This is the first cross sectional study that investigated the prevalence of LTBI in a large number of patients receiving either long-term PD or HD. Using QFT as a diagnostic tool for LTBI in this immuno-compromised population, 4.7% may have an indeterminate result, especially those with anemia, hypoalbuminemia, and who have been receiving HD for a long time. There is a high prevalence (21.3%) of LTBI in the long-term dialysis population, especially in the elderly, current smokers, and those with prior TB history. However, the use of either PD or HD is associated with similar risks of LTBI.
Though IGRA-positive is not 100% equivalent of LTBI, it has several advantages over tuberculin skin test (TST) in terms of convenience and accuracy 
. The TST has a significant limitation in Taiwan due to BCG vaccination 
and the high prevalence of NTM disease 
. In QFT, by incorporating positive control (mitogen) and negative control (no antigen) tubes, the true immune reaction against Mycobacterium tuberculosis
-specific antigens can be differentiated from false-positive result due to non-specific activation and false-negative result due to immuno-suppression. Thus, IGRA is a better screening test for LTBI than TST while implementing public health policy, especially for an immune-compromised host.
Previous studies using IGRA report an LTBI prevalence of 21–40% in HD patients 
. The QFT-positive rate in the present study is within this range and lower than 40%, as reported in a study conducted in south Taiwan 
. This is probably because the incidence of TB has been decreasing in Taiwan 
. However, the prevalence in the present study (21.3%) is similar to that reported in household contacts in large-scale studies (11~30%) 
and much higher than the results of new health-care staff in the study institute at the same period (13 [5.7%] QFT-positive in 229 [unpublished data]). As such, it can be posited that dialysis patients in Taiwan have a much higher prevalence of LTBI than the general population and should be a priority group for targeted screening for active TB disease, especially IGRA-positive patients. If IGRA is unavailable, focus should be on older patients, current smokers, and those with prior TB history. Quite interestingly, the three predictors for LTBI identified in the present study are also risk factors of active TB disease 
. Different combinations of these predictors may be useful to select the target population for preventive therapy for LTBI. However, the cost and benefit of preventive therapy in this special population should be further evaluated.
The population of chronic renal failure patients receiving long-term dialysis is increasing worldwide and TB is a commonly associated infectious disease 
. It has been previously assumed that because HD patients frequently visit the HD room, they are more likely to acquire Mycobacterium tuberculosis
infection than PD patients via airborne transmission. Only a report of PD patients in Spain has shown a comparable LTBI prevalence of 18% 
. By simultaneously enrolling HD and PD patients, this is the first study to demonstrate similar LTBI prevalence in the two patient groups, thereby challenging the hypothesis of occult transmission in the HD room. Although the two dialysis groups are different in more ways than just dialysis place and duration, this observation suggests that transmission of TB to HD patients within crowded dialysis facilities may be similar to PD patients at home 
. This suggests that the study institute has an effective TB infection control policy on early detection, prompt treatment, and rapid isolation.
Although 71.4% of QFT-indeterminate patients have definite results after repeat testing, 4.7% of the initial QFT tests with an indeterminate result are associated with hypoalbuminemia, anemia, HD, and longer dialysis duration. The association between anemia and indeterminate status has been shown before 
. Along with hypoalbuminemia, these predictors suggest that malnutrition attenuates immune response and compromises the performance of IGRA 
. The current finding that HD, but not PD, is associated with QFT-indeterminate is interesting and worth discussing further. A previous study reveals that while both HD and PD patients have lower but insignificant HLD-DR expression on peripheral blood monocytes compared to healthy controls, HLA-DR expression is significantly higher in PD than in HD patients 
. This implies that continuous dialysis like PD can attenuate immune dysfunction compared to intermittent modes like HD. Moreover, longer duration of dialysis in dialysis patients has been correlated with worse cellular immunity 
. This may explain how a much higher percentage (95%) of QFT-indeterminate results in the present study may come from low mitogen responses, compared to 51% in a public health clinic setting 
. For diagnosing LTBI in such patients, IGRA should be meticulously applied. Repeating IGRA or using alternative tests may be necessary.
In contrast to a previous report 
, female gender is not an independent factor of QFT-indeterminate results in the present study. This may be due to different patient characteristics, such as age, race, and prevalence of HIV infection, between studies. Further large-scale investigations are necessary to confirm this finding and investigate possible reasons.
The present study has several limitations. First, this study was conducted in a tertiary referral center and its branch, so patients had more underlying co-morbidities and the LTBI prevalence might be higher. Second, without detailed contact investigation, the epidemiologic link and biological implication of QFT-positivity cannot be confirmed. Lastly, this is a cross-sectional study. Further prospective studies with long-term follow-up on the development of active TB are needed.
In conclusion, patients receiving PD have a similar prevalence of LTBI as those receiving HD (19% and 22%, respectively). The prevalence of LTBI in long-term dialysis patients is even higher in the elderly, current smokers, and those with prior TB history. These risk factors can be used to select a target group for cost-effective LTBI screening. Patients receiving HD or long duration of dialysis, and those with anemia or lower albumin level are likely to have a QFT-indeterminate result. For such patients, repeat IGRA or alternative test may be necessary to detect LTBI.
Parts of the study results have been presented as a poster in the 2011 Congress of the Asia Pacific Society of Respirology and the 2012 International Conference of the American Thoracic Society.