The TST is not a routine investigation for TB infection; however, it is helpful for the identification of remote or occult TB infection, particularly in persons at risk of tuberculosis infection (i.e., those who need immunosuppressant therapy or have a history of close contact with persons with TB). TB infection is an infection that has been detected in SSc patients [11, 12]. The diagnosis of pulmonary tuberculosis infection is difficult in SSc patients with previous chest symptoms related to their underlying SSc disease. Our study reported the possible role of the TST for TB infection detection in SSc, which skin thickness might limit TST evaluation.
The respective sensitivity and specificity of the results depends on the community area, history of BCG vaccination, and host. The specificity is high in persons who have not had the BCG vaccination (97%; 95%CI 95–99) [13]. Jamil et al. reported the respective sensitivity and specificity—with a cut-off of ≥ 10 mm—for active pulmonary TB in persons having had a BCG vaccination was 86% and 74% versus 77% and 74% in persons with extrapulmonary TB [14]. Yaacob et al. reported a respective sensitivity and specificity of the TST for active pulmonary TB of 86% and 90% [15]. A positive TST is not necessarily due to active TB but can simply indicate exposure to Mycobacterium infection as the test cannot differentiate between a latent and an active TB infection.
According to our analysis, the TST is not sufficiently sensitive for detecting TB infection in SSc patients. Eighty percent of the SSc patients had a TST skin reaction size of < 5 mm despite having a previous history of infection or recent TB. The sensitivity of the TST in our study was lower than in previous studies;5,6 this may be due to the specific SSc population and the difference in the prevalence of TB infection in the general population of other countries. Most of the previous studies represented general populations suspected of having TB infection, where the prevalence of TB infection was 18–24%. Our study population comprised SSc patients with a prevalence of TB of 10%, which is lower than in previous reports. The severity of skin thickness negatively correlated with indurated skin reaction in our SSc patients, so the skin reaction for the TST for TB infection in SSc patients was not apparent as presumed. The severity of the skin thickness can be a factor that affects the sensitivity of the TST. According to the results, we do not recommend using the TST as a screening tool for diagnosing TB infection among patients with SSc. The sensitivity of other test methods (i.e., IGRA) should be investigated as a screening tool for diagnosing TB infection.
False positives and negatives are limitations of the TST. A false positive can occur in persons who have had the BCG vaccination and/or are infected with a non-tuberculosis mycobacterium [16]. Meanwhile, false negatives have been reported in children, the elderly, persons with severe hypoalbuminemia, disseminated TB, and persons with HIV [16, 17]. A meta-analysis by Wang et al. included 26 published articles on the TST and revealed that a previous BCG vaccination was associated with a positive result (Relative risk 2.12, 95%CI 1.50–3.00) compared to persons without a BCG vaccination [18]. The effect of BCG on the TST is reported to be no greater than 10–15 years after vaccination, so false positives are uncommon in communities with a low prevalence of TB infection [19]. Skin indurations of > 15 mm are more likely to indicate a TB infection (remote, recent, or active TB infection) than a BCG vaccination [18, 20]. In general, an indurated skin reaction size of > 10 mm is considered TST-positive in children and adults; however, a reaction size of < 10 mm does not exclude TB [21].
Our findings reveal that a TST skin reaction size of ≥ 15 mm is suggestive of a TB infection (specificity 97.4%). Notably, the sensitivity was low (around 6%), and around 3% (4 from 151 cases) of patients with no history of TB contact and/or no TB infection also had a skin reaction size of ≥ 15 mm. The false-positive TST among those with no history of TB contact and/or with no TB infection might be explained by the effect of the BCG vaccination, but we did not have a TST comparison with a control of ‘no BCG vaccination’. Since all of our SSc patients received a BCG vaccination at birth—in compliance with the national immunization program—our interpretations are limited to SSc patients who had had a BCG vaccination.
Extensive skin thickness—evaluated using the mRSS—affected the TST indurated skin reaction. The majority of our SSc patients were the dcSSc subtype, and our study demonstrated that a high mRSS was negatively correlated with the indurated reaction size to the TST (Rho -0.23, p = 0.003). The mRSS at the right forearm, where TST was performed, did not significantly influence the outcome of the TST skin reaction size. The skin reaction size according to the TST would not be due to local skin thickness but to the overall severity of the disease.
