Isabella Carter
Tuberculosis (TB) is a serious, highly contagious, and infectious disease that primarily affects the lungs and respiratory system. A TB test is a blood or skin test that checks for TB infection. A positive TB skin test indicates the presence of TB germs in the body, but it does not always mean the patient has an active TB infection. A positive TB blood test, on the other hand, confirms TB infection. However, it is important to note that previous vaccination with the Bacillus Calmette-Guerin (BCG) vaccine can cause false-positive results in skin tests. Therefore, it is crucial to consider the patient's risk factors and perform additional tests to determine the presence of active TB infection.
| Characteristics | Values |
|---|---|
| Type of test | Skin test or blood test |
| Skin test procedure | A small amount of fluid (tuberculin or PPD) injected under the skin on the forearm |
| Blood test procedure | Blood sample mixed with antigens and controls, reaction measured using immunoassay |
| Skin test result interpretation | Positive if a bump of a certain size is present where the fluid was injected |
| Blood test result interpretation | Positive if TB germs detected in the blood |
| Induration size interpretation | =5mm: positive in people with immunosuppression or previous TB; >=10mm: positive in people from TB-endemic countries or high-risk environments |
| False-positive risk factors | Previous BCG vaccination or infection with nontuberculous mycobacteria |
| False-negative risk factors | Concurrent infections |
| Further tests after positive skin test | Blood test, chest X-ray, sputum test |
| Further tests after positive blood test | Chest X-ray, sputum smear or culture test |
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What You'll Learn
TB skin test procedure
The TB skin test, sometimes called a Mantoux test, tuberculin skin test (TST), or PPD, is a simple, harmless way to find out if you have a latent TB infection. The test requires two or three visits with a healthcare provider. During the first visit, they will clean and disinfect the skin on your lower inner forearm and inject a small amount of fluid (called tuberculin or PPD) with a tiny needle under the skin. The fluid contains some inactive TB protein. You should feel a small prick from the needle. After two or three days, you must return to the clinic to have the skin test read. A trained healthcare provider will measure the size of the bump or reaction. A positive or negative test result depends on the size of the bump or reaction. The test is "positive" if there is a bump of a certain size where the fluid was injected, which means you probably have TB germs in your body. Most people with a positive TB skin test have a latent TB infection. To be sure, your doctor will examine you and give you a chest x-ray. You may need other tests to see if you have active TB disease.
It's important to remember that the TB skin test can show if you've been exposed to the tuberculosis infection, but it can't determine if your infection is active or latent. You might have a false-positive skin test result if you've received a TB vaccine (BCG). If you've had a positive TB skin test in the past, you'll be expected to always test positive in the future. Once you have had a positive skin test, let your doctor know. They are usually not repeated because the swelling can be worse each time.
A two-step TB skin test is required for baseline TB screening of healthcare workers and patients in boarding care facilities, correctional facilities, and nursing homes. Two-step TSTs are not recommended for patients in other settings. A two-step TB skin test can lower the chance that a boosted reaction from an old TB infection will be misinterpreted as a recent infection. If the reaction to the first-step TB skin test is negative, a second-step TB skin test is given one to three weeks after the first test is read.
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TB blood test procedure
A TB test is a blood or skin test that checks for tuberculosis infection. The TB skin test is also called a Mantoux test or tuberculin skin test (TST), while a TB blood test is called an Interferon-Gamma Release Assay (IGRA).
The TB blood test procedure involves drawing a blood sample from the patient and sending it to a laboratory for testing. The laboratory scientists then mix the blood sample with antigens and controls and measure the reaction using a method called immunoassay. The TB blood test, also known as the QuantiFERON™ TB test, measures a person's immune reactivity to M. tuberculosis. It is a modern alternative to the skin test, offering quicker, more reliable, and accurate results. The test results will come back as either positive, negative, or indeterminate.
A positive TB blood test suggests that M. tuberculosis infection is likely, indicating the presence of TB germs in the body. However, it does not distinguish between a latent (inactive) or active TB infection. Therefore, additional tests, such as chest X-rays, sputum smear or culture tests, and further examinations, are necessary to confirm the status of the infection.
A negative TB blood test result indicates that M. tuberculosis infection is unlikely. However, in rare cases, immunocompromised individuals may experience false-negative results. An indeterminate result indicates uncertainty regarding TB infection and may warrant a repeat test.
It is important to note that certain factors, such as receiving a TB vaccine (BCG) or other vaccines like MMR, Varicella, or Yellow Fever, may affect the test results. Therefore, it is recommended to either get the TB blood test on the same day as receiving a vaccine or wait 4-6 weeks after vaccination.
TB Skin Test Procedure
The TB skin test, on the other hand, involves injecting a small amount of fluid called tuberculin or PPD (purified protein derivative) under the skin on the lower inner forearm. The patient then needs to return to the healthcare provider after two or three days to assess the skin's reaction to the injection. If there is a raised bump or swelling at the injection site, it indicates a positive test, suggesting the presence of TB germs in the body. However, similar to the blood test, further tests are required to determine if the infection is latent or active.
