Assessment of the prevalence of latent tuberculosis infection in hemodialysis patients using tuberculin skin test

Implication for health policy/practice/research/medical education: Early detection of latent tuberculosis infection and initiation of preventive treatment for active tuberculosis infection especially in individuals with immune deficiency is crucial. A screening test should be able to differentiate between asymptomatic patients and healthy individuals. It seems necessary that the World Health Organization (WHO) seeks a cheap, fast, and reliable screening method. Please cite this paper as: Zafar Mohtashami A, Amiri A, Hadian B, Nasiri P. Assessment of the prevalence of latent tuberculosis infection in hemodialysis patients using tuberculin skin test. J Renal Inj Prev. 2022; 11(x): x-x. doi: 10.34172/jrip.2022.xx.


Introduction
According to the World Health Organization (WHO), in 2019, about ten million people were infected with active tuberculosis (TB) and about 1.4 million deaths occurred from it (1). TB is caused by Mycobacterium TB with a significant incidence of morbidity and mortality, especially in people with impaired immune systems (2). About one-third of the world population has latent TB infection (LTBI), meaning they have TB bacteria in their body without any symptoms or signs of active TB (3). LTBI may become active TB over time. Patients infected with the human immunodeficiency virus (HIV), transplant recipients and hemodialysis patients are more prone to develop active TB (4). The prevalence of chronic kidney disease (CKD) is estimated to be about 8 to 16% in the general population. CKD is associated with oxidative stress, inflammation, vitamin D deficiency, and malnutrition, all of which cause immune deficiencies (5). In patients with CKD, due to immunodeficiency status, a high frequency of infectious diseases and subsequent mortality is observed (6). Immune system deficiencies in patients with CKD, especially end-stage renal disease (ESRD) which requires dialysis, result from disorders in T cells and antigen presenting cells, and increased cytokines produced by monocytes (7)(8)(9). According to immune system disorders, the incidence and prevalence of TB in patients undergoing dialysis is higher than the general population (10)(11)(12). Therefore, to prevent evolution of LTBI into active TB and its transmission to others, it is recommended to screen for LTBI in these patients (13).

Objectives
There are two common methods for diagnosing LTBI; the tuberculin skin test (TST) and interferon-gamma release assays (IGRAs) blood tests. Since the TST skin test is less sensitive and specific for LTBI and may be false positive in people who have been infected with non-tuberculous mycobacteria or have a history of Bacillus Calmette-Guérin (BCG) vaccination, IGRA blood tests are the preferred diagnostic method for patients receiving the BCG vaccine (14). We evaluated the prevalence of LTBI in hemodialysis patients in Khorramabad teaching hospitals, Lorestan province, Iran.

Study design
This study is cross-sectional study that was conducted in 2018. All eligible hemodialysis patients of the two dialysis centers of Lorestan university of medical sciences in Khorramabad participated. Inclusion criteria were age over 15 years and at least three months of dialysis. Exclusion criteria were patients being dialyzed for acute kidney injury and those with a history of known TB. Written informed consent was obtained from patients. Demographic data such as age, gender, education and place of residence and clinical diagnostic data according to the WHO guidelines on the management of LTBI were assessed and recorded in a questionnaire. Additionally, the proposed diagnostic algorithm for LTBI in the aforementioned guideline was considered as a basis for our practice (Figure 1) (15).
The size of induration reaction to TST at the injection site was also recorded. According to this algorithm, hemodialysis patients, which are considered an atrisk group, were first assessed for TB by taking history and physical examination for determined symptoms. Participants who had one or some of the TB clinical signs were further evaluated for TB according to the national guideline of TB in Iran, which mainly includes sputum smear investigation and chest X-ray. For others, a TST was performed to diagnose LTBI.
For this test, 5 ml vials of human tuberculin purified protein derivative (PPD) manufactured by Razi vaccine and serum institute, Tehran, Iran were used. The expiration date of the vials was checked and the necessary cold chain was prepared. The TST was conducted by intradermal injection of 0.1 mL of PPD into the inner surface of the forearm by a professional vaccinator, then the injection site was marked. Forty-eight to 72 hours after injection, the indurated area was measured perpendicular to the length axis of forearm. Induration size equal to or greater than 10 mm was considered as positive (10). Fortunately, our study was concomitant with the routine periodical chest X-ray check-ups of patients.
Although evidence of abnormality was observed in chest X-rays for eight patients, none were in favor of TB. In other patients, the lung image was considered as normal. Chest X-rays were interpreted by both a radiologist and a pulmonologist concerning clinical signs.

