What constitutes high-risk exposure to tuberculosis (TB)?

High-Risk Exposure to Tuberculosis

High-risk exposure to tuberculosis is defined by close contact with infectious pulmonary or laryngeal TB patients (positive sputum smear or culture), particularly in settings with prolonged duration, poor ventilation, and high bacillary concentration in the air. 1

Defining High-Risk Exposure Characteristics

Exposure factors that constitute high risk include:

• Close contacts of patients with pulmonary or laryngeal TB who have positive AFB sputum smears or positive M. tuberculosis cultures 1
• Household and family members living with infectious TB patients 1
• Shared bedroom exposure with an index TB patient, which significantly increases transmission risk 2
• Prolonged or intensive contact in congregate settings including prisons, homeless shelters, migrant shelters, and indoor spaces like schools or offices 1
• Exposure duration is critical—longer contact time correlates directly with higher infection risk 1, 3
• Environmental factors including crowding, poor ventilation, and small enclosed spaces that increase bacillary concentration in the air 1, 4
• Exposure to multiple index patients rather than a single source case 2

Highest Priority Contacts Requiring Immediate Evaluation

The following contacts require the most rapid follow-up and highest priority for testing:

• Children younger than 5 years (especially those under 3 years) who have the greatest risk for rapid progression to severe disease and should be considered high-risk contacts regardless of other factors 1, 5
• HIV-infected persons who have substantial risk for disease progression (35-162 cases per 1000 person-years) 1, 5
• Immunosuppressed individuals including those on prolonged corticosteroid therapy (≥15mg/day prednisone for ≥1 month), patients receiving dialysis, transplant candidates, and those on anti-TNF therapy 1, 5, 6
• Persons with fibrotic changes on chest radiograph consistent with prior TB 5, 6

Quantifying Transmission Risk

The risk of TB transmission depends on four key determinants:

• Concentration of tubercle bacilli in the air—index patients with positive sputum smears or cavitary disease on chest radiograph are most infectious 1
• Airflow and ventilation—poor ventilation dramatically increases transmission risk, as demonstrated in outbreak investigations where low ventilation rates contributed to massive spread 4
• Duration of contact—longer exposure correlates with higher infection rates, with close contacts showing 46.30% prevalence of MTB infection at baseline 7
• Susceptibility of the contact to infection—host immune status significantly affects both infection and progression risk 1

Temporal Risk Following Exposure

The timing of disease development after exposure follows a predictable pattern:

• First year post-exposure carries the highest risk, with 2.15% cumulative incidence of active TB among close contacts 7
• Peak risk occurs in the first 5 months following infection, with some studies showing cumulative hazard of 14.5% at 1,650 days when accounting for censorship 8
• Children under 5 years have extraordinarily high risk reaching 56.0% cumulative hazard, while children aged 5-14 years show 27.6% risk 8
• Risk declines over time but remains elevated, with 2-year cumulative incidence of 1.21% and 5-year incidence of 1.11% 7

Special High-Risk Exposure Scenarios

Beyond traditional household contacts, high-risk exposures include:

• Social networks and congregate activities such as church choirs, card games, and other group activities where transmission may not be initially recognized 1
• Healthcare settings where patients receive care before diagnosis or where aerosol-generating procedures are performed on undiagnosed TB patients 9, 4
• Nosocomial exposures in cancer wards or among immune-compromised hospitalized patients, which can result in rapid progression and high mortality 4
• Prison environments where closed spaces, prolonged cohabitation, poor ventilation, and cold weather keeping inmates indoors all accelerate transmission 4

Critical Pitfalls to Avoid

Common errors in assessing high-risk exposure:

• Failing to identify non-traditional transmission sites beyond work and home, missing social networks and congregate activities that facilitate spread 1
• Underestimating risk in brief exposures—secondary TB cases have occurred with contact duration of less than 5 minutes in healthcare settings 4
• Delaying contact investigation—visits to exposure sites should be conducted as soon as possible, as delayed identification of source patients results in secondary cases 1
• Overlooking environmental assessment—failure to observe crowding, ventilation, and other transmission factors at the exposure site 1
• Not considering drug resistance patterns of the source case when evaluating contacts of MDR-TB or XDR-TB patients, who require specialized evaluation 1

Testing Thresholds for High-Risk Contacts

TST interpretation is risk-stratified based on exposure level:

• ≥5mm induration is positive for close contacts of TB cases, HIV-infected persons, immunosuppressed individuals, and those with fibrotic chest radiograph changes 1, 5, 6
• ≥10mm induration is positive for residents of congregate settings, healthcare workers, immigrants from high-burden countries, and persons with medical conditions increasing TB risk 5, 6
• Recent converters (≥10mm increase within 2 years for those <35 years; ≥15mm increase for those ≥35 years) are considered high-risk regardless of absolute value 6