Choosing the Right TB Test

Andrew Berry

Residents of the United States face significantly lower risk of tuberculosis (TB) infection than those in other parts of the world, including Africa, Asia, and Central and South America.1 Despite these lower risks, TB cases in the U.S. have seen a slight rise since hitting a low point during the COVID-19 pandemic. The Centers for Disease Control and Prevention (CDC) continues to recommend, and states require, that both paid and unpaid health care personnel undergo TB testing upon their hire. There are two forms of TB testing, the tuberculin skin test and the interferon gamma release assay (IGRA) blood tests. Both test types have their advantages, though businesses may prefer the expedited timeline and more precise results of an IGRA test like the accurate and quick QuantiFERON®-TB Gold Plus (QFT-Plus). A strong occupational health partner, like Concentra®, can help employers choose a testing method that best fits their workforce’s needs.

Current TB snapshot

A recent report by the University of Minnesota found that there were 9,615 reported TB cases in the United States in 2023 – a ten year high and a 16 percent increase from 2022.2 Cases rose by nine percent in U.S.-born residents and 18 percent in non-U.S.-born residents. The U.S. has one of the lowest rates of TB infections in the world; the U.S. incidence rate of 2.9 cases per 100,000 persons is well-below the global rate of 133 cases per 100,000 persons.1

Who is at risk?

The CDC, National Tuberculosis Controllers Association, and state departments of health recommend or require pre-placement TB testing for paid and unpaid health care personnel.3,4 In 2019, the CDC issued updated guidelines that no longer recommended annual TB screenings while continuing to recommend screenings after a documented exposure.5 The specific health care settings the CDC recommends for employee screening include the following:

  • Inpatient settings
  • Outpatient settings
  • Laboratories
  • Emergency medical services
  • Home-based health care and outreach settings
  • Long-term care facilities
  • Clinics in homeless shelters

The CDC also recommends testing for employees who work in places where TB is known to easily spread, including homeless shelters, nursing homes, and correctional facilities.5 Additionally, some school districts and daycares may recommend or require TB testing.6

Latent vs. active infection

Across the globe about 1.8 billion people, or a quarter of the world’s population, are infected with Mycobacterium tuberculosis, the scientific name for TB.6  Most of these people, however, have latent TB, which has no symptoms and is not contagious. All TB tests screen for TB infection, but a positive test does not distinguish between active and latent TB infection. All people with a positive TB test must have a chest X-ray to determine if there are any signs of active TB disease. Sputum (phlegm) tests are needed if the chest X-ray is concerning for active TB disease or when a patient patient has symptoms of active TB disease even if the chest X-ray is negative. A person with a positive test but negative chest X-ray and no symptoms consistent with active TB disease is diagnosed with latent TB infection and is not infectious. As per CDC recommendations, most people with latent TB infection should be treated to significantly decrease the probability of developing active TB disease.

Test Types

Skin test

The TB skin test is the more common and widely available screening tool for TB. During a TB skin test, a person receives an injection of tuberculin solution into their dermis, the second layer of skin just below the epidermis. Tuberculin is a purified protein derivative taken from lab-produced Mycobacterium tuberculosis bacteria.7 If a person has TB, their body will react to the tuberculin by forming a raised, hard bump at the injection site.

This reaction must be measured, in millimeters, by a health care professional between 48 and 72 hours after the injection. In otherwise healthy people born in the U.S., a bump measuring more than 15 millimeters indicates infection. A smaller bump, at five to ten millimeters could indicate infection in people born in countries where TB vaccination is more common, people who have had a direct exposure or those who have certain health conditions.8 At least 2 visits to a health care provider are required.

Although cheaper than TB blood tests, TB skin tests are also less reliable and more open to bias. Disadvantages of TB skin testing include:7

  • Requirement for two or more patient visits
  • False-positive results for those who have received the Bacille Calmette-Guérin (BCG) TB vaccine
  • Booster phenomenon (false-negative results in older adults)
  • Subject to biases and errors with TB skin test placement and reading

The CDC manages a full list of the causes for both false-positive and false-negative readings associated with skin tests, but false-positive reactions associated with the BCG vaccine, which is widely used in countries where TB is common, is the most prevalent. There is no reliable way to distinguish a positive TB skin test reaction caused by BCG vaccination from a reaction caused by true TB infection – blood testing or chest x-rays may be required.9

Blood tests

TB blood tests are the more accurate, and for patients, the less time-consuming option for TB screening. There are currently two TB blood test that are approved by the Food and Drug Administration (FDA) and commercially available: T-Spot TB (T-Spot) and QuantiFERON®-TB Gold Plus (QFT-Plus). Both blood tests are interferon gamma release assay (IGRA) tests, which means they measure the body’s immune response to TB antigens.10

In IGRA testing, blood is drawn and then mixed with peptides that simulate antigens derived from TB bacteria and with control substances. The specific antigens come from the mycobacterium tuberculosis complex but are absent from the BCG TB vaccine, helping to avoid false-negatives. If a person has been infected with TB, their body sends white blood cells to fight the antigens. Both TB blood tests measure the interferon-gamma (IFN-γ) released by these blood cells, hence the name interferon gamma release assay.

Overall, IFN-γ measurement is more reliable than skin testing, with several advantages:

  • Requires only one patient visit to do the test, including 2-Step testing
  • Not impacted by booster phenomenon (unlike the TB skin test)
  • Not subject to biases and errors associated with TB skin test placement and reading
  • Results can be available within 24 hours
  • Is over 95 percent accurate

Most IGRA test will be positive or negative, although some may be borderline (T-SPOT only), indeterminate (QFT-Plus only), or invalid (T-spot only).11  Negative tests mean that TB infection is unlikely, though more testing may be needed if a person has TB symptoms. Positive tests are highly accurate and can indicate likely active TB infection with a chest X-ray.

A report on QFT-Plus found that it has been shown to be more accurate than the skin testing in identifying people who may have latent TB infection. Additionally, it is more reliable than skin testing in identifying those who may progress to active TB infection.12

Tackle your TB testing

Your employees deserve the best when it comes to their health screenings, especially with serious illnesses like TB. Concentra offers multiple TB testing methods, including skin testing, T-Spot, and the highly accurate QFT-Plus. Learn more about our TB testing services packages and then find a Concentra medical center near you to schedule TB testing for your employees. 


NOTES:

  1. TB Incidence,” World Health Organization, n.d.
  2. US reports 2023 rise in TB cases, incidence,” by Chris Dall. University of Minnesota, March 29, 2024.
  3. Frequently Asked Questions About TB,” Texas Health and Human Services, n.d.
  4. Preventing Occupational Exposure to Tuberculosis,” New Jersey Department of Health,January 10, 2017.
  5. Clinical Testing Guidance for Tuberculosis: Health Care Personnel,” Centers for Disease Control and Prevention, n.d.
  6. Learn About Tuberculosis,” American Lung Association, June 7, 2024.
  7. Clinical Testing Guidance for Tuberculosis: Tuberculin Skin Test,” Centers for Disease Control and Prevention, n.d.
  8. TB (Tuberculosis) Tests,” WebMD, n.d.
  9. Tuberculin Skin Test (TST),”Minnesota Department of Health, November 9, 2022.
  10. TB (Tuberculosis) Tests,” WebMD, n.d.
  11. Clinical Testing Guidance for Tuberculosis: Interferon Gamma Release Assay,” Centers for Disease Control and Prevention, n.d.
  12. Tuberculosis Test - QuantiFERON (QFT),” Labcorp, n.d.