A false-positive TB test means the result suggests tuberculosis (TB) infection even though a person is not infected with TB bacteria. It can happen with a TB skin test after BCG vaccination, from exposure to certain related bacteria, or because the skin reaction was measured or interpreted incorrectly. A positive result needs follow-up; it does not by itself show whether TB disease is active.
How Can a TB Test Give a False-Positive Result?
TB testing helps determine whether a person has been infected with the bacteria that cause tuberculosis. However, a positive result does not always reflect a true infection. False-positive results can occur with skin tests and blood tests, although the possible reasons depend on the type of test used.
The Centers for Disease Control and Prevention (CDC) identifies several possible causes of false-positive TB skin test results, including previous vaccination, exposure to certain other bacteria, and errors in measuring or interpreting the skin reaction.
Common Reasons for a False-Positive TB Skin Test
- Previous BCG vaccination: The bacille Calmette-Guérin (BCG) vaccine helps protect against serious forms of tuberculosis, particularly in children. It can cause a positive reaction on a TB skin test even when the person has not been infected with TB bacteria.
- Exposure to related bacteria: Some bacteria from the same broader family as TB bacteria, known as nontuberculous mycobacteria, can cause a positive skin test reaction without infection from the bacteria that cause tuberculosis.
- Incorrect test reading: A skin test must be measured and interpreted correctly. Errors in assessing the firm swelling at the injection site can lead to an incorrect result.
- Test administration or technical problems: Issues with how the test is given or which testing material is used can affect the reliability of the result.
Having received the BCG vaccine does not mean that every positive skin test is false. Healthcare professionals still assess the result according to the person's risk of TB infection and other relevant information.
Can a TB Blood Test Be False Positive After BCG Vaccination?
BCG vaccination can affect TB skin test results, but it does not cause a positive result on the standard TB blood tests used to detect infection. These blood tests are called interferon-gamma release assays (IGRAs). According to the CDC's guidance on TB blood testing, blood tests are preferred for people who have received the BCG vaccine because the vaccine does not interfere with their results.
TB blood tests and skin tests are different methods of checking for an immune response to TB bacteria. Neither result should be interpreted without considering the person's exposure history, previous test results, and clinical circumstances. An unexpected positive blood test may need further assessment to determine whether it reflects a true infection.
Does a False-Positive TB Test Mean You Have Active Tuberculosis?
No. A false-positive result means the test suggests TB infection when the person is not actually infected. A true-positive result, however, does not automatically mean active tuberculosis disease is present.
When someone is genuinely infected with TB bacteria, the infection may be inactive, also called latent TB infection, or it may cause active TB disease. Inactive TB infection does not cause symptoms and is not contagious. Active TB disease can cause illness and may spread to others when it affects the lungs or throat.
The CDC's tuberculosis diagnosis guidance explains that a positive skin or blood test needs further evaluation to determine whether active disease is present. The test result alone cannot make that distinction.
What Should You Do After a Positive TB Test?
A positive result should be reviewed by a healthcare professional rather than assumed to be either correct or false. The next steps depend on which test was performed, whether you have symptoms or known exposure to TB, and whether you have previously tested positive or received treatment for tuberculosis.
1. Review Your Test and Medical History
Tell your healthcare professional if you received the BCG vaccine, have lived in or travelled to an area where TB is common, have spent time with someone who has active TB, or have previously had a positive TB test. Mention any past treatment for TB infection or disease, too. These details can help determine how the result should be interpreted.
According to MedlinePlus's guidance on tuberculosis screening, a healthcare professional may use a TB blood test to help clarify a positive skin test result in appropriate circumstances. Whether another test is useful depends on the individual situation and the testing method already used.
2. Find Out Whether Further Testing Is Needed
If your healthcare professional cannot confidently interpret the result, additional testing may be recommended. Depending on your circumstances, this may include another type of TB test, a chest X-ray, or laboratory testing of sputum (the mucus coughed up from the lungs). These tests serve different purposes and help assess whether TB infection is present and whether active disease needs to be ruled out.
The CDC's clinical and laboratory diagnosis guidance states that evaluation may include medical history, a physical examination, a TB test, chest imaging, and laboratory examination of samples when indicated. Not everyone needs every test; the evaluation depends on the findings and clinical circumstances.
3. Do Not Rely on Repeat Testing Without Guidance
Repeating a TB test is not always the best next step. People with documented previous positive results or a history of treatment for TB disease generally should not undergo routine repeat skin or blood testing, because positive results often persist and may not provide useful new information. Ask your healthcare professional how to proceed if a previous result is already documented.
When Should You Seek Medical Evaluation?
Arrange a medical evaluation after a positive TB test, even if you feel well, so that a possible infection can be assessed appropriately. Seek prompt medical attention if you also have a persistent cough, chest pain, fever, night sweats, unexplained weight loss, or coughing up blood. These symptoms can occur with active TB disease but are not specific to tuberculosis.
Do not dismiss a positive result solely because you received BCG vaccination or have no symptoms. A careful review of the test, your history, and any additional findings helps distinguish a false-positive result from a true infection and determines whether further evaluation is needed.