BCG Vaccine Impact on TB Testing

Tuberculosis has been stalking humankind for hundreds of years, and has been quite deadly until very recently. A vaccine developed in the early part of the last century, known as the BCG vaccine, is still given to people in many countries. In the USA, where PPD testing is a common diagnostic tool to determine if someone has active TB, a history of BCG vaccination may complicate matters.

TB Testing & the BCG Vaccine: What Every Employer Needs to Know

Tuberculosis (TB) has been a scourge on the planet for centuries. In the first half of the 20th century, one in seven Americans and Europeans died from the disease or related infections. With the discovery of several medicines that effectively treated TB beginning in the early 1900s, the mortality rate dropped drastically across the world. Today, the Bacille Calmette-Guérin (BCG) vaccine has saved countless lives as the only approved pre-emptive medical option available to protect against the disease.

The BCG vaccine revolutionized the medical world but is far from perfect. It can cause BCG-vaccinated people to produce false test results when screened through certain methods. Even more, it has been found to have highly variable success rates across regions and even cause some vulnerable populations to become infected.

As an employer, understanding the dangers of TB and the testing issues caused by the BCG vaccine can help you institute the necessary testing procedures and policies to maintain a safe workplace.

What is tuberculosis?

Tuberculosis (TB) is the disease caused by the highly contagious bacteria Mycobacterium tuberculosis. TB spreads through the air and usually attacks the lungs first, but can cause a fatal infection in any part of the body. People infected with TB may have active or latent symptoms, but those with latent infection are not usually contagious. Although several different medicines have been created to treat TB, drug-resistant strains are becoming a problem.

Today, TB still concerns public health officials globally and is seen as an active health risk in some countries. In regions where there is still a high prevalence of TB infections or related miliarial diseases, children and health workers are vaccinated with the BCG vaccine resulting in a tell-tale raised scar.

This vaccine is not recommended for people who do not fall into one of these two groups because of possible complications and false positive testing. Researchers have also not been able to identify why the vaccine appears to be significantly more effective amongst some populations than others, raising serious questions about how both the vaccine and bacteria function.

How does TB testing work?

If you conduct occupational health screenings at your company, you need to understand how TB testing works so that you can correctly interpret your employees’ test results. Although most American-born citizens have not been vaccinated against TB, many health workers and people born outside the U.S. have received the BCG vaccine.

As protection against a fatal and highly contagious disease, TB testing is a common part of many types of occupational health screenings. There are two types of TB tests— a skin test and a blood test.

The tuberculin skin test (TST) is the most commonly used method and identifies if a person has ever had the TB bacteria in their body. Because of the sensitivity of this test, it can produce false-positive results in people who have received the BCG vaccine.

The TB blood test is an Interferon Gamma Release Assay (IGRA), measuring a person’s immune response to the TB bacteria. This methodology allows for quick results within 24 hours and is not impacted by the BCG vaccine. In the U.S., the Oxford T-Spot and QuantiFERON Gold blood tests have been approved by the FDA.

What choices do employers have for TB screening?

If TB testing is a part of your company’s occupational health screening, make sure to include a question about whether the applicant or employee has received the BCG vaccine in your pre-test medical survey. While you are free to give the majority of your workforce the TST, BCG-vaccinated employees should be given an IGRA to avoid false-positive results.

If someone received a positive result from a TST, it might seem logical to then request an IGRA to confirm the results. Because of the mechanics behind how the TST and IGRA work, however, the CDC recommends that no person take both a TB skin and blood test.

As a result, any job applicant or employee who tests positive on either type of TB test will be required to complete further testing to confirm the findings. In addition, TSTs and IGRAs can only provide positive or negative results, not determine whether a person has latent or active tuberculosis. Whether hoping to disprove an expected false positive or determine whether a suspected infection is latent or active, doctors usually confirm the presence of TB through a chest x-ray or sputum test.

Clinics with occupational health screening expertise, including those set up by Health Street, allow you to choose between TST or IGRA testing as part of your overall screening package. While the blood test is generally a better option, it can also be more expensive. Therefore, some employers opt for the PPD for employees that have not had a TB vaccination while setting up the IGRA blood test for people who were born in countries where the BCG is routinely provided.


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