Index cases
From November 2018 until March 2019, TB patients newly admitted to the Issyk-Kul regional TB Center, Issyk-Kul region, Kyrgyz Republic, were enrolled as index cases when they were (i) over 18 years of age; (ii) independently capable of making decisions; (iii) living in the region for more than 3 months; (iv) diagnosed with bacteriologically confirmed pulmonary TB; and (v) had anti-TB treatment initiated within previous 2 months. Index cases were categorized as either positive (ss+) or negative (ss-) for sputum smear microscopy, and underwent a standardized interview regarding potential contacts and living conditions.
Contact persons
Individuals the index case reported to have had contact with over the past 3 months prior to hospitalisation were enrolled when they (i) lived in the Issyk-Kul region; (ii) agreed to be visited; (iii) were accompanied by parent or legal guardian if under 18 years old; and (iv) had provided informed written consent. The frequency, intensity and duration of the exposure were key factors in the categorization of contacts. A streamlined approached was employed and contacts were categorized as home contacts when living or regularly staying in the same household as the index case (at a minimum 3 months prior to the index case having symptoms or having a presumptive TB diagnosis), or close contacts when having had regular contact in closed or poorly ventilated rooms for more than an hour several times per week over the past 3 months prior to the TB diagnosis. For all contact persons, medical conditions fostering TB infection (below 6 years of age, living with HIV, immuno-suppression, renal failure, diabetes) were queried; however, results were only presented to the doctor and not available for analysis. The household risk was assessed as “high” when two or more people on average lived in one room.
Home visit
A representative of the local branch of the national sanitary and epidemiological services (SES) visited enrolled participants who had home or close contact with the index case and established a history of symptoms (cough, thoracic aches, fever, nigh-sweats, wasting), performed a clinical examination, and for participants over the age of five blood collection for QuantiFERON; for children below the age of six a TST was performed. Symptomatic contacts were presented to local TB physicians who ordered digital chest x-rays in two planes and further TB diagnostics according to local standards. The QuantiFERON result was interpreted following the instructions of the manufacturer (Qiagen, Germany) at the National Reference Laboratory (NRL) in Bishkek. The TSTs were read by the representative of the local SES and assigned “reactive” with indurations of more than 5 mm after 48 to 72 h.
Sample transport
All blood samples for QuantiFERON were transported from the collection site to one of two regional hospitals located in either Balykchy or Karakol within 6 h of collection (Fig. 1). The 16 to 24 h incubation of the test tubes, as per manufacturer instruction, was performed at these hospitals before being collected by the laboratory services AquaLab as per a private public partnership agreement, and transported to the NRL in Bishkek. All following QuantiFERON analyses was performed at the NRL.
Diagnosis
Active TB was diagnosed based on bacteriological confirmation by doctors in the Kyrgyz Republic who are encouraged to follow the diagnosis algorithm for active TB as recommended by the WHO [14]. LTBI was assigned when the patient was not diagnosed with active TB and had a positive immune response measured by QuantiFERON or by TST (when valid Quantiferon-TB Gold plus results were not available).
Statistical analyses
All statistics were performed using STATA 16.0 (StataCorp LP, College Station, TX, USA). Statistical significance of predictors for either QuantiFERON positivity or symptoms were analysed using logistic regression modelling. Predictors for QuantiFERON positivity included type of contact (home or close), household risk (high or low), presence of symptoms (yes or no), and index case smear microscopy results. Predictors were selected based on the likelihood of transmission from the index case to the contact; household contacts are in close proximity to the index case, household risk indicates density of household living, presence of symptoms has been shown to increase IGRA response, and positive microscopy results of the index case indicate an infectious individual. Two additional logistic regressions were also performed with the outcome defined as either “presence of symptoms” or “TST positive result”. Stratified analysis was separately conducted for adults (≥15 years) and children (< 15 years) due to an association of QuantiFERON positivity with age [15]. Analysis was further stratified by index case smear results [16]. Significant differences were those with a p ≤ 0.05.
Ethical clearance
Permission to conduct the study was obtained from the Ethical Committee of the Kyrgyz Ministry of Health.