Tuberculosis (TB) testing

2. What’s the difference between the TB skin test and TB blood test?

If you were born outside of the US, it is recommended that you get a TB blood test, not a TB skin test. The BCG vaccine, given outside of the US, can cause a false positive result on a TB skin test, but does not impact the results of a TB blood test.

The TB skin test (PPD) is a two part test. At your first appointment, a small amount of liquid is placed just under the skin on your forearm. After 48-72 hours has passed, you will come back to the doctor for a second appointment and the nurse will check your forearm. If your TB skin test is positive, you will need a chest x-ray to rule out active TB disease.

The TB blood test (QFT or TSPOT) is just like other blood tests you have had done at check-ups with your doctor. You will visit the doctor or lab once and a healthcare worker will draw your blood. Your blood will be tested by the lab and the doctor or lab will call to tell you your results. If your TB blood test is positive, you will need a chest x-ray to rule out active TB disease.

4. What’s the difference between TB disease and latent TB infection?

A positive result on a blood test (called IGRA, QFT, or TSPOT) and a normal chest x-ray, along with a provider's evaluation, means that you have latent TB infection. You can have latent TB infection if you don't have symptoms and aren't sick. Latent TB infection means you have "sleeping" TB germs in your body that could “wake up” and become TB disease, making you sick. Once you have TB disease, you will have symptoms and can spread the infection to other people, like family and friends.

Lots of people have latent TB infection. Some doctors may not want you to take medicine for latent TB infection unless you have certain health conditions that suppress your immune system. Even if you are healthy, latent TB infection can turn into TB disease.

6. Why should I take medicine for latent TB infection if I’m not sick?

If latent TB infection becomes TB disease it can cause a lot of problems. You will have to isolate from family and friends, stay home from work, and take medicine for a long time. Taking medicine for latent TB infection decreases your change of getting TB disease in the future. This protects you and also your family and friends.

8. I took treatment for latent TB infection or TB disease in the past, do I need to take treatment again?

If you have already taken medicine for latent TB infection or TB disease, you usually don’t have to take treatment again but some people need repeat treatment in a special circumstance. Call your doctor if you have questions.