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.
FAQs
A TB test checks to see if you have been infected with tuberculosis (TB). There are two types of TB tests: a skin test and a blood test. TB skin and blood tests can show if you've ever been exposed to the bacterium that causes TB, but they can't determine if you have a latent (dormant) or active TB infection.
What is the purpose of the TB test? ›
A tuberculosis screening test checks to see if you have the bacteria (germs) that cause tuberculosis (TB) in your body. TB is a bacterial infection that mainly attacks the lungs. But it can also affect other parts of the body, including the brain, spine, and kidneys.
What do the results of a TB blood test mean? ›
A “positive” TB blood test result means you probably have TB germs in your body. Most people with a positive TB blood test have latent TB infection. To be sure, your doctor will examine you and do a chest x-ray.
What is tuberculosis analysis results? ›
A positive TB blood test result or TB skin test result usually means TB infection. More tests, such as a chest radiograph, are needed to rule out TB disease. Some people with TB disease may have a negative TB blood test or TB skin test result.
What is the purpose of tuberculosis screening for healthcare workers? ›
Health care personnel with untreated latent TB infection should receive a yearly TB symptom screen to detect early evidence of TB disease and to reevaluate the risks and benefits of treatment for latent TB infection.
What is the purpose of the TB culture test? ›
Culture test
This test uses your sputum or tissue sample to grow any TB bacteria that may be there. It tells doctors how infectious you are and also whether your TB is resistant to any antibiotics. This helps ensure they put you on a combination of drugs that will cure you.
Why do employers require TB tests? ›
current CDC recommendations. Pre-employment and post-exposure TB testing of staff is crucial to help stop the spread of TB and keep employees and communities safe. The CDC recommends TB screening and testing of all US healthcare personnel upon hire as part of a TB Infection Control Plan.
How to read TB test results? ›
If you have an Mtb infection, the skin around the injection site should start to swell and harden within 48 to 72 hours . This bump, or induration, may also change color. But the induration's size, not the color, will determine your results. An induration of less than 5 millimeters (mm) is a negative result.
What does a positive TB test read 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.
What does it mean if you fail a TB test? ›
If your skin test is POSITIVE: You have TB germs in your body. The test does not tell whether you have inactive TB or active TB disease. You will need other tests, such as a physical examination, chest x-ray, and lab work to determine if you have active TB disease.
A positive test result for TB infection means you have TB germs in your body. If you test positive for TB infection, your health care provider will do other tests to determine if you have inactive TB (also called latent TB infection) or active TB disease.
How to diagnose tuberculosis? ›
A health care provider may diagnose you with inactive TB or active TB disease based on your:
- Medical history.
- Physical examination.
- Test for TB infection (TB blood test or TB skin test)
- Chest x-ray.
- Laboratory tests to see if TB germs are present (sputum smear and culture)
- Laboratory tests for drug resistance.
There are two primary objectives of screening for TB disease: the first objective is to ensure that TB disease is detected early and treatment is initiated promptly, with the ultimate aim of reducing the risk of poor treatment outcomes, health sequelae and the adverse social and economic consequences of TB; the second ...
What is the most common screening test for tuberculosis? ›
Tuberculin Skin Testing (TST) is one of the available testing methods. There is also blood-based interferon-gamma release assays (IGRAs) used for TB testing such as QuantiFERON-TB Gold, QuantiFERON TB Gold In-Tube or T-SPOT. IGRAs have been approved for use in adults in all circ*mstances where a TST would be used.
What is the mandatory test for tuberculosis? ›
There are two tests that are used to determine if a person has been infected with TB bacteria: TB blood tests (Interferon Gamma Release Assay [IGRA]) and the TB skin test (Mantoux tuberculin skin test).
What is the point of care test for TB? ›
Point of care test target product profile
The POC biomarker test (for non sputum samples) should enable the diagnosis of both pulmonary and extra pulmonary tuberculosis, paediatric TB or at early stages of the disease. It would need to be at least as sensitive as other POC tests in sputum (Xpert), portable and.
What is the purpose of the T spot TB test? ›
The T‑SPOT. TB test is an indirect test for Mycobacterium tuberculosis infection (including disease) and is intended for use in conjunction with risk assessment, radiography and other medical and diagnostic evaluations.
How long is a TB test good for? ›
How long are TB tests good for? This can depend on the employer but most TB test are renewed every year.
What causes tuberculosis? ›
Tuberculosis (TB) is caused by a bacterium (or germ) called Mycobacterium tuberculosis. When a person breathes in TB germs, the germs can settle in the lungs and begin to grow. From there, they can move through the blood to other parts of the body, such as the kidney, spine, and brain.
