Latent tuberculosis infection (LTBI) is  defined by the World Health Organization(WHO) as a state of persistent immune response to stimulation by Mycobacterium tuberculosis (MTB) antigens without evidence of clinically manifested active tuberculosis (TB)(1). The US Centers for Disease Control and Prevention defines LTBI as a person who is infected with Mycobacterium tuberculosis but has no clinical signs or symptoms of active tuberculosis(2). It is estimated that 23% of the world's population is infected with MTB and at risk of developing active TB. Latent tuberculosis  infection  can progress to active TB disease in roughly 5% to 15% of infected individuals over their lifetimes(3) .While most people with latent infection never develop the disease, the risk of progression is highest within the first 2 years after initial infection. Patients with latent TB infection are asymptomatic. Active tuberculosis disease must be excluded before testing for latent TB infection.

This  hospital based retrospective  study was done to find out the burden of LTBI among contacts of Tuberculosis patients .Chauhan A et al  published a systemic review and metaanalysis  in the Indian Journal of Medical Research in the year 2023,they analyzed data from 77 studies (including 31 cohort studies) from 2013-2022, there was pooled LTBI prevalence ,irrespective of risk, estimated at 41% (95% CI: 29.5-52.6%) based on community-based cohort studies while the general population prevalence,excluding high-risk was estimated at 36% (95% CI: 28-45%). They depicted a high LTBI burden among groups such as diabetes mellitus, smokers, malnourished, and alcoholics(4).Also in a cross-sectional study done  in Rumtek monastery  in east Sikkim ,  the overall LTBI rate was 44.2% (5).

Contact investigation is an essential component of the WHO’s TB management algorithms to detect those with LTBI among high-risk groups, and to initiate tuberculosis preventive treatment (TPT). Individuals of all ages who are in contact with patients with active TB infection are at a significantly increased risk of developing a new TB infection. WHO advocates that trained staff should elicit the required information from the index TB patients and counsel patients and their family members on the importance of LTBI testing, TPT initiation and completion.

WHO recommends the tuberculin skin test (TST) or interferon-gamma release assays (IGRA) to detect LTBI. IGRA results in fewer false-positive results than with TST (8). IGRA  results are not affected by Bacille Calmettee Gue´rin (BCG)-vaccination and by the majority of environmental mycobacteria; moreover, only one patient-visit is required. However, since these assays are based on an immune response detection, they have a poor sensitivity in children and in immune-compromised subjects furthermore, they do not discriminate between active TB and LTBI and poorly correlate with the risk of developing active disease. Exclusion of active TB is to be done by sputum Catridge based nucleic acid amplification test (CBNAAT)  before  the diagnosis of latent TB infection is made . Identifying and treating patients with latent tuberculosis infections is critical for NTEP goals.

METHODOLOGY:

This present study was  done in Intermediate Reference Laboratory in a tertiary care hospital.The case report form data containing  age,sex,demographic details of the contact, past history , any treatment history  was collected for analysis  from IRL  laboratory register   from January 2022 till December 2025 for a period of 4 years  maintained in the laboratory. Statistical analysis:Data  analysis was done using excel .Categorical variables were presented as percentages .

Institutional Ethics Committee clearence was taken for the study.

RESULTS :

There was a total of 724 blood samples from  suspected contact during the period of four years from January 2022 till December 2025. Out of  724 only 295 (40.74%) blood sample was positive IGRA while out of remaining 429 (59.25%) there was 91 ( 12.56%) was indeterminate and 338 (46.68%) was negative . (Table 1). In the year 2022, out of 299 samples tested for LTBI, 126 (42.14%) were positive, 161 (53.84%)were negative and 12 (4.01%) were indeterminate.In the year  2023, out of 103 samples tested for LTBI, 34 (33.00%) were IGRA positive, 26 (25.24%) were IGRA negative and 43 (41.74%) were IGRA indeterminate.In the year  2024, out of 132samples tested for LTBI, 53 (40.15%) were IGRA positive, 69 (52.27%) were IGRA negative and 10 (7.57%) were IGRA indeterminate.while in the year 2025, out of 190 samples tested for LTBI, 82 (43.15%) were IGRA positive, 82 (43.15%) were IGRA negative and 26( 13.68%) were IGRA indeterminate.

The age group was ranging from 4 years upto 89 years among IGRA positive  during the entire four years period.The median age was 33years,29 years,29 years,39years among  the 126,34,53,82 IGRA positive in the year 2022,2023,2024 and 2025 respectively.(Table no 2)

The overall male to female sex ratio was (1:1.25) .In the year 2022, out of the 126 IGRA positives, 52/126 (41.26 %) were males, 74/126 (58.73%) were females. In the year 2023, out of the 34 IGRA positives, 16/ 34 ( 47.05% ) were males, 18/ 34( 52.94%) were females. In the year 2024,out of the 53 IGRA positives,16/53(30.18%) were males, 37/53 (69.81%) were females.In the year  2025,out of the 82IGRA positives,47/82 ( 57.31% ) were males, 35/82  ( 42.68% ) were females.(Table no.3)

Table 1.Total  blood samples tested for IGRA from January 2022 till December 2025

Table no 2. Age distribution among IGRA Positives :

Table no 3. Sex Distribution

DISCUSSION:

The national tuberculosis  prevalence survey done in the year 2019 till 2021 estimates that the prevalence of LTBI in India  was found to be 33%(6). Studies from Vietnam, Indonesia and Philippines reported LTBI rates from 30% to 50% in high risk groups(7). Similarly a meta analysis done by Chauhan A et al where 41% prevalence was seen  in a community based cohort study in population with high risk groups(4). However in our retrospective hospital based  study,  the LTBI positivity was  around 40.74%.  

The proportion of infection rises with age especially in Africa and south east region, where there was more LTBI  in the younger age groups while in Europe and American regions older age group was affected(8).An increasing trend of TBI was observed with increasing age in India(4).However our  study also shows Latent bacterial tuberculosis infection mostly among  adult age group.

For detecting LTBI infection ,there are three  standard  tests namely Tuberculin skin test (TST),  Interferon-gamma (IFN-γ) release assays (IGRA) and Cy-TB test. In the present study IGRA was used for diagnosing Latent Tuberculosis infection.

Majority of the LTBI  individuals (around 90 − 95%) remain latent throughout their life and rest develop active cases of TB. However, deficiency in immunity level can lead an individual  to develop active TB(9).LTBI testing and treatment  in high-risk groups, such as people living with HIV, and close contacts of TB cases will reduce the burden and subsequent re activation to TB disease. Risk of contracting TBI is significantly greater among household contacts compared to general population (10,11,12).

CONCLUSION:

The present study showed the positivity rate of Latent Tuberculosis  Infection was  40.74% among contacts of tuberculosis patients by IGRA. Hence our study can guide the physician the need for prompt diagnosis and treatment of LTBI  thereby  reducing the burden of LTBI in the contacts and  the possibility of re activation to TB disease .

REFERENCES:

1. World Health Organization. Latent TB Infection (2018): Updated and Consolidated Guidelines for Programmatic Management. Geneva, Switzerland: World Health Organization:WHO/CDS/TB/2018.4
2. CDC (2025), Clinical overview of Latent Tuberculosis Infection .Apr;17
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