Lessons learned from continued TB outbreaks in a high school

We investigated the aftereffects of confirmatory QuantiFERON testing (QFT) added to a positive tuberculin skin test (TST). We reviewed the pre and post course of sequential tuberculosis (TB) outbreaks in a high school where massive 43 active TB cases had been found within one year before delayed contact investigation. And we investigated the TB development in relation to initial TST and QFT during mean follow-up of 3.9 ± 0.9 years. After delayed contact investigation for two subsequent TB outbreaks, 925 contacts were divided into the following 3 groups: TST- (n = 632), TST+/QFT+ (n = 24), TST+/QFT- (n = 258). QFT- was more prevalent than QFT+ in contacts with 10mm ≤ TST <15mm (158, 61.2%) compared with TST ≥15mm (100, 38.8%) among the TST+ reactors (P < 0.001). Among the 258 TST+/QFT- subjects, 256 received no latent TB infection (LTBI) treatment, but 7 contacts developed TB during follow-up. Among these 7 patients, 4 had initial TST ≥15mm and 3 had 10mm ≤ TST <15mm. In conclusion, the delayed contact investigation for LTBI in a high school resulted in continued TB developments. False-negative QFT performed late among the TST+ reactors should not be considered criteria for LTBI treatment. Additionally, the contacts only with TST ≥15mm should be considered for LTBI treatment in congregate settings of intermediate-burden countries.


Introduction
To eliminate tuberculosis (TB), rapid diagnosis and treatment of infectious TB patients in TB high-burden countries and control of latent TB infection (LTBI) for TB contacts in low-burden countries are important main strategies [1,2]. However, how to control LTBI in intermediate-burden countries such as South Korea where the incidence rate of active TB is 80/100,000 [3] is not well understood, although WHO recommends the tuberculin skin test (TST) and interferon-gamma release assay (IGRA) guidelines based on TB burden with a cut-off level of 100/100,000 that discriminates the high from low burden in addition to economic status [4,5]. PLOS  Traditionally, the TST has been used to identify LTBI preferentially in children and adolescents. The WHO recommends that IGRA should not replace TST in low-income countries, but can replace the TST in high-income and upper middle-income countries with estimated TB incidence less than 100/100,000 [5]. Recently, the United States recommends the IGRA single test can be used for the diagnosis of latent TB in all cases except for subjects less than 5 years of age [6] and the United Kingdom recommends the IGRA can be offered as an additional test for individuals 2-17 years of age with an initial negative TST result [7]. However, in several countries, a two-step strategy consisting of an initial TST and confirmatory IGRA was utilized which was useful in high-income countries [8]. The Korean LTBI policy, which adopted a two-step strategy for contact investigation during school outbreaks has been expanded with few accumulated evidences following the changing trend in world LTBI guidelines.
Even though IGRA have many merits compared with TST in the aspect of convenience and interpreters' errors except high costs, other disadvantages of using confirmatory IGRA added to a positive TST should be considered during public contact investigations. Missed identification of true LTBI due to false-negative IGRA in TB outbreaks of schools or military communities consisting of 2-3 years of community life can result in continued TB outbreak situations accompanied by socioeconomic loss [8]. When using a two-step screening strategy, performing an IGRA test within 3 days of administering the TST is safe to prevent spontaneous conversion and reversion [9]. In addition, variability of Quanti-FERON-TB Gold In-Tube test (QFT; Cellestis Ltd, Carnegie, VIC, Australia) results can occur due to technical errors caused by impact of blood volume and tube shaking, therefore these errors should be avoided [10].
In this study, we identified the disadvantages of the two-step strategy (a TST followed by an IGRA for TST+ reactors) and the meaning of strong positive TST results after a TB outbreak in congregate settings such as high schools.

Change in Korean TB control policy for contact investigations
The contact investigation policy for TB outbreaks was reinforced after 2011 when the standardized protocol for contact investigation was implemented [11]. In 2013, a systematized contact investigation team from Korean Centers for Disease Control and Prevention (KCDC) was launched. Before the update of the 2017 Korea LTBI guideline recommending a TST or IGRA as a diagnosis tool for TB close contacts 5-18 years of age, a TST followed by an IGRA (the two-step strategy) for contact investigation of a TB outbreak was the conditionally adopted strategy.

TB outbreaks and subsequent approach
In 2007, multiple TB outbreaks in middle and high schools occurred in the local city of South Korea where one tenth of 700,000 residents were immigrants and the TB incidence rate (314/ 100,000) was significantly higher than the average rate for South Korea (97/100,000) during the same period [12].
Initially, a TB outbreak in a high school was reported to the public health care center. During an epidemiologic investigation, the isolated strains from this TB outbreak were the same strains based on restriction fragment length polymorphism (RFLP). All close contacts from the TB outbreaks were adolescents in the same school; all 947 students were analyzed for this retrospective review. Using the data from the 2005-2007 TB outbreaks, we reviewed the TB outbreak courses of a high school and identified the TB development associated with the initial TST and QFT results using the Korean national claims database.

