Applied Evidence

Tuberculosis testing: Which patients, which test?

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The most appropriate test to identify latent TB depends on the patient’s risk for developing active TB and other factors. This review provides practical guidance on who to test, how, and when.


 

References

PRACTICE RECOMMENDATIONS

› Test for latent tuberculosis (TB) infection by using a tuberculin skin test (TST) or interferon gamma release assay (IGRA) in all patients at risk for developing active TB. B
› Consider patient characteristics such as age, previous vaccination with bacille Calmette-Guérin (BCG), and whether the patient will need serial testing to decide whether TST or IGRA is most appropriate for a specific patient. C
› Don’t use TST or IGRA to make or exclude a diagnosis of active TB; use cultures instead. B

Strength of recommendation (SOR)

A Good-quality patient-oriented evidence
B Inconsistent or limited-quality patient-oriented evidence
C Consensus, usual practice, opinion, disease-oriented evidence, case series

CASE 1  Judy C is a newly employed 40-year-old health care worker who was born in China and received the bacille Calmette-Guérin (BCG) vaccination as a child. Her new employer requires her to undergo testing for tuberculosis (TB). Her initial tuberculin skin test (TST) is 0 mm, but on a second TST 2 weeks later, it is 8 mm. She is otherwise healthy, negative for human immunodeficiency virus (HIV), and has no constitutional symptoms. Does she have latent tuberculosis infection (LTBI)?

CASE 2 A mom brings in her 3-year-old son, Patrick. She reports that a staff member at his day care center traveled outside the country for 3 months and was diagnosed with LTBI upon her return. She wants to know if her son should be tested.

More than 2 billion people—nearly one-third of the world’s population—are infected with Mycobacterium tuberculosis.1 Most harbor the bacilli as LTBI, which means that while they have living TB bacilli within their bodies, these mycobacteria are kept dormant by an intact immune system. These individuals are not contagious, nor are they likely to become ill from active TB unless something adversely affects their immune system and increases the likelihood that LTBI will progress to active TB.

Two tests are available for diagnosing LTBI: the TST and the newer interferon gamma release assay (IGRA). Each test has advantages and disadvantages, and the best test to use depends on various patient-specific factors. This article describes whom you should test for LTBI, which test to use, and how to diagnose active TB.

Why test for LTBI?

LTBI is an asymptomatic infection; patients with LTBI have a 5% to 10% lifetime risk of developing active TB.2 The risk of developing active TB is approximately 5% within the first 18 months of infection, and the remaining risk is spread out over the rest of the patient’s life.2 Screening for LTBI is desirable because early diagnosis and treatment can reduce the activation risk to 1% to 2%,3 and treatment for LTBI is simpler, less costly, and less toxic than treatment for active TB.

Whom to test. Screening for LTBI should target patients for whom the benefits of treatment outweigh the cost and risks of treatment.4 A decision to screen for LTBI implies that the patient will be treated if he or she tests positive.3

The benefit of treatment increases in people who have a significant risk of progression to active TB—primarily those with recently acquired LTBI, or with co-existing conditions that increase their likelihood of progression (TABLE 1).5

Screening for latent TB infection is desirable because early diagnosis and treatment can reduce the activation risk to 1% to 2%.

All household contacts of patients with active TB and recent immigrants from countries with a high TB prevalence should be tested for LTBI.6 Those with a negative test and recent exposure should be retested in 8 to 12 weeks to allow for the delay in conversion to a positive test after recent infection.7 Health care workers and others who are potentially exposed to active TB on an ongoing basis should be tested at the time of employment, with repeat testing done periodically based on their risk of infection.8,9

Individuals with coexisting conditions should be tested for LTBI as long as the benefit of treatment outweighs the risk of drug-induced hepatitis. Because the risk of drug-induced hepatitis increases with age, the decision to test/treat is affected by age as well as the individual’s risk of progression. Patients with the highest risk conditions would benefit from testing/treating regardless of age, while treatment may not be justified in those with lower-risk conditions. A reasonable strategy is as follows:10
• high-risk conditions: test regardless of age
• moderate-risk conditions: test those <65 years
• low-risk conditions: test those <50 years.

Children with LTBI are at particularly high risk of progression to active TB.5 The American Academy of Pediatrics (AAP) recommends assessing a child’s risk for TB at first contact with the child and once every 6 months for the first year of life. After one year, annual assessment is recommended, but specific TB testing is not required for children who don’t have risk factors.11 The AAP suggests using a TB risk assessment questionnaire that consists of 4 screening questions with follow-up questions if any of the screening questions are positive (TABLE 2).11

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