TB Contact Investigation

In tuberculosis (TB) contact investigation or tracing, the terms "case," "index case," and "source case" are used to identify individuals involved in the chain of transmission. Here’s what each term means:

1. Case: A case refers to any individual who has been diagnosed with TB, particularly active TB disease. In the context of contact tracing, this term typically denotes someone with pulmonary TB, as this form is more likely to be infectious.

2. Index Case: The index case is the first diagnosed person within a specific setting (such as a household, workplace, or social environment) that triggers the contact investigation. In other words, the index case is the first TB case that healthcare providers identify and investigate. Importantly, the index case is not always the person who initially introduced TB into the group or setting; they are simply the first person identified.

3. Source Case: The source case is the individual responsible for transmitting TB to others. The source case is often identified through contact tracing as someone who may have had TB for a longer period and who could have exposed the index case and others to the infection. Sometimes the index case and the source case are the same person, but often, especially in cases involving children or young people, the source case is a different person who was infectious before the index case developed symptoms.

In tuberculosis (TB) contact tracing and investigation, the terms "contact," "converter," and "window period" refer to aspects of the exposure, infection, and timing in the context of TB transmission. Here's what each term means:

1. Contact: A contact is any person who has been exposed to someone with active TB disease. Contacts can include family members, coworkers, classmates, or anyone else who has spent significant time with a person diagnosed with infectious TB. The goal of contact tracing is to identify these individuals, assess their risk of infection, and provide appropriate testing and follow-up care.

2. Converter: A converter is a person who initially tests negative for TB infection (usually with a tuberculin skin test (TST) or interferon-gamma release assay (IGRA)) but later tests positive, indicating recent TB infection. The conversion typically indicates that the person was recently exposed to TB and has developed a latent TB infection. In contact investigations, identifying converters can help determine the timing and extent of TB transmission within a group.

3. Window Period: The window period is the time between the last known exposure to TB and when an individual can reliably be tested for TB infection. It generally lasts about 8-12 weeks. During this time, the person's immune response to TB may not yet be detectable on a TST or IGRA. If an exposed contact initially tests negative, healthcare providers often recommend retesting after the window period to ensure that infection hasn't been missed.

To prioritize these individuals in terms of urgency for TB contact investigation and treatment, consider factors such as infectiousness, potential for transmission, presence of vulnerable contacts, and severity of the disease. Here’s an order with reasons for each:

Priority Order:

1. C. 25 y.o. man with bilateral infiltrates on CXR, 3+ AFB sputum smear, culture positive

   • Reason: This individual has a high priority due to the 3+ AFB smear result, indicating a high bacillary load and thus a high level of infectiousness. Bilateral infiltrates on the chest X-ray suggest extensive disease, which increases the potential for TB transmission. Since he is highly infectious, contact tracing should begin immediately to prevent further spread.

2. B. 37 y.o. woman with RUL cavity on CXR, AFB sputum smear negative, culture positive; lives with her 2 children aged 3 and 5

   • Reason: Although the smear is negative, this individual has a cavity in the right upper lobe on chest X-ray, which is associated with a higher risk of transmission, especially in culture-positive cases. Additionally, she lives with two young children, who are vulnerable to TB infection and complications. This case has a high priority for contact tracing due to both the presence of young household contacts and her cavitary disease.

3. E. 50 y.o. man with AFB sputum negative, culture positive, partner is HIV-positive

   • Reason: This patient has a lower infectiousness due to the negative smear, but his culture positivity indicates active TB. Additionally, his partner is HIV-positive, making them highly vulnerable to TB infection and at greater risk for TB-related complications. Therefore, this individual is prioritized to prevent potential transmission to an immunocompromised contact.

4. A. 25 y.o. man with bilateral infiltrates on CXR, AFB sputum smear negative, culture positive

   • Reason: Although this patient has extensive disease on chest X-ray and is culture-positive, he is likely less infectious than a smear-positive case. His lower infectious potential means he is a lower priority for immediate contact tracing compared to others on this list, especially those with vulnerable contacts. However, his condition should still be monitored closely.

5. D. 62 y.o. man with abnormal CXR, AFB sputum and cultures negative

• Reason: This individual has an abnormal chest X-ray, but both his AFB smear and cultures are negative. This suggests that he may not have active TB or may not be infectious. He has the lowest priority for contact tracing due to the lack of laboratory evidence of active TB or transmissibility. Further investigation may still be warranted to clarify his condition.