TB (Tuberculosis) Test: Purpose, Procedure & Results (2023)

Overview

What is a TB (tuberculosis) test?

A TB test checks to see if you have been infected with Mycobacterium tuberculosis complex, which is the bacteria that causes tuberculosis (TB). There are two types of latent tuberculosis infection (LTBI) screen tests: a skin test and a blood test.

A TB skin test is also called a Mantoux test or tuberculin skin test (TST). A TB blood test is also called an Interferon-Gamma Release Assay (IGRA), but you might sometimes hear “QuantiFERON” (the most commonly used commercial test). TB skin tests are more common and are the preferred type for children under age 5. But blood tests for TB are becoming more common, especially for healthcare workers and those who have compromised immune systems.

TB skin and blood tests can show if you’ve ever been infected with M. tuberculosis complex, but they can’t distinguish between a latent (dormant) or active TB infection. You’ll need additional tests (like a sputum test) to see if you have an active infection.

What is tuberculosis?

Tuberculosis is a serious bacterial infection that mainly affects your lungs, but it can also affect virtually every organ including your brain, spine and kidneys. TB spreads from person to person through coughing or sneezing. It’s also sometimes spread through transplant with a donated organ.

Not everyone infected with TB gets sick with symptoms. Approximately 25% of the world's population has latent tuberculosis infection (LTBI). When you have latent TB, you don't feel sick and can't spread the infection to others.

While many people with latent TB will never feel any symptoms of the condition, others, especially those who have or develop weakened immune systems, can experience the active form of this dangerous infection, called active TB.

Because of this, LTBI screening through tests is a common and essential tool for public health.

Who should be screened for TB?

You may need a TB test if you have symptoms of an active TB infection or if you’re at higher risk for getting TB. In addition, some childcare centers, hospitals and other facilities require you to be tested for TB in order to work there.

Symptoms of an active TB infection include:

• Bad cough lasting longer than two weeks.
• Pain in your chest.
• Coughing up blood or sputum (mucus).
• Fatigue or weakness.
• Loss of appetite.
• Unexplained weight loss.
• Chills and fever.
• Night sweats.

You may be at higher risk for getting TB if you:

• Are a healthcare worker who interacts with and cares for people.
• Live or work in a place with a high rate of TB infection, including homeless shelters, nursing homes and prisons.
• Have been exposed to someone who has an active TB infection.
• Have a medical condition or take medication that weakens your immune system.
• Use intravenous drugs.
• Have traveled or lived in an area where TB is endemic, including countries/regions other than USA, Canada, Australia, New Zealand, Japan, Northern and Western Europe.

Who performs a TB test?

Any healthcare provider who is specially trained in performing and assessing the results of a TB skin test can perform the test.

A healthcare provider called a phlebotomist usually performs blood draws, including those for a TB blood test, but any healthcare provider who is trained in drawing blood can perform this task. They then send the sample to a lab where a medical technologist prepares the samples and performs the test.

Test Details

How does a TB (tuberculosis) test work?

Both types of TB tests — the skin test and blood test — work by measuring your immune system’s response to antigens taken from the bacterium that causes TB: Mycobacterium tuberculosis. An antigen is any substance that causes your immune system to produce antibodies against it. This means your immune system doesn’t recognize the substance and is trying to fight it off.

How a TB skin test works

A TB skin test measures your immune system’s response to a purified protein derivative (PPD) solution injected under your skin. PPD is a protein that comes from the bacteria that causes TB — Mycobacterium tuberculosis.

It’s not live bacteria, and it won’t make you sick. If you’ve ever been exposed to the TB bacteria, your skin will react to the antigens by developing a firm, red bump at the site of the injection within three days.

How a TB blood test works

In most cases, if you’ve been infected with TB, blood cells from your blood sample will release a protein called interferon-gamma (IFN- γ) when mixed with antigens derived from Mycobacterium tuberculosis in a laboratory.

To conduct the tests, laboratory scientists mix your blood sample with antigens and controls and measure the reaction using a method called immunoassay. They’ll interpret your results as negative, positive or indeterminate.

What do I need to do to prepare for a TB test?

You don't need to do anything special to prepare for a TB skin test or blood test.

What should I expect during a TB (tuberculosis) test?

The TB (tuberculosis) testing process is different based on what kind you get — the skin test or the blood test.

TB skin test process

A TB skin test requires two or three visits with a healthcare provider. During the first visit, they inject fluid into your arm. On the second visit, the healthcare provider assesses your skin’s reaction to the injection. If still negative, the third visit (72 hours) is needed.

