Latent Tb Infection Vs Active Tb Disease
If you have tuberculosis, you might have a latent TB infection or active TB disease.
With a latent TB infection, you have the TB bacteria, but you don’t feel sick and you have no symptoms. You can’t spread TB to anyone else. The only sign that you have a TB infection is a positive TB skin test or blood test.
With latent infection, your chest X-ray will not show active TB and your sputum cultures will be negative for TB.
About 5% to 10% of people who have a latent TB infection will eventually get active TB disease. Your chance of it depends on your medical history.
In some cases, the TB bacteria overcome your body’s immune system and multiply. This becomes TB disease. You’ll have symptoms that can include:
If you have active TB, you can spread it to other people. Active TB is diagnosed by the presence of symptoms, TB tests, sputum cultures and imaging.. Work with your doctor if you get these test results. TB disease is serious and needs treatment.
What If I’ve Had Bcg Vaccine
Even if you have had BCG vaccine, you can have a TB skin test.
- People who have had BCG vaccine still can get latent TB infection and active TB disease.
- BCG vaccine may help protect young children from getting very sick with TB. This protection goes away as people get older.
- BCG vaccine sometimes causes a positive TB skin test reaction. But if you have a positive reaction to the TB skin test, it probably is from TB germs in your body – not from your BCG vaccine.
You can have a TB skin test, even if you have had BCG vaccine!
What Happens If Your Tb Test Is Positive
If your TB test is positive and you have symptoms or are considered at high risk of TB exposure, a doctor will likely prescribe medication to clear the infection and relieve symptoms.
If youre at low risk of TB exposure but have a positive test, a doctor may recommend you take a TB blood test to confirm the diagnosis. The blood test is more accurate than the skin test, but like the skin test, it cant differentiate between active TB disease and a latent TB infection.
The doctor may order a chest X-ray or CT scan to determine if you have active TB disease or latent TB, as well as a sputum test to identify the bacteria in your body and choose the most effective medication.
| 15 mm or more | positive |
An induration of less than 5 millimeters is a negative result. If you have symptoms or know youve been exposed to someone with TB, a doctor may recommend you get another test.
If the induration is at least 5 mm, it is considered positive in people who:
- have had recent contact with a person with TB
- are HIV-positive
- have had an organ transplant
A doctor may interpret a 5 mm induration as a positive result if you are taking immunosuppressant medications or previously had TB.
An induration of at least 10 mm may be considered a positive result if you recently immigrated from a country with a high prevalence of TB.
An induration of 15 mm or more is considered positive in anyone, even people who dont think theyve been exposed to TB.
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All You Need To Know About Tb Skin Tests
TB skin tests are an effective way to detect the presence of TB bacteria in a persons body. However, there are some potential risks and benefits associated with the procedure. Here are some things to consider before getting a TB skin test.
Who Should Get a TB Skin Test? It is recommended that anyone who has had contact with someone with TB, or who is at risk of contracting TB, should get a TB skin test. It is also recommended that children, pregnant women, and people with weakened immune systems should get tested.
What Are the Benefits of Getting a TB Skin Test? The primary benefit of getting a TB skin test is that it can help detect TB early, which can reduce the risk of complications. Early detection can also help prevent the spread of TB.
Are There Risks Associated with Getting a TB Skin Test? Although TB skin tests are generally safe, there is a small risk of infection or allergic reaction at the injection site. If you experience any redness, swelling, or discomfort at the injection site, contact your healthcare provider immediately.
When Should I Call My Doctor About Tb
If you have symptoms of TB , such as a persistent cough, chest pain and fever, or think youve been exposed to someone with an active TB infection, its important to see your provider as soon as possible.
TB can be deadly if its not properly treated. Most cases of TB can be cured if you take antibiotics as directed by your healthcare provider. Both active and latent TB should be treated, but the type and medications and how long you need them are different if you have active vs. latent TB.
A note from Cleveland Clinic
TB testing and screening are essential for public health. Since TB can be deadly and spreads from person to person easily, its important to get tested if you have symptoms, are at high risk for developing active TB or are at high risk of being exposed to the infection. Once you know your results, your healthcare provider will let you know if you need to undergo further tests. Dont be afraid to ask your provider questions. Theyre here to help you.
