)13 ( 10%)0.061received bcg vaccination34 ( 63%)116 ( 64%)0.867received tb treatment0 ( 0%)29 ( 16%)0.008laboratory data  wbc count ( cells/l)4620    5434058    5180.350  lymphocyte count ( cells/l)1036    92916    870.402  total protein ( g / dl)6.9    1.06.4    0.80.346  albumin ( g / dl)3.7    0.83.3    0.60.367with hemodialysisall patients received corticosteroid therapy clinical characteristics of 50 screening contacts with latent tuberculosis infection ( ltbi ) and 180 patients with active tuberculosis ( tb ) disease all patients received corticosteroid therapy concerning the comparison of qft-2 g test results related to combined and separate responses to esat-6 and cfp-10 antigens for ltbi and active tb disease , the positive response rate for both esat-6 and cfp-10 antigens was significantly lower in community contacts with ltbi than that in patients with active tb disease . the positive response rates for esat-6 only and for cfp-10 only were lower in patients with active tb disease than in screening contacts with ltbi ( table  4).table  4comparison of the qft-2 g test results related to the combined and separate responses to esat-6 and cfp-10 antigens for patients with ltbi and active tb diseaseqft-2 g testltbi ( n  =  50)active tb disease ( n  =  180)p valueesat-6 only positive22 ( 44%)49 ( 27%)0.077cfp-10 only positive24 ( 48%)51 ( 28%)0.048esat-6 and cfp-10 positive4 ( 8%)50 ( 28%)0.026total50 ( 100%)150 ( 83%)0.564qft-2 g quantiferon tb-2 g comparison of the qft-2 g test results related to the combined and separate responses to esat-6 and cfp-10 antigens for patients with ltbi and active tb disease qft-2 g quantiferon tb-2 g the ifn- level in the response to esat-6 and cfp-10 antigens for screening contacts with ltbi and patients with active tb disease is shown in table  5 . the ifn- levels for esat-6 and cfp-10 antigens were significantly higher in patients with active tb disease ( ifn- level for esat-6 : 1.24    0.30  iu / ml , ifn- level for cfp-10 : 1.58  iu / ml ) than in screening contacts with ltbi ( ifn- level for esat-6 : 0.61    0.20  iu / ml , ifn- level for cfp-10 : 0.72    0.21  iu / ml).table  5ifn- level ( u / ml ) in response to esat-6 and cfp-10 antigens for patients with ltbi and active tb diseaseqft-2 g testltbi ( n  =  50)active tb disease ( n  =  180)p valueesat-60.61    0.20 ( 0.354.25)1.24    0.30 ( 0.2010.62)0.010cfp-100.72    0.21 ( 0.354.60)1.58    0.36 ( 0.2411.90)0.008qft-2 g quantiferon tb-2 g ifn- level ( u / ml ) in response to esat-6 and cfp-10 antigens for patients with ltbi and active tb disease qft-2 g quantiferon tb-2 g we noted a significant reduction in the positive response rate for both esat-6 and cfp-10 antigens in screening contacts with ltbi than in patients with active tb disease , and the quantitative t - cell responses to esat-6 and cfp-10 antigens were significantly higher in patients with active tb disease than in those with ltbi using the qft-2 g assay . in previous reports , janssens et al .   and chee et al .   reported a significant difference in the quantitative values of t-spot.tb responses between active tb cases and contacts with positive t-spot.tb responses , with a large overlap in the range of values . although chee et al .   did not identify a significant difference in the quantitative values using the qft gold - in - tube assay in previous reports , we found a significant difference in quantitative qft-2 g responses between the two groups in this study , consistent with the findings in a previous report by higuchi et al . . higuchi et al .   also indicated that the geometric mean of the ifn- production level in the active tb disease group was significantly larger than that in the ltbi group , and the combined parameters of esat-6 and cfp-10 significantly contribute to the active tb disease risk for infected subjects using the qft-2 g test . differences in the sensitivity of t-spot.tb and qft-2 g assays in immunocompromised patients have been well reported . although both assays measure t - cell ifn- responses to the mycobacterium tuberculosis ( mtb)-specific antigens esat-6 and cfp-10 after overnight stimulation , the measurement of ifn- levels in whole blood in the qft-2 g assay [ as opposed to measuring the number of ifn--producing t cells in a standardized number of peripheral blood mononuclear cells ( pbmcs ) using t-spot.tb ] may render this assay less sensitive than t-spot.tb . regardless of this hypothesis , there was a significant difference in quantitative qft-2 g responses between active tb cases and screening contacts with ltbi in this study . while the median ifn- level in response to esat-6 and cfp-10 antigens in patients with active tb disease was more than two times higher than that of screening contacts with ltbi , the large overlap in the range of values between these two groups suggests that isolated qft-2 g results would not be useful as a diagnostic method to discriminate between active tb disease and ltbi . first , because there is no gold standard for the diagnosis of ltbi , cases with active tb disease may have been missed among screening contacts with presumed ltbi . however , this is highly unlikely since all of the subjects demonstrated normal clinical and radiological findings . indeed , there were no missed cases of active tb disease detected during the subsequent 3  years of follow - up . second , because there were significant differences in the frequency of patients with underlying disease and mean age of both groups ( table  3 ) , these factors have influenced the quantitative response of ifn- for both mtb - specific antigens in this study . third , the present definition of ltbi is arbitrary . indeed , there is no gold standard for defining ltbi . igra results correlate better with exposure than tst results ; the lifetime risk of active tb disease based on igra results only currently remains unknown . the clinical situation resulting from the present cdc recommendations was investigated ; subjects considered as having probable ltbi were detected by a positive qft-2 g test result despite negative tst results . although the t-spot.tb test has been used as an igra , janssens et al .   indicated that studying only t-spot.tb-positive subjects or t - spot.tb- and tst - positive subjects did not significantly alter the results according to prevailing guidelines . fourth , we used the qft-2 g test as an igra in this study because the qft-2 g assay is currently the only igra commercially available in japan . newer commercial assays such as t-spot.tb ( oxford - immunotec ) and quantiferon - tb gold in - tube ( cellestis ) have many differences and may provide higher levels of sensitivity . indeed , harada et al . reported that the sensitivity of the qft gold in - tube test ( 93% ) was significantly higher than that of the qft-2 g test ( 81% ) , although the specificities of both tests were the same ( 99% ) . we think that the qft gold in - tube or t-spot.tb should be approved and made commercially available for the supportive diagnosis of active tb disease and ltbi . in conclusion , we demonstrated significant differences in the quantitative responses of ifn- to mtb between patients with active tb disease and those with ltbi in this study . however , there is an overlap in the ifn- levels between active tb disease and ltbi groups . therefore , it would be difficult to use the qft-2 g test to completely discriminate active tb disease from ltbi . a large - scale longitudinal study that investigates this important potential use of igras test is awaited .