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 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.
How can you tell the results of a TB test? ›
Your health care provider MUST check your arm 2 or 3 days after the TB skin test, even if your arm looks OK to you. If you have a reaction to the test, it will look like a raised bump. Your health care provider will measure the size of the reaction. If there is a bump, it will go away in a few weeks.
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 does a failed TB test look like? ›
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 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.
How to know if a TB test is positive? ›
If you have a raised, hard bump or there's swelling on your arm, you have a positive test. That means TB germs are in your body. But it doesn't always mean you have active tuberculosis disease.
When should TB results be read? ›
The skin test should be read between 48 and 72 hours after administration. A patient who does not return within 72 hours will probably need to be rescheduled for another skin test. The three cut points below should be used to determine whether the skin test reaction is positive.
How long is a TB test good for? ›
If you have a negative skin test, you need a repeat test at least once every four years. If you have a documented positive skin test, you must have an initial chest X-ray. After that, you still need to be screened every four years.
Can a tuberculosis test make you sick? ›
Side effects are uncommon from the Mantoux skin test. However, a person who has been exposed to TB germs may occasionally have a sizable reaction, which may cause some mild itching, swelling or irritation. These kinds of reactions should disappear in one to two weeks.
What is the point of care TB test? ›
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.
Like all case-finding strategies, systematic screening for TB has three primary goals: 1. to ensure the early detection and initiation of appropriate treatment for those with active TB; 2. to reduce the risk of poor treatment outcomes, health sequelae and the adverse social and economic consequences of TB; and 3. to ...
What is the test for tuberculosis? ›
Skin test. A tiny amount of a substance called tuberculin is injected just below the skin on the inside of one forearm. Within 48 to 72 hours, a health care worker will check your arm for swelling at the injection site. The size of the raised skin is used to determine a positive or negative test.
What color is a negative TB test? ›
Redness alone at the skin test site usually means you haven't been infected with TB bacteria. A firm red bump may mean you have been infected with TB bacteria at some time. The size of the firm bump (not the red area) is measured 2 to 3 days after the test to find out the result.
How to read TB blood test results? ›
Negative results suggest that there is not TB infection. In patients with high suspicion of exposure, a negative test should be repeated. A positive test indicates infection with Mycobacterium tuberculosis. Among individuals without tuberculosis infection, a positive test may be due to exposure to M.
Can a TB test be done wrong? ›
False-negative results
Some persons may have a negative result from a TB skin test even though they are infected with TB bacteria. The reasons for these false-negative results may include, but are not limited to, the following: Anergy (inability to react to a TB skin test because of a weakened immune system)
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.
What happens if I test positive for TB? ›
TB germs can live in your body for years without causing symptoms. Without treatment, inactive TB can develop into active TB disease and make you sick. If you have been diagnosed with active TB disease, you will need to take and finish all your TB medicine.
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.
