Tuberculosis (TB) is an infectious disease that attacks the lungs, though it can affect almost any body area. Two types of tests can identify a TB infection, but how often you should get a tuberculosis test depends on your exposure risk.

Tuberculosis (TB) is a serious disease that mainly affects the lungs.

How often you should receive a TB test depends on your risk of getting and transmitting the TB infection. It also depends on your risk of developing active TB disease following a TB infection.

There are two types of tests for TB infection: the TB blood test and the TB skin test. A healthcare professional will determine the most appropriate test type on a case-by-case basis.

The TB blood test involves taking a small sample of blood and mixing it with TB proteins. This allows the healthcare team to see how your body’s immune system reacts to the bacteria that causes TB.

The TB skin test involves injecting a small amount of a fluid called tuberculin into the skin of the arm. A healthcare worker then assesses the injection site 48 to 72 hours later to check for a reaction, such as a skin bump, also called a skin induration. A TB skin test is not 100% accurate, sometimes resulting in a false-positive or false-negative result. Because of this, you may need a follow-up TB skin test to assist with the diagnosis.

You should have a TB test if any of the following apply:

  • you’re at increased risk of acquiring TB infection
  • you’re at increased risk of developing active TB following a TB infection
  • you have recently been in contact with someone who has active TB
  • you’re experiencing symptoms of TB
  • you require TB screening for employment, school, travel, or immigration purposes

Factors that increase the risk of a TB infection

Factors that could increase your risk of TB infection include:

  • being born in or frequently visiting a country where TB is common
  • living in, or having previously lived in, a group setting where TB is more common, such as a homeless shelter or jail
  • working in a place where TB is more likely to spread, such as a hospital or nursing home

Factors that increase the risk of active TB disease

You should also have a TB test if you’re at increased risk of developing active TB disease following a TB infection. Factors that can increase this risk include:

  • being elderly
  • having a weakened immune system due to an underlying health condition, such as:
  • having been diagnosed with a TB infection in the past 2 years
  • not completing appropriate treatment for TB in the past
  • injecting illegal drugs

Children under 5 years of age should also receive a test for TB as their immune systems are not fully developed.

How often you need testing for TB depends mainly on your risk of exposure to TB and your risk of passing the infection on to others.

Recommendations for healthcare workers

If you work in healthcare, you should follow your state’s regulations on TB screening. Health facilities may recommend annual TB screening for staff who are at increased risk of infection, such as:

If you work in healthcare and have knowingly been exposed to TB disease, you should receive a TB symptom screen and testing. If you have previously tested negative for TB, you should receive this test immediately and should have a follow-up test 8 to 10 weeks after your last known exposure. For consistency, all tests should be of the same type.

If you have had a positive TB test in the past, you do not need another test following exposure to TB disease. You should instead receive a TB symptom screen to determine if you have the disease again.

Recommendations for those who live or work in care homes

If you’re admitted to a care home for the elderly or you begin working in a care home, you should receive a TB skin test as soon as your residency or employment begins. However, the test is not necessary if you have documentation to show you have had a positive TB test result in the past.

Different care facilities will have different levels of TB risk and different recommendations on the frequency of TB testing.

A TB test measures how effectively your immune system responds to the bacterium that causes TB. A positive result indicates that the bacterium is present in your body. Further tests are then necessary to determine whether you have inactive TB or active TB disease.

A positive reaction on a TB blood test or a TB skin test indicates a TB infection, but it does not show whether the disease is active or inactive. The only way to tell the status of the disease is with further tests, such as:

  • a physical examination and symptom assessment
  • imaging scans to check for visible signs of TB in the lungs
  • a sputum test to check for the Mycobacterium tuberculosis bacterium in mucus

Below is some additional information on what TB test results mean.

TB blood test results

There are two types of TB blood tests doctors may use to diagnose TB:

  • QuantiFERON®-TB Gold Plus (QFT-Plus)
  • T-SPOT®.TB test (T-Spot)

For either test, a positive result means that the bacterium that causes TB is in your body. A negative result means that it is unlikely you have inactive or active TB. However, your doctor may order further tests to confirm this, especially if you:

  • are showing signs or symptoms of active TB disease
  • have recently been in contact with someone who has active TB
  • have HIV

TB skin test results

When interpreting the results of a TB skin test, doctors will consider the size of the skin induration along with other risk factors to determine whether the test is positive or negative. For example;

  • Induration measuring less than 5 millimeters (mm) is positive if:
    • you have HIV and have been in close contact with someone who has active TB
  • Induration measuring 5 mm or more is positive if any of the following apply:
    • you have a weakened immune system due to an underlying health condition or from medications
    • you have received an organ transplant
    • medical scans show signs of TB in your lungs
    • you have been in close contact with someone who has active TB
  • Induration measuring 10 mm or more is positive if any of the following apply:
    • you’re from a country where TB is endemic, or there’s a high incidence of infection
    • you have a chronic medical condition like cancer, diabetes, or kidney failure, which increases your risk of reactivating TB you had in the past
    • you weigh less than 90% of your ideal body weight
    • you live or work in a high-risk setting
    • you inject drugs
  • Induration measuring 15 mm or more is positive if:
    • you’re healthy and have a low likelihood of TB infection

You should tell your doctor if you have previously received the BCG vaccine to help protect against TB, as this vaccine can cause a false positive TB skin test reaction. The vaccine is uncommon in the United States, but is common in countries where there is a high risk of TB.

If you have had the BCG vaccine, your doctor will order you a TB blood test instead, as the results are more reliable.

What happens if the results are positive?

If you receive a positive TB test result, your healthcare professional will carry out further tests to determine whether you have active or inactive TB disease.

If tests show that you have active TB, your doctor will prescribe a 6- to 12-month course of antibiotic medications to help keep the infection under control, treat the symptoms, and prevent you from spreading the infection to others. It’s important that you complete the full course of antibiotics, even if you begin to feel better. Not completing the full course can lead to drug-resistant TB, and this can be fatal.

If tests show that you have inactive TB, your doctor will prescribe a 6- to 9-month course of antibiotic medications to help keep the disease under control and prevent it from becoming active.

The length of time that your TB test results remain valid depends on different factors, including:

  • your state or local regulations
  • the regulations of the facility where you study or work
  • the regulations of any country you’re travelling to

Tuberculosis (TB) is an infectious disease that mainly attacks the lungs. There are two types of tests that can identify a TB infection: a TB blood test and a TB skin test. Both tests can detect the presence of the TB infection in the body, though neither can determine whether the disease is active or inactive.

You should receive a TB test if you’re at increased risk of acquiring TB infection or you’re at increased risk of developing active TB disease following TB infection. You should also receive a TB test if you have knowingly been in contact with someone who has TB, or you develop TB symptoms.

Different states, facilities, and employers may have different recommendations on how frequently you should test for TB.