During the first visit:

• A healthcare provider will clean and disinfect the skin on your lower inner arm.
• They’ll inject a small amount of fluid (called tuberculin or PPD) with a tiny needle under the skin on your lower inner arm.
• The provider may mark the injection site by circling the area with a pen. This makes it easier for them to find the area when assessing the reaction.
• A small bump will form where they injected the fluid. It should go away in a few hours.

You’ll then return to your provider within 48 to 72 hours so that they can look for a reaction on your arm where they injected the fluid. Firm swelling and redness at the site may indicate a TB infection. It’s important to go to your second appointment to get an official result. If you don’t return within 72 hours, you’ll need to take the test again. Immunocompromised individuals and those with frail skin conditions may not have reliable skin test results.

TB blood test process

You can expect to experience the following during a blood test, or blood draw:

• You’ll sit in a chair, and a healthcare provider will check your arms for an easily accessible vein. This is usually in the inner part of your arm on the other side of your elbow.
• Once they’ve located a vein, they’ll clean and disinfect the area.
• They’ll then insert a small needle into your vein to take a blood sample. This may feel like a small pinch.
• After they insert the needle, a small amount of blood will collect in a test tube.
• Once they have enough blood to test, they’ll remove the needle and hold a cotton ball or gauze on the site to stop the bleeding.
• They’ll place a bandage over the site, and you’ll be finished.

The entire procedure usually takes less than five minutes.

After a healthcare provider has collected your blood sample, they’ll send it to a laboratory for testing. Once the test results are back, your healthcare provider will share the results with you.

Does a TB test hurt?

For both a TB skin test and blood test, you may feel a small pinch from the needle, but this will be brief. For a skin TB test, the liquid the healthcare provider injects should not burn or feel uncomfortable.

What are the risks of a TB (tuberculosis) test?

There are very few risks to having a TB skin test or blood test.

For a TB skin test, you may feel a pinch when you get the injection. For a TB blood test, you may have slight tenderness or a bruise at the site of the blood draw, but this usually resolves quickly.

Results and Follow-Up

When should I know the results of my TB (tuberculosis) test?

It takes 48 to 72 hours (two to three days) to complete a skin TB test. Once you have your second visit with your healthcare provider, they’ll be able to tell you if your skin’s reaction indicates that you may or may not have a TB infection.

In most cases, you should get the results of your TB blood test within one to two days, though it could take longer.

What type of results do you get from a TB (tuberculosis) test?

The results of a TB skin test or blood test will be either negative or positive. It’s important to remember that these tests only show if you’ve been exposed to the TB (tuberculosis) infection — not if your infection is active or latent. Those who have received TB vaccine (BCG) may show false-positive skin test results.

Positive TB test

If your TB skin test or blood test is positive, it likely means you’ve been exposed to the bacterium that causes TB. Your healthcare provider will likely order more tests to help make a diagnosis. Tests that diagnose active TB infections include chest X-rays and laboratory tests on a sputum sample. Sputum is the thick mucus that you cough up from your lungs. It’s different than spit or saliva. You’ll need to be treated for TB, whether it’s latent or active.

Negative TB test

If your TB skin or blood test is negative, you likely haven’t been exposed to the TB bacterium. A negative TB skin test means your skin didn’t react to the liquid. You may need further testing if your results were negative, but you have symptoms of TB.

What does a positive TB test look like?

A positive TB skin test will look like a firm, red bump. Your provider will measure the size of the firmness of the bump (induration). Your provider considers your particular risk factors to determine the results of your test. You’re considered positive if your bump is:

• Greater than or equal to 5 millimeters (mm) and you have a compromised immune system.
• Equal to 10 mm and you’re a healthcare worker.
• Equal to 15 mm and you don’t have any risk factors.

Even if you think you know the result of your test based on your skin’s appearance after 48 to 72 hours of when you were first injected with the liquid, it’s important to see your healthcare provider for the second visit to get a proper assessment. They’re specially trained to observe your skin for a reaction and need to document your result.

When should I call my doctor about TB (tuberculosis)?

If you have symptoms of TB (tuberculosis), such as a persistent cough, chest pain and fever, or think you’ve been exposed to someone with an active TB infection, it’s important to see your provider as soon as possible.

TB can be deadly if it’s not properly treated. Most cases of TB can be cured if you take antibiotics as directed by your healthcare provider. Both active and latent TB should be treated, but the type and medications and how long you need them are different if you have active vs. latent TB.

A note from Cleveland Clinic

TB (tuberculosis) testing and screening are essential for public health. Since TB can be deadly and spreads from person to person easily, it’s important to get tested if you have symptoms, are at high risk for developing active TB or are at high risk of being exposed to the infection. Once you know your results, your healthcare provider will let you know if you need to undergo further tests. Don’t be afraid to ask your provider questions. They’re here to help you.