Last reviewed by a Cleveland Clinic medical professional on 04/15/2022.
References
- Centers for Disease Control and Prevention. Interferon-Gamma Release Assays Blood Tests for TB Infection. Accessed 4/15/2022.
- Centers for Disease Control and Prevention. Testing for TB Infection. Accessed 4/15/2022.
- de Lima Corvino DF, Shrestha S, Kosmin AR. Tuberculosis Screening. . In: StatPearls . Treasure Island : StatPearls Publishing 2022. Accessed 4/15/2022.
- MedlinePlus. Tuberculosis Screening. Accessed 4/15/2022.
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Interferon Gamma Release Assay
The interferon gamma release assay is a blood test for TB that’s becoming more widely available.
The IGRA may be used to help diagnose latent TB:
- if you have a positive Mantoux test
- if you previously had the BCG vaccination the Mantoux test may not be reliable in these cases
- as part of your TB screening if you’ve just moved to the UK from a country where TB is common
- as part of a health check when you register with a GP
- if you’re about to have treatment that will suppress your immune system
- if you’re a healthcare worker
Page last reviewed: 12 November 2019 Next review due: 12 November 2022
What Do The Tuberculin Test Results Mean
After 2 to 3 days of having the tuberculin skin test, the laboratory technician will measure your skins reaction to the tuberculin. If more than 3 days have passed before you get the site checked, the assessment will not be reliable and you will need another skin test.Interpretation of the test result depends on a number of factors, including whether you are known to have been in contact with somebody who has TB, whether you have previously had a BCG vaccine, your age and your medical history.
If your tuberculin test is negative, you may be advised to either:
- have a repeat test in 12 weeks
If your injection site shows a reaction, you may be advised to either:
- have treatment for latent TB
- have further tests for TB disease
- avoid further tuberculin tests
- consult with your usual doctor
- have no further follow-up.
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When Should I Get A Tb Skin Test
A TB skin test may be recommended to screen a person who is at an increased risk of TB infection. People whose job or living condition puts them at an increased risk of TB infection include those who live or work in group settings where tuberculosis is more common, such as:
- Health care settings
- Countries where TB infection is common, including Mexico, India, and China
If a patient is showing symptoms of TB disease, a TB skin test may be ordered to assist in making a diagnosis. Symptoms of TB disease include:
- A bad cough that lasts longer than 3 weeks
- Coughing up blood and mucus
- Lack of appetite or weight loss
- Fever, chills, or night sweats
Diagnosis Of Latent Tb Infection Or Tb Disease
If a person is found to be infected with TB bacteria, other tests are needed to see if the person has TB disease. TB disease can be diagnosed by medical history, physical examination, chest x-ray, and other laboratory tests. TB disease is treated by taking several drugs as recommended by a health care provider.
If a person does not have TB disease, but has TB bacteria in the body, then latent TB infection is diagnosed. The decision about taking treatment for latent TB infection will be based on a persons chances of developing TB disease.
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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 an exposure to TB
- 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 health care provider.
Learn more about laboratory tests, reference ranges, and understanding results.
How Is A Tb Skin Test Test Different From An Igra Tb Test
Another test used to detect TB infections, the interferon gamma release assay is a blood test that requires a blood sample. An IGRA TB test may be used instead of a TB skin test for a number of reasons, including the test setting, the cost of testing, and test availability. Providers may also recommend an IGRA TB test because this test only requires one visit. However, both tests are acceptable for detecting TB infections.
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When Should I Know The Results Of My Tb Test
It takes 48 to 72 hours to complete a skin TB test. Once you have your second visit with your healthcare provider, theyll be able to tell you if your skins reaction indicates that you may or may not have a TB infection.
In most cases, you should get the results of your TB blood test within one to two days, though it could take longer.
