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 tuberculosis test? ›
Understanding Your Test Results. If your blood test result is positive, it means you have TB germs in your body. A doctor or healthcare provider should examine you. Most people with positive TB tests feel just fine and have no signs of illness, because they have latent (or “hidden”) TB infection.
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 is the purpose of tuberculosis screening for healthcare workers? ›
Conduct an annual TB symptom screen
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.
How to know if a TB test is positive? ›
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.
What happens if I test positive for TB? ›
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. This is to help you feel better and prevent other people from getting sick.
How to detect tuberculosis? ›
How Is TB Diagnosed? Your doctor will start by collecting a patient history to determine if you may have been exposed. During a physical exam, they will use a stethoscope to listen to your lungs and check the lymph nodes in your neck for swelling. If your doctor suspects TB, they may order a skin or blood test.
What is the normal value of TB test? ›
The reference range of the induration is 0-15 millimetres at an increment of 5 mm, 10 mm and 15 mm. An induration size of more than 15 mm is considered positive in a normal patient.
Tuberculosis (TB) is a bacterial infection spread through inhaling tiny droplets from the coughs or sneezes of an infected person. It is a serious condition, but can be cured with proper treatment. TB mainly affects the lungs. However, it can affect any part of the body, including the glands, bones and nervous system.
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.
What is the purpose of the TB blood test? ›
The tuberculosis (TB) blood test, also called an Interferon Gamma Release Assay or IGRA, is a way to find out if you have TB germs in your body.
What is the aim of tuberculosis test? ›
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 size is a positive PPD test? ›
Induration of 15 mm or more is considered positive in the following cases: Healthy individuals older than four years of age with a low likelihood of TB infection[14]
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 require TB tests? ›
Tuberculosis (TB) is one of the leading causes of infectious disease morbidity and mortality worldwide1 and is an ongoing concern for employers managing the health of their employees.
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 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.
