What is a tuberculosis (TB) blood test?
A tuberculosis blood test or 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. TB spreads from person to person when someone who is sick with TB coughs or sneezes.
TB germs cause two types of conditions:
- Latent TB infection happens when you have TB germs in your body, but you’re not sick because the germs aren’t active. You don’t have symptoms and you can’t spread the disease to others. But the germs could start to grow (multiply) and make you sick in the future. So, latent TB infection is usually treated to prevent you from getting sick with active TB.
- Active TB disease means that TB germs are actively growing in your body and making you sick. If TB germs grow in your lungs or throat, you can spread the disease to other people. TB disease can almost always be cured with antibiotics. But without treatment, it can cause serious illness or even death.
A TB screening test checks to see if you have TB germs in your body. But it can’t show whether you have a latent TB infection or active TB disease. If a TB screening test finds signs of TB germs, you’ll need other tests to find out which type of TB you have so you can get the right treatment.
There are two types of TB tests used for screening: a TB skin test and a TB blood test. Your health care provider or your local health department can let you know which test is best for you.
Other names: TB test, TB skin test, purified protein derivative (PPD) test, latent TB infection test, Mantoux tuberculin skin test; IGRA test, TB blood test, QuantiFERON gold.
What is it used for?
TB screening is used to look for signs of TB germs in people who have been exposed to someone with active TB disease or who have a high risk of exposure. The screening can show whether you have TB germs in your body and need more testing. It does not show if you have a latent infection or active disease.
Why do I need a TB screening?
You may need a TB skin test or TB blood test if you were exposed to someone with active TB disease or if you have a high risk of exposure.
Your risk of exposure is higher if you:
- Are you a healthcare worker?
- Work or live in a place where TB is more common, such as jails, nursing homes, and shelters for people without homes
- Were born in or often travel to countries where TB disease is common
You may need a TB screening test if have symptoms of active TB disease, including:
- A cough that lasts longer than three weeks
- Coughing up blood or sputum (a thick mucus from the lungs)
- Chest pain
- Night sweats (heavy sweating during sleep)
- Losing weight without trying
- Loss of appetite
- Weakness or fatigue
You may need a TB screening test if you have a health condition that increase your risk for getting active TB disease. These conditions include:
- HIV or another disease that weakens your immune system.
- Having an organ transplant
- Use of illegal drugs that are injected with needles.
- What happens during a TB screening?
You may get a TB screening test at your provider’s office, a health clinic, or hospital. You will have either be a TB skin test or a TB blood test, depending on your health and medical history. TB skin tests are used more often, but blood tests for TB are becoming more common.
For a TB skin test (also called a Mantoux tuberculin skin test), you will need two visits to complete the test. The first visit is for doing the test and the second visit is for reading the results.
On the first visit:
A healthcare professional will use a tiny needle to place a small amount of fluid under the skin in the lower part of your arm. The fluid is called tuberculin. It contains a protein that comes from the bacteria that cause TB. It cannot make you sick.
The fluid will make a small bump on your arm.
The test spot will be left uncovered.
You will need to avoid touching the test spot or putting lotions on it. It’s okay to get it wet. If it itches, you can put an ice cube or cold cloth on it.
After two to three days, you will return for the second visit. A healthcare professional will look at the test spot on your arm to see if your skin reacted. If there is a bump of hard skin, the professional will measure the bump. The meaning of the bump depends on how large it is, your risk of exposure to TB germs, and your risk for developing active TB disease. The professional will let you know whether or not the bump means that you’re likely to have TB germs in your body.
For a TB test in blood (also called an IGRA test), a healthcare professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.
About QuantiFERON gold (IGRA) test:
QuantiFERON is a powerful Premolecular diagnostic technology that delivers results with high sensitivity, enabling the detection of M. tuberculosis infection even in early or latent stages. The QFT-Plus test employs a peptide cocktail simulating ESAT-6 and CFP-10 proteins to stimulate cells in heparinized whole blood. Detection of IFN-γ by ELISA is used to identify in vitro responses to these peptide antigens, which are associated with M. tuberculosis infection. Premolecular Diagnostics that employ QuantiFERON technology, like QFT-Plus, are able to detect infection at an earlier stage than is possible with DNA- and RNA-based molecular diagnostics.
Will I need to do anything to prepare for the test?
You don’t any special preparations for TB skin test or a TB blood test.
Are there any risks to the test?
There is very little risk of having a TB skin test or blood test. For a TB skin test, you may feel a pinch when the fluid is placed under your skin.
For a blood test, you may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.
What do the results mean?
Your TB skin or blood test results will usually be positive or negative.
A positive result means that you have been infected with TB bacteria. You will need more tests to find out if you have a latent TB infection or TB disease. These tests may include a chest x-ray or a sputum culture. If you had a positive result on a TB skin test, you may have a TB blood test to confirm the result.
A negative result means that your skin or blood did not react to the test. You are unlikely to have a latent TB infection or TB disease. But you may still need more testing if you:
- Have symptoms of TB
- Were tested sooner than six to eight weeks after exposure to TB
- Have HIV
- Had a TB skin test after being around someone with TB disease.
Sometimes a TB blood test result will be “borderline,” which means the test could not show for sure whether you have a TB infection. If this happens, you will likely be tested again.
Overall, TB screening tests tend to be accurate. But TB blood tests are more accurate than TB skin tests. Your provider will consider whether anything about your health history might affect the accuracy of your test results.
If you have questions about your results, talk with your healthcare provider.
Is there anything else I need to know about a TB screening?
Both active TB disease and latent TB infections should be treated. The treatment for both conditions is antibiotics. To make sure you get rid of all the TB germs in your body, you’ll need to follow the directions for taking your medicine. Treatment may last a few months to a year. Stopping treatment too soon can cause the infection to come back and make it harder to treat.
Source: MedlinePlus, National Library of Medicine
MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.