Data analysis
Graphs and tables were used to show data distribution and summary measures for descriptive analysis. For comparing Figure 1. WHO algorithm for latent tuberculosis infection screening in high risk groups. *Any symptoms of TB include any one of: cough, hemoptysis, fever, night sweats, weight loss, chest pain, shortness of breath and fatigue. HIV test could be offered based on national or local guidelines or clinical judgment. Similarly chest radiographs can be conducted if efforts are intended also for active TB case finding. ** Clients for whom LTBI treatment is not indicated should be provided information about TB inducing the importance of seeking care if symptoms of TB developed. *** National TB guidelines should be followed while investigating for TB. In addition, those individuals in whom TB is excluded after investigations (inducing individuals with fibrotic radiologic lesions) can be considered for LTBI treatment.
groups with statistical tests, chi-square test was conducted for categorical (nominal) and t test for quantitative data. A statistical significance level of 0.05 was considered. SPSS version 18 software was used for statistical analysis.

Results
One hundred and nineteen eligible patients including 74 men (62.2%) and 45 women (37.8%) participated in the study. The patients had a mean age of 58.55 ± 16.04 years, a mean duration of dialysis of 37.66 ± 32.09 months and a hemodialysis schedule of three sessions per week on average. Eighty-one patients (68.1%) were diabetic and 84 patients (86.6%) were hypertensive. Two patients (1.6%) had hepatitis B. None had hepatitis C or HIV. Thirtytwo of the participants were either currently or former smokers. None of the patients or his/her family had a history of TB. None of the patients had a history of TB or a family history of TB.
After an initial history taking and physical examination of the patients various symptoms such as cough, sputum, fever, night sweats were illustrated in (Table 1).
Then the symptomatic patients were then clinically examined by a pulmonologist who suspected seven of them as TB patients and referred for sputum smear test for TB bacillus and other necessary examinations. However, none of them was confirmed having TB.
According to the TST results, 97 patients (81.5%) with an induration equal to or greater than 10 mm were considered positive ( Table 2). Mean of induration size for patients with positive results was 22.11 ± 12.51 mm and for all participants was 18.77 ± 13.97 mm.
After performing additional diagnostic tests in patients who had symptoms indicating TB and ruling out active TB, 97 hemodialysis patients (81.5%) were diagnosed as having LTBI (Figure 2). The cases of LTBI and non-LTBI were compared in terms of other variables (such as gender, age, duration of dialysis, history of smoking, diabetes and hypertension). A statistically significant relationship was observed only between gender and LTBI, since it is more common in males than females in our study (Table 3).
Without any preventive treatment, none of the participants developed active TB until about two years after the end of the study.

