Alan Fein MD, Jeffrey Fein BA, Lawrence Shulman, DO
Affecting more than one third of the global human population, Tuberculosis (TB) is the most common infectious disease in the world. Ten percent of diagnosed TB cases progress to the disease’s active form.
Thankfully, in the United States, TB is much less common than in most other parts of the world, only several thousand cases are seen annually. However, because of increased tourism and globalization of business practices, there is increased risk of exposure to TB.
Doctors are frequently asked to evaluate people who may have had exposure to individuals with TB or who have latent (inactive) tuberculosis. In the United States, TB evaluation is most often required during regular checkups or pre-employment health screenings.
Who should be screened?
Screening every person for TB is impractical. Screening tests are reserved for those with a higher likelihood of exposure to the disease. These include:
- Close contacts of people with tuberculosis.
- Foreign-born individuals.
- Homeless persons.
- IV drug users.
- Prisoners.
The screening of lower-risk individuals is generally based on occupation. In schools and hospitals, for example, widespread public health consequences could result from an active case of TB. Screening of school and hospital employees is therefore appropriate.
Selecting the Right TB Test:
Tuberculosis screening has long involved an intra-dermal (beneath the skin) test. In such a test, a purified protein derivative (PPD) composed of tubercular proteins is injected just beneath the skin. If an individual has had exposure to TB, a localized edema (swelling) forms at the site of injection within three days.
Although the PPD is the traditional TB screening tool, it does have certain drawbacks. The patient must return 2-3 days after injection for reading of the PPD. Additionally, the PPD is subject to variability in its administration and interpretation. Methods for determining the test’s results, typically involving measurement with a ruler and a pen, are not always precise.
Recently, medical advancements have offered an alternative way of testing for TB. This innovative new blood test can quickly and reliably detect if an individual has a TB infection, even if that infection is latent. These tests rely on detection of a protein produced by the TB carrier’s immune cells.
One such blood test, the Quantiferon Assay, can now be ordered at ProHealth. This test is less prone to misreading than the older PPD test but may also be less sensitive than the PPD. Still, the Center for Disease Control (CDC) has stated that TB blood testing may be used instead of the skin test.
These blood-based assays are especially useful when it comes to screening specialized populations:
- Those who have received the BCG tuberculosis vaccine.
- Those with MAI (a non-contagious infection related to TB infection).
- Those with a high likelihood of TB but are skin test negative.
- Immuno-compromised individuals.
- Children, who sometimes display a false-negative PPD, despite latent or even active tuberculosis.
Interferon-based assays may also be viewed as complimentary to the PPD skin test, as positive results have similar import. The advantages and disadvantages of both tests are highlighted below.
| TST | Interferon Assays | |
| Advantages | Simple to performLower Cost | No follow-up visitAvailable < 24 hoursNot affected BCG |
| Disadvantages | Requires injectionHigher false positive and false negativeSkin reactions | Requires blood drawHigher costReduced sensitivity in HIV and active TB |