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Difference Between Positive TB Skin Test and Negative TB Skin Test
Tuberculosis, commonly known as TB, is a bacterial infection caused by the bacterium Mycobacterium tuberculosis. TB is a serious infectious disease that primarily affects the lungs but can also impact other parts of the body. The most common way to test for TB is through a skin test, also known as the Mantoux test. This test can result in either a positive or negative TB skin test result, and the difference between these two results can have significant implications for an individual's health and treatment plan.
Positive TB Skin Test
A positive TB skin test result means that an individual has been exposed to the bacterium that causes TB. The test itself involves injecting a small amount of protein from the bacterium under the skin, and if an individual has been exposed to the bacterium, their immune system will react to the injection, causing a red bump to appear at the site of the injection. This bump is measured after a certain amount of time has passed, usually 48 to 72 hours after the injection. If the bump measures 5mm or larger, it is considered a positive result.
A positive TB skin test does not necessarily mean that an individual has active TB disease. It simply means that they have been exposed to the bacterium at some point in their life, and their immune system has responded to the injection. This could be due to past exposure, such as coming into contact with someone who has active TB disease, or it could be due to the Bacillus Calmette-Guérin (BCG) vaccine, which is used in some countries to prevent TB.
However, a positive TB skin test result does mean that an individual is at a higher risk of developing active TB disease in the future. This is because the bacterium can remain dormant in the body for years, and if the immune system becomes weakened, the bacterium can become active and cause disease. Therefore, individuals with a positive TB skin test result may need to undergo further testing, such as a chest x-ray, to determine if they have active TB disease.
Negative TB Skin Test
A negative TB skin test result means that an individual has not been exposed to the bacterium that causes TB, or that their immune system has not responded to the injection. A negative result is typically interpreted as a sign that an individual is not infected with TB, although it is important to note that the test is not 100% accurate.
A negative TB skin test result can be particularly reassuring for individuals who have been in close contact with someone who has active TB disease, as it means that they have not been infected. However, it is important to remember that the test is not foolproof, and if an individual develops symptoms of TB in the future, they should seek medical attention immediately.
Limitations − Unfortunately, there are some limitations to this test in certain situations. For instance, individuals who have HIV or are very ill may not react to the skin test in the way that is expected. This means that it is possible to have false negative test results in these situations.
Recommended action − If tuberculosis is still suspected based on symptoms then either the TB skin test can be repeated or further diagnostic testing can be done. Blood can be tested, sputum can be analyzed and chest X-rays can be done to further test if TB is present. If TB is not suspected due to the person being in a low-risk category and having no symptoms, then nothing further needs to be done. If symptoms do appear later a person can always be retested for TB at a later date.
Differences: Positive vs. Negative TB Skin Test
The following table highlights the major differences between Positive and Negative TB Skin Tests −
Characteristics |
Positive TB Skin Test |
Negative TB Skin Test |
|---|---|---|
Definition |
A positive TB skin test is when the Mantoux tuberculin skin test indicates that a person has the bacteria or has been exposed to M. tuberculosis. |
A negative TB skin test indicates that a person has not been exposed to M. tuberculosis. |
Raised area |
A raised area is always present in a positive skin test. |
This is not always the case with a negative skin test. |
Diameter of the raised area |
In a positive test, the raised area will be more than 5 or 10 mm in diameter in high-risk people, or more than 15 mm in low-risk people. |
In a negative test any raised area will be less than 5 mm in diameter. |
Limitations |
In a positive skin test, false positives can occur if one has had the BCG vaccine recently, and a test cannot indicate if an infection is active or dormant. |
In a negative skin test, false negatives can occur in people with HIV or who are very ill. |
Recommended action |
If a person has a positive TB skin test, then once the diagnosis is confirmed treatment with antibiotics should begin. |
If a person has a negative TB skin test but has symptoms, then further testing should be done. |
Conclusion
In conclusion, the difference between a positive and negative TB skin test result can have significant implications for an individual's health and treatment plan.
A positive result means that an individual has been exposed to the bacterium that causes TB and may be at risk of developing active disease in the future, while a negative result means that they have not been infected.
It is important for individuals to understand the meaning of their test results and to seek medical attention if they develop symptoms of TB or have concerns about their health.
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