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Interpreting TB skin test results
Understanding the Test Procedure
The TB skin test is typically administered in two or three visits with a healthcare provider. During the first visit, the provider will inject the fluid, and on subsequent visits, they will assess the skin's reaction to the injection. It is important to return for the official reading within 48 to 72 hours, as specified by the healthcare provider.
Positive Test Result Indications
A positive TB skin test result indicates the presence of TB germs in the body. This is typically characterised by a raised, hard bump or swelling at the injection site. The bump or swelling indicates a positive result if it meets or exceeds a certain size. The World Health Organization (WHO) and medical sources consider a bump size of ≥10 mm induration as a positive result. This means that the bump is firm and raised, measuring at least 10 millimetres in diameter.
Negative Test Results
A negative TB skin test result suggests that the individual has likely not been exposed to the TB bacterium. It is important to note that false-negative results can occur due to various factors, including improper vaccine storage, administration errors, or interpretation mistakes. Therefore, ensuring proper testing procedures and interpretation guidelines are crucial to obtaining accurate results.
Further Testing and Considerations
A positive TB skin test does not differentiate between a latent (dormant) or active TB infection. Additional tests are necessary to determine the status of the infection. These may include chest X-rays, blood tests, and sputum tests, where a sample of the mucus coughed up from the lungs is examined for the presence of TB bacteria. It is important to remember that even with a negative skin test, other tests may be warranted based on an individual's symptoms, risk factors, and exposure history.
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Interpreting TB blood test results
TB tests check for tuberculosis infection in the body. There are two types of TB tests: a skin test and a blood test. Both tests can show if you’ve been infected with TB germs, but they can’t distinguish between a latent (dormant) or active TB infection.
TB blood tests (also called interferon-gamma release assays or IGRAs) use a blood sample to find out if you are infected with TB germs. The tests measure how your immune system reacts when a small amount of your blood is mixed with TB proteins.
The results of a TB blood test will be either negative or positive. A positive blood test result for TB infection means you have TB germs in your body. A negative blood test result for TB infection means inactive TB or active TB disease is unlikely. However, your healthcare provider may do more tests, especially if you have HIV, were recently exposed to TB germs, or have symptoms consistent with TB disease.
It's important to remember that a positive TB blood test result does not necessarily mean you have active TB disease. Additional tests, such as a chest radiograph or X-ray, are needed to rule out TB disease and determine if the infection is latent or active.
TB blood tests are the preferred method of testing for people who have received the TB vaccine, have difficulty returning for a second appointment, or have compromised immune systems.
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TB test limitations
TB skin tests are simple and harmless, but they have several limitations. Firstly, they cannot distinguish between a latent (dormant) and active TB infection. A positive skin test result indicates the presence of TB germs in the body, but it does not confirm whether the infection is active or latent. To determine the status of the infection, additional tests, such as chest X-rays, laboratory tests on sputum samples, or blood tests, are required.
Secondly, the TB skin test requires multiple visits to a healthcare provider, usually two or three. During the first visit, the healthcare provider injects fluid into the patient's forearm. The second and third visits are necessary to assess the skin's reaction to the injection. This multi-step process can be inconvenient for patients and may pose challenges in terms of access and adherence to the testing protocol.
Thirdly, the TB skin test can produce false-positive results in individuals who have received the BCG (Bacillus Calmette-Guerin) vaccine. The BCG vaccine is known to cause a positive skin test reaction, which may lead to incorrect interpretations of TB infection status.
Furthermore, the TB skin test may not always be accurate, especially if the exposure to TB was a long time ago or if the individual's immune response to the infection is weak. In such cases, the test may show a negative result even when the person has been exposed to TB.
Lastly, individuals who have had severe skin reactions to previous TB skin tests are generally not recommended to undergo another TB skin test due to the risk of scarring and other potential complications. This limitation underscores the importance of alternative testing methods, such as blood tests, for individuals who cannot tolerate the skin test.
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Frequently asked questions
A TB test is a blood or skin test that checks for tuberculosis infection.
A TB skin test is positive if there is a bump of a certain size where the fluid was injected. The bump indicates the presence of TB germs in your body. The size of the bump and your infection risk determine if the test is positive.
A positive TB skin or blood test result means you've been exposed to the TB bacterium. However, it does not indicate whether the infection is active or latent. Further tests, such as a chest X-ray or sputum test, are needed to determine the status of the infection.
Yes, previous BCG vaccination can cause false-positive results on skin tests. TB blood tests are preferred for individuals who have received the BCG vaccine.
If you have a positive TB skin or blood test, your doctor will likely order additional tests to determine if the infection is active or latent. Treatment is required for both latent and active TB infections.
