Timeframe for Symptom Development After TB Exposure
Most people exposed to tuberculosis never develop symptoms at all, but among those who do progress to active disease, approximately 54% develop symptoms within the first year after infection and 82% within the first two years. 1
Understanding the Natural History After Exposure
The Immune Response Window (0-8 Weeks)
The immune system requires 8-10 weeks after M. tuberculosis exposure to mount a detectable response on tuberculin skin testing (TST) or interferon-gamma release assay (IGRA). 2 This is the critical "window period" during which testing may be falsely negative despite true infection.
During this window, exposed individuals—especially children under 5 years and immunocompromised persons—should begin preventive therapy immediately after excluding active disease, even with negative initial testing. 2
Peak Risk Period for Active Disease (First 2 Years)
Among individuals who develop latent TB infection (LTBI), the risk of progression to active disease is highest in the first year: 12.9 cases per 1,000 person-years in year one, dropping dramatically to 1.6 cases per 1,000 person-years in subsequent years. 1
In a large British study of tuberculin converters, 54% who eventually developed clinical TB did so within the first year after infection, and 82% developed disease within two years. 1
Lifetime Risk and Latent Infection
The majority of infected individuals (85-95%) never develop active disease at all—they remain in a state of latent TB infection (LTBI) characterized by positive TST/IGRA but no symptoms, normal chest X-ray, and negative sputum studies. 3, 4
The overall lifetime risk of progression from LTBI to active disease is 5-15%, with most cases occurring within the first five years after initial infection. 3, 5
Clinical Implications for Contact Investigation
Immediate Actions After Exposure
Test all exposed contacts as soon as possible with TST or IGRA, but recognize that a negative result within 8 weeks does not rule out infection. 2, 6
Perform chest radiography and clinical evaluation immediately to exclude active disease in all contacts, regardless of test results. 2
Mandatory Repeat Testing
All contacts with initial negative TST/IGRA must undergo repeat testing 8-10 weeks (ideally 12 weeks) after the last exposure to capture delayed conversions. 2, 6
A positive result on this second test indicates infection occurred during the exposure period and warrants full treatment for latent TB infection. 2
High-Risk Groups Requiring Immediate Prophylaxis
Children younger than 5 years must start preventive therapy immediately after excluding active disease, even with negative initial testing, due to their vulnerability to severe disseminated TB and TB meningitis. 2
HIV-infected and other immunocompromised contacts should receive immediate prophylactic treatment after active disease is excluded, regardless of initial test results. 2
Critical Pitfalls to Avoid
Never assume a negative test within the first 8 weeks post-exposure rules out infection—this is the most common and dangerous error in TB contact management. 2, 6
Do not delay prophylactic treatment in high-risk contacts (especially young children and immunocompromised individuals) while waiting for the 8-10 week repeat test. 2
Always counsel all exposed contacts to monitor for TB symptoms (persistent cough ≥2-3 weeks, hemoptysis, night sweats, weight loss, fever) and seek immediate evaluation if any develop, even years after exposure. 2, 3
Defining the Source Patient's Infectious Period
For contact investigation purposes, the infectious period is estimated to begin 3 months before TB diagnosis, though in cases of prolonged illness or large lung cavities, an even earlier start should be used. 1
The infectious period closes when effective treatment has been given for >2 weeks, symptoms have diminished, and mycobacteriologic response is documented (e.g., decreased sputum smear positivity). 1