The skin reaction to the TST is a cell-mediated delayed hypersensitivity (Type IV) reaction mediated by skin macrophages, monocytes, and T cells. T cells—sensitized by a previous TB infection—move to the skin test site and release lymphokines. T cells— particularly CD4 + T cells—play a role in the reaction. The study of TST in adults infected with HIV revealed that the patients who were TST-positive had higher CD4 + T cell counts than those who were negative [22]. CD4 + T cells also take part in the pathogenesis of SSc. Activated CD4 + T cells have been reported in the skin, lung, and stomach in SSc patients, and the cells express pathogenic cytokines in both serum and tissue of SSc patients [23, 24].
Despite CD4 + T cells being detected in the skin of SSc patients, the skin reaction to the TST was not as pronounced as expected, particularly in patients who had extensive skin thickness. Therefore, the finding presumes that CD4 + T cells in the skin of SSc patients will respond to some antigen in the skin and cannot move or release the lymphokines related to a TB infection. Nevertheless, the role of CD4 + T cells in the mechanism of cell-mediated delayed hypersensitivity in SSc remains unclear. Therefore, further research is needed to test the hypothesis and study the sensitivity of TST among patients with the lcSSc subset and those who have less skin thickness.
Current steroid use did not influence our SSc patients’ skin reaction size after the TST. We know that steroids can suppress the TST reaction [25], and the reaction size is negatively associated with a higher dose of steroid treatment [26]. Another study found that low-dose steroid treatment did not influence the skin reaction size of the TST among patients with rheumatoid arthritis [27]. Most of our patients received a low to moderate dose of steroid because we wanted to avoid a steroid-induced renal crisis; this might explain why steroid did not affect skin reaction size to the TST in SSc patients. We conclude that low-dose steroid did not influence TST reaction size in SSc patients as it does in rheumatoid arthritis patients. This result gave us confidence in interpreting the TST in SSc patients receiving low-dose steroid during the evaluation. Owing to the unavailability of data on the cumulative dose and the duration of steroid treatment, we cannot comment on their respective contribution to the effect on the TST in SSc patients.
There is currently no test established for the definite diagnosis of occult or latent TB infection. The TST, however, can demonstrate whether or not there is an immune reaction response to a previous mycobacterium exposure. The cut-off skin reaction size for the TST for diagnosis of latent TB infection has been classified according to the risk of exposure and the risk of developing active TB based on the WHO guidelines [28]. Since—by our analysis—skin thickness affects the skin reaction size of the TST, the skin reaction size among SSc patients might not be interpreted as in the general population or even according to the WHO guidelines. Even though some patients had a history of TB contact, we cannot determine the cut-off skin reaction size, the sensitivity, or specificity for diagnosing latent TB infection among SSc patients.
Interferon-gamma assay (IGRA) is another screening tool for diagnosing both latent [29] and active TB infection [30]. Although a study showed the concordance between TST and IGRA results [31], the accuracy of TB infection tests trended to improve when using IGRA (i.e., there was a low rate of false positives) [16, 32]. IGRA is more reliable than the TST in cases of BCG vaccination because a BCG vaccination does not confound the IGRA [31]. In addition, the cross-reactivity with non-tuberculosis Mycobacterium infection is low with the IGRA test [16]. Notwithstanding, there is limited data on IGRA testing in various settings and populations, so the results must be interpreted in concurrence with clinical, radiography, and risk assessment [32]. Moreover, the IGRA is much more expensive than the TST and is available at only a few centers, while the TST is widely available.
The current study had some limitations: (a) the low prevalence of TB infection in patients with SSc, which would affect the sensitivity of the test; (b) the absence of patients without BCG vaccination, so the sensitivity and specificity of the TST in such patients could not be determined; (c) the lack of a comparison of the sensitivity and specificity of the TST among SSc patients with/without the BCG vaccination; (d) we cannot provide the TST reaction after boosting in patients who received immunosuppressants due to a shortage of tuberculin and the crossectional study design; and, (e) because of financial limitations, none of our patients received biological DMARDs, so we cannot conclude whether biological DMARDs affect TST reaction. The strengths of our study were (a) the use of the TST, which is a widely available, feasible, and practical tool for diagnosing TB infection in resource-limited areas, making the results of value to daily practice; and, (b) the inclusion of parameters of interest, particularly local skin thickness where the TST would be injected. The severity of skin thickness and current steroid/immunosuppressant therapy can influence the TST skin size reaction in SSc. The findings of our study are valuable for evaluating TB infection among SSc patients and for planning future studies.