What is the minimum duration of exposure to Tuberculosis (TB) required to be considered at risk of contracting it?

From the Guidelines

You are considered at risk of contracting TB if you have been exposed to an infectious person for at least 8 hours in close proximity, as recommended by the CDC 1. The risk of contracting tuberculosis (TB) depends on several factors, including the intensity, frequency, and duration of exposure to an infectious person, as well as the ventilation of the space where the exposure occurred.

• The likelihood of infection increases with longer, closer, and more frequent contact with an infectious person, particularly in poorly ventilated spaces.
• Brief, casual encounters, such as passing someone on the street, rarely lead to infection, while sharing living space with an infectious person for days or weeks significantly increases risk.
• According to the guidelines for the investigation of contacts of persons with infectious tuberculosis, the optimal cut-off duration for exposure is undetermined, but a cut-off of 120 hours of exposure per month is suggested for pulmonary TB without lung cavities 1.
• However, for any specific setting, index patient, and contacts, administratively determined durations derived from local experience are recommended, with frequent reassessments on the basis of results.
• The CDC considers close contacts who have spent at least 8 hours in close proximity to an infectious TB patient to be at higher risk and recommends they be tested 1.
• Risk factors that increase susceptibility to TB include having a weakened immune system, young age, and malnutrition.
• If you've had significant exposure to someone with active TB, you should get tested even if you don't have symptoms, as early detection and treatment are crucial for preventing disease progression.

From the Research

Duration of Exposure to TB

• The duration of exposure to TB is a significant factor in determining the risk of contracting the disease 2.
• A study found that the prevalence of latent tuberculosis infection (LTBI) increased with increasing exposure duration, with an incremental prevalence increase of 8.2% per 250 exposure hours 2.
• The study suggested that there may be a risk threshold of 250 hours of exposure to a patient with infectious TB 2.
• Other factors such as age, sex, birthplace, and proximity to the index patient also play a role in determining the risk of LTBI 2.

Risk Factors for LTBI

• A study identified several risk factors for LTBI, including:
  • Origin from a TB endemic country 3
  • Recent stay in a TB endemic country 3
  • Duration of TB exposure 3, 2
  • Previous TB disease 3
  • Shared bedroom with an index patient 2
  • Exposure to more than one index patient 2

Diagnosis and Treatment of LTBI

• Interferon-gamma release assays (IGRAs) are more specific than the tuberculosis skin test (TST) in diagnosing LTBI 3, 4.
• IGRAs are associated with higher treatment completion and persistence rates compared to TST 4.
• A study found that receiving an IGRA or a TST followed by an IGRA was associated with increased treatment persistence and completion rates compared to receiving a TST alone 4.