- Case discussion
- Diagnosing and treating latent TB in low-incidence countries
- UW Peds Outpt guidelines for TB screening
- CDC guidelines for interpreting PPD tests
Key take-home points:
- What are the rates of TB in the US and what are the risk factors among children? TB has been declining in the US and reached an historic low of 3.2 cases per 100,000 in 2012. The biggest risk is being born outside the US or traveling to another country, especially for >1 week, and staying with family. For children, additional risks include living among family members or visitors born in endemic countries, or living with high risk adults, including those affected by homelessness, incarceration, drug use or HIV. Those with chronic diseases, immunodeficiency, and/or using high-dose steroids are also at higher risk of developing TB.
- Who do we need to screen in clinic? It’s recommended to start screening for latent TB infection (LTBI) from the first time we meet patients and annually at well visits, or 10 weeks after return from travel (although considered acceptable to wait for annual check-up). To assess LTBI risk factors, there are 4 validated questions: 1) Has a family member or contact had TB? 2) Has a family member had a positive TB test? 3) Was the child born in a high-risk country (i.e., outside US, Canada, Australia, New Zealand or Western Europe)? 4) Has child traveled to a high-risk country for more than 1 week? (and SCH ID team add: or has child had household visitors from a high-risk country?)
- Which screening tests do we use? Screening tests vary by age group: tuberculin skin test (TST or PPD) is still preferred for children less than 5. Interferon gamma release assays (IGRAs, including QuantiFERON -TB Gold) measure interferon-gamma response to mycobacterial antigens. IGRAs are now the preferred test for ages 5 and older as they are relatively specific to M. tuberculosis, do not require return visit, and are not cross-reactive with BCG vaccine. We can use both tests to help establish diagnosis when there are indeterminate results, or concern for false positives or negatives.
- What happens if there is a positive TB screen? To establish a diagnosis of latent TB, rule out active disease through a chest x-ray, history and exam. The initial preferred treatment for positive latent TB is with isoniazid (INH) for 9 months (there are alternative schedules to this based on special patient needs).
- How common is BCG vaccine? How does BCG vaccine affect interpretation? Bacille Calmette -Guerin (BCG) immunization is widely used in TB endemic countries; the WHO estimates that 83% of the world’s population has received this vaccine. Most countries recommend giving the vaccine at birth, and the majority of children receive it before age 5. Because of the varying effects of BCG on interpreting TB tests, we use a conservative approach and BCG status is not used in interpreting PPD reactions. Quantiferon gold testing is not affected by cross-reactivity with BCG.