How Does A Tb Skin Test Detect Tuberculosis
Now that we have a better understanding of how a TB skin test works, lets take a closer look at how it actually detects TB bacteria. When a person is exposed to TB bacteria, their body produces an immune response in the form of antibodies. These antibodies bind to the TB proteins, which triggers an inflammatory response in the injection site. This response is what the healthcare provider looks for when interpreting a TB skin test result.
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What Is The Tuberculosis Skin Test
The tuberculosis skin test determines if someone has developed an immune response to the bacterium that causes tuberculosis . This response can occur if someone currently has TB, if they were exposed to it in the past, or if they received the BCG vaccine against TB . Estimates indicate that one-third of the world’s population has latent TB, and around 1.3 million people worldwide die of TB each year. The tuberculin test or PPD test are other names for the tuberculosis skin test.
The tuberculin skin test is based on the fact that infection with M. tuberculosis bacterium produces a delayed-type hypersensitivity skin reaction to certain components of the bacterium. Medical professionals extract the components of the organism from TB cultures and are the core elements of the classic tuberculin PPD . This PPD material is used for skin testing for tuberculosis. Reaction in the skin to tuberculin PPD begins when specialized immune cells, called T cells, sensitized by prior infection, are attracted by the immune system to the skin site where they release chemical messengers called lymphokines. These lymphokines induce induration through local vasodilation leading to fluid deposition known as edema, fibrin deposition, and attraction of other types of inflammatory cells to the area.
Are Test Results Accurate
The TB skin test is a widely used test. There are known circumstances that can lead to false negative and false positive test results.
There are several factors that can contribute to false positive test results, in which a person has a positive test result despite not having an infection. Known causes for false positive test results include:
- Previous vaccination with the bacille Calmette-Guérin TB vaccine
- An infection from another type of bacteria from the same family as Mycobacterium tuberculosis
False negative results, in which a person has a negative test result despite having a TB infection, can occur for several reasons, including:
- Recent vaccination using the live-virus measles or smallpox vaccine
- A recent TB infection acquired within 8 to 10 weeks before a TB skin test
- Testing on infants
- A phenomenon known as anergy, which describes a lack of normal immune response to the test fluid
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What To Think About
- The results of a tuberculin skin test alone cannot confirm active TB. Other tests, such as a chest X-ray, sputum cytology, and sputum culture, may be done to confirm active TB when a skin test is positive. A person who has a positive skin test or chest X-ray, but no TB symptoms, is usually thought to have a TB infection that cannot be passed to others .
How Is The Tuberculin Skin Test Done
The tuberculin skin test involves having a small amount of liquid, called tuberculin, injected into the very top layer of the skin on your forearm. This is done with a small needle and syringe. The test, which is done by specially trained laboratory technicians, does not contain live bacteria and cannot cause TB.
- You may get a small blister at the injection site, but this usually disappears within the first 30 minutes.
- Sometimes the site may bleed a little and the nurse may cover it with a cotton wool swab. This can be removed after 10 minutes and discarded.
- Do not apply creams or band-aids to the injection site and avoid scratching it. To relieve an itchy reaction, apply ice or something cold.
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Taking A Tb Skin Test
A TB skin test is performed by injecting a test fluid under the skin on the inside of the forearm, between the wrist and the elbow.
The test takes two separate visits to complete. The first visit is to administer the test, which typically takes about 5 minutes. The second visit is to interpret the test results. Test results must be read within a 48 to 72 hour window to be considered valid. If the test is not read within that time frame, another TB skin test can be administered as soon as possible.
Exploring The Science Behind Tb Skin Tests
Before we dive into the details of a TB skin test, its important to understand the basics of TB and how it is detected. Tuberculosis is an infectious disease caused by Mycobacterium tuberculosis, a type of bacteria. It is spread through the air when an infected person coughs, sneezes, or talks. Symptoms of TB can include a persistent cough, chest pain, fever, night sweats, and weight loss.
In order to detect the presence of TB bacteria, doctors use a variety of methods, including chest X-rays, sputum tests, and blood tests. However, the most common method of detecting TB is through a TB skin test. A TB skin test is a simple and relatively inexpensive way to determine whether someone has been exposed to the TB bacteria.
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