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? ›
What is a tuberculosis (TB) screening? 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 are the results of a TB test? ›
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. You may need other tests to see if you have latent TB infection or active TB disease.
What is the purpose of tuberculosis screening for healthcare workers? ›
The purpose of screening for TB using a signs and symptoms assessment questionnaire is to ensure active TB is identified early. Anyone with symptoms of TB should be referred for medical evaluation.
What are the results of TB investigation? ›
Interpreting test 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 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.
What is the purpose of the T spot TB test? ›
T-SPOT TB is an in vitro diagnostic test for the detection of effector T cells that respond to stimulation by Mycobacterium tuberculosis antigens and is intended for use as an aid in the diagnosis of tuberculosis infection.
What is the test for TB screening? ›
The TST (or Mantoux test) indicates whether a person has been infected with Mycobacterium tuberculosis (the bacteria causing TB) by measuring if the body's immune response to TB has been stimulated. A TST may be used to diagnose latent TB infection.
What is the main purpose of a contact investigation? ›
The goals of a contact investigation are to successfully stop TB transmission and prevent future cases and outbreaks of TB disease. It is important to remember that every TB case started as a TB contact.
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.
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 ...
Which test confirms 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 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 are the results from a TB test? ›
The test is "positive" if there is a bump of a certain size where the fluid was injected. This means you probably have TB germs in your body. Most people with a positive TB skin test have latent TB infection. To be sure, your doctor will examine you and give you a chest x-ray.
How is a tuberculosis test done? ›
Positive TB test
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.
How to read TB blood test results? ›
Interpreting test results
The results from a TB blood test are typically reported in one of three ways: A positive result means that there was an immune reaction to the TB bacteria and that infection is likely. A negative result means that there was no immune reaction and that TB infection is not likely.
What is the point of care test for TB? ›
Point-of-care tests (POCTs) facilitate early detection, immediate treatment, and reduced transmission of TB disease. This Review evaluates current diagnostic assays endorsed by the World Health Organization and identifies the gaps between existing conventional tests and the ideal POCT.
Why do employers do 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 you have TB? ›
If TB of the lung is not treated early or if treatment isn't followed, long-lasting (permanent) lung damage can result. TB can also cause infection of the bones, spine, brain and spinal cord, lymph glands, and other parts of the body.
How does one get tuberculosis? ›
Tuberculosis (TB) germs spread through the air from one person to another. TB germs can get into the air when someone with active TB disease coughs, speaks, or sings. People nearby may breathe in these germs and become infected. People with inactive TB, also called latent TB infection, cannot spread TB germs to others.