TST and QFT
The TST was administered by intradermal injection (0.1 ml) of2 tuberculin units of purified protein derivative (RT 23; Statens Serum Institute, Copenhagen, Denmark) into the anterior surface of the forearm with a disposable syringe and a 27-gauge needle by using the Mantoux technique. Induration was measured after 48-72 hours with a ruler or a caliper by an expert nurse. The QuantiFERON TB Gold In Tube test (QFT; Cellestis Ltd, Carnegie, VIC, Australia) was performed as a second-step test among TST+ (cut-off !10mm) reactors, and interpreted according to the manufacturer's instructions. Whole blood was collected by venipuncture from each subject and incubated for 16-24 hours in 3 separate conditions. A QuantiFERON value of 0.35 international units or more was deemed positive. Contact investigation with LTBI treatment policies in TB outbreak are mandatory under the Korean national tuberculosis control program (NTP) since 2007 [11]. Therefore, the requirement for informed consent was waived and this study was approved by the Institutional Review Board of the Korea University Ansan Hospital, Ansan, South Korea.

Statistical analysis
All analyses were performed using SPSS software, version 20.0 (SPSS Inc., Chicago, IL, USA). The χ2 tests were used to determine the differences between groups based on the TST cut-off value. Logistic regressions using different variables were done for the prediction of TB development. And, ROC (Receiver Operating Characteristic) curves were constructed to establish the optimal cut-off points of TST induration size. All tests for significance were 2-tailed and Pvalues < 0.05 were considered statistically significant.

Continued TB outbreaks without LTBI diagnosis and treatment
From November 2005, when infectious source cases of TB developed, a total of 3 active TB cases developed until January 2006 in a high school. Four months after this event, 12 more sequential active TB cases with positive AFB culture were identified and 28 more clinical TB cases with abnormal chest X-rays (CXRs) were diagnosed from May 2006 to December 2006 based on the results from mass screening with CXR and sputum AFB smears and cultures ( Fig  1). After RFLP fingerprinting, a total of 15 active TB cases with positive AFB cultures were identified as the same strain (Fig 2). Belated contact investigation with TST and QFT was conducted in January 2007.

TB among the TST+/QFT-contacts
Using the database, we investigated the progression of TB among TB contacts within a 3-year follow-up period after TST. Seven contacts developed TB during the follow-up (Fig 3, Table 3). All were TST+/QFT-and received no LTBI treatment. One patient was sputum smear-positive for TB with cavitary lung lesions on CXR and previous TST size of 15mm. One patient was sputum culture-positive for TB with cavitary lung lesions on CXR and previous TST size of 15mm. Four patients had culture-negative clinical TB with cavitary lung lesions on CXR and previous TST sizes of 18mm, 15mm, 11mm and 13mm. One patient had culture-negative clinical TB with pleurisy on CXR and a previous TST size of 10mm. In total, 4 patients had previous strong positive TST results !15mm (Table 3). Among subjects with TST induration !15mm and QFT-contacts, the rate of TB progression was 4/100 (4.0%) and among subjects with TST induration size !10mm and <15mm, the rate of TB progression was 3/158 (1.9%).
The ROC (Receiver Operating Characteristic) curve was constructed to establish the optimal cut-off points of TST induration size after exclusion of the contacts who received LTBI  treatment. ROC curve for the prediction of TB cases in this study showed that the criteria of TST !10 mm (AUC 0.86) was more accurate than that of TST !15 mm (AUC 0.73) to predict TB cases for all contacts. However, for the contacts with TST+/QFT-results, the overall accuracy was the most highest with the criteria of TST !15 mm (Fig 4).   [13], although the immune status of the host [14] or completion of LTBI treatment can affect the IGRA dynamicity [15]. In the same context, low positive rate of QFT (8.5%, 24/282) among the TST positive reactors and the false-negative QFT results even in the strong positive TST (!15mm) reactors apparently seem to originate from spontaneous reversion of the QFT test rather than technical error [16,17] because QFT tests were delayed by more than 3 months after the initial TB outbreak; Nevertheless our results showed a strong positive TST (TST !15mm) tended to be associated with QFT+ compared with weak positive TST (10mm TST < 15mm) ( Table 2) [15].
Although the initiation of the infectious period cannot be determined, an assigned initiation of 3 months before a TB diagnosis is recommended based on the TB patient's index characteristics such as cavity on CXR [18]. Therefore, QFTs conducted for a retracement of contacts after an active TB should not be preferred to TST due to possible reversion of the QFT [18].
Recently in the United States, the 2016 American Thoracic Society, Centers for Disease Control and Prevention, and Infectious Diseases Society of America recommends the use of IGRA or TST for the diagnosis of latent TB in all cases, but recommends IGRA for subjects over 5 years of age [6]. In the United Kingdom, the NICE guideline have been recently updated and recommends using IGRA alone for subjects 18-65 years of age if a large number of people require screening. In particular, for individuals 2-17 years of age, LTBI should be screened only using TST for contacts with infectious TB patients and when the initial TST is negative, additional IGRA tests should be performed after 6 weeks and the TST repeated [7]. In the same context, based on the results from our report, the two-step strategy should be modified for Korean children and adolescents using additional evidence from large scale studies in South Korea.
The present study had several limitations. First, we could not determine if the TB strains from the 7 contacts that developed TB during follow-up were the same as the TB strain from the index case. Second, in this study, QFT was significantly delayed after first TB outbreak and the findings were limited to only a single school, therefore, the findings cannot be generalized. To propose a basis for amending Korean guidelines, the confirmatory IGRA test for the TST positive reactors should be reverified based on concrete evidence from studies where QFT is performed on a large number of subjects.
In conclusion, false-negative QFT among TST+ reactors could cause continuous sequential TB outbreaks with the use of confirmatory IGRA tests among the TST+ reactors (two-step strategy) for LTBI treatment in school TB outbreaks. Furthermore, the contacts with a strong induration size of TST (!15mm) should be considered for LTBI treatment without a confirmatory IGRA test especially in intermediate-burden countries.