Discussion
The aim of this study was to determine the frequency of LTBI in patients undergoing hemodialysis. In the study, the gender distribution of the patients was 74 (62.2%) male, which is almost the same as that for ESRD patients (16).
Studies show that LTBI diagnosed by either TST or IGRA can predict the development of active TB in dialysis patients (17) and therefore, requires preventive treatment (3). However, the relative risk of developing TB in dialysis patients with LTBI in comparison to those without LTBI is estimated to be about 1 (21). In Taiwan in 2013, 2016, and 2020, the ESRD patients were examined for LTBI with IGRA blood test and the prevalence of LTBI was reported to be 25%, 20.5%, and 19.2%, respectively (22,23).
On the whole, in all of the aforementioned studies, the prevalence declared by both IGRA and TST test results were clearly different from our results. To be sure about the correct technique of performing TST, we employed a health professional with more than 15 years of experience in TST practice. All other necessary aspects regarding expiration date and maintaining cold chain for PPD vials were fully controlled.
The percentage of positive TST tests in our patients was much higher than in other studies, whether performed with TST or IGRA. TST is a valuable test but false positivity is of concern. In low-risk individuals, most positive reactions are, in fact, false positives due to the low-specificity of the TST test (24,25).
Numerous studies, including several systematic reviews, have been conducted to compare the TST and IGRA tests and nearly all of them acknowledge that the IGRA test has more sensitivity and specificity and is more reliable than the TST in diagnosing LTBI in dialysis patients and ESRD (26,27).
The Pasteur Institute of Iran started manufacturing the BCG vaccine in 1974 and vaccination of certain groups, including primary school students was begun. In 1988, the BCG vaccine was produced on a large scale in the country (28). More than half of the dialysis patients in our study also had a history of BCG vaccination. The debate over BCG vaccination and its impact on the TST test result has been going on for years. BCG vaccination may result in a false positive TST test for years (27)(28)(29)(30).
The World Health Organization notes that if BCG is given at birth, it will have little effect on the specificity of the TST test. Therefore, it points out that BCG vaccination history should not be a determining factor in test selection (15). However, BCG administration after infancy or repeated administration causes more severe and larger TST reactions (31).
The results of several systematic reviews, including the 2008 study by Pai et al, and the 2011 study by Diel et al, and the guidelines of the US Centers for Disease Control and Prevention (CDC) suggest that in individuals with a history of BCG vaccination the TST may cause a false positive reaction, therefore IGRA is more specific and is the preferred test (32)(33)(34)(35)(36)(37).
Another reason to use an IGRA test instead of a TST test in individuals with a history of BCG vaccination is to avoid the inappropriate administration of isoniazid (38). It is important to know even in confirmed LTBI cases, the effectiveness of existing drugs is about 60% to 90% (15). However, some studies indicate that the history of BCG vaccination does not have a significant effect on TST size (39).

Conclusion
The prevalence of LTBI and the risk of active TB is high in hemodialysis patients and it is necessary to make an effort to diagnose LTBI and apply a preventive treatment.
Most of the patients in our study who are TST positive may be infected with non-TB mycobacteria and/or have a history of BCG vaccination which may be false positives. Even we are not sure that the negative results are all true. Although WHO algorithm for LTBI screening has helped us to assure the participants who have not active TB, we are in doubt about the LTBI, while it is difficult to make a decision about preventive treatment of them.
Even though IGRA blood tests are more sensitive and specific than TST, their results are not absolutely reliable. Therefore, for achieving the global end TB goal, our recommendations are listed below; In the future, we may need to expand LTBI screening to a larger population. In long-term strategic planning, it is suggested that World Health Organization attempts to set goals for innovating cheaper and more accessible diagnostic methods. More reliable tests that help health professionals to administer treatment with less doubt are necessary.
In WHO algorithm, TST and IGRA are assumed to be equal for screening LTBI in hemodialysis individuals. However several studies indicate low-specificity of TST and suggest the IGRA test to be as the preferred method. It is suggested to consider history of BCG vaccination in the algorithm and explain it as a determining factor in test selection. After analyzing data, some colleagues advised us to conduct IGRA for TST confirmation. According to such doubtful results, using TST for LTBI screening needs further cost-effectiveness analysis.

Limitations of the study
For some patients, the researcher had to visit several times to contact the patient. No definite officially recorded history of BCG vaccination was present.
Authors' contribution AZM, BH were the principal investigators of the study.
AA was the consultant physician for clinical differential diagnosis and deciding on final diagnosis. PN was the research assistant and helped in preparing the design and methodology of the study. All authors participated in preparing the final draft of the manuscript and critically revised the manuscript contents. All authors have read and approved the content of the manuscript and confirmed the accuracy or integrity of any part of the work.

Conflicts of interest
The authors declare that they have no conflict of interest.

Ethical issues
This study followed the ethical standards of the Helsinki Declaration. The ethics committee of Lorestan University of Medical Sciences approved this study (IR.LUMS. REC.1397.078). Written informed consent was obtained from all participants before any intervention. This study was extracted from the M.D thesis of Pardis Nasiri at that university (Thesis #899). Additionally, ethical issues related to publishing articles including plagiarism, data fabrication, double publication have been completely observed by the authors.