Difference Between Positive TB Skin Test and Negative TB Skin Test (With Table)

Tuberculosis is among the most dangerous diseases in the world. One-fourth of the world’s population has been impacted by this disease. It is transmissible and a matter of utmost concern. Often referred to as TB, it is an infection that is caused by bacteria and is transferred through the air.

TB skin test stands for Tuberculosis skin test. It is also known as the Purified Protein Derivative (PPD) test or Tuberculin test. This test discovers whether a person has evolved a resistant reaction to the bacterium that seeds Tuberculosis or not.

Since the bacteria are sleeping, the people who are infected with Tuberculosis, might not even have any symptoms of the disease.

Positive TB Skin Test vs Negative TB Skin Test

The difference between a positive TB skin test and a negative TB skin test is the presence of TB causing bacteria. If tuberculosis causing bacteria are present, it is a positive TB skin test and if tuberculosis causing bacteria are absent, it is a negative TB skin test.

 

Comparison Table Between Positive TB Skin Test and Negative TB Skin Test

Parameter of ComparisonPositive TB Skin TestNegative TB Skin Test
DefinitionIt is a positive TB skin test when a person has been exposed to tuberculosis causing bacteria.It is a negative TB skin test when a person has not been exposed to tuberculosis causing bacteria.
Raised areaIf the TB skin test is positive, there will be a raised area.If the TB skin test is negative, there might not be a raised area.
Raised area diameterThe raised area will be 6mm-10mm in diameter in a positive skin test.The raised area will be less than 5mm in a negative skin test.
False resultsA person may have false positives if he had taken the BCG vaccine earlier.A person may have false negatives if he is very sick or diagnosed with HIV.
RecommendationOnce the result of a positive skin test has been confirmed, treatment should begin.If a person has symptoms but the result is negative, testing should be done again.

 

What is Positive TB Skin Test?

When Tuberculosis causing bacteria is present, it is a positive TB skin test.  The result doesn’t state if the diseased person has a latent infection, which is often referred to as LTBI or has active TB. Further testing is required to indicate that.

A liquid named Tuberculin is put into the forearm’s skin for this test. A health professional, after 2-3 days checks that part for a raised area that can show the presence of TB bacteria.

If the bacteria are present, the skin where the liquid is put in will start to harden and swell. It might become red as well.

It is the size of the raised area that decides the results. If the diameter of the raised area is less than 5mm, the person is negative for TB. If its size is at least 5mm, it is a positive TB result and it may signify that the infected person had TB previously or was HIV-positive, or he had an organ transplant or he had been in contact with someone who had TB.

If the size of the bump is at least 10mm, it may signify that the infected person is a child under four, or the person was in an environment that was at high risks such as a medical laboratory or hospital. The size of the bump can also be up to 15mm, which is also a positive TB test.

 

What is Negative TB Skin Test?

It is a negative TB skin test when a person has not been exposed to tuberculosis causing bacteria. In such a case, there might not be a raised area, or if there is one, its diameter will be less than 5mm.

Sometimes, there are cases when a person is infected with TB, but the result is negative. Directing a test incorrectly or wrongly interpreting the result may head us to a false negative TB test.

For children, it might show a negative TB result even if they are infected with TB.  An organ transplant or certain conditions of the immune system may also indicate a false negative test.  A person may not test positive for the test if he has been infected with TB in the past few days.

If a person has symptoms but the result is negative, he should get himself tested again. He can either go for a second skin test or a blood test also.


Main Differences Between Positive TB Skin Test and Negative TB Skin Test

  1. If tuberculosis causing bacteria are present, it is a positive TB skin test and if tuberculosis causing bacteria are absent, it is a negative TB skin test.
  2. In the case of a positive TB skin test, there will be a raised area also known as a bump in the injected area, which is not the case in a negative TB skin test.
  3. If the size of the bump is less than 5mm, it might be a negative TB skin test and if its diameter is more than 5mm, it is a positive TB test.
  4. The result might be a false positive if the person had taken the BCG vaccine earlier.
  5. A person may have false negatives due to various reasons such as being very sick or
  6. being diagnosed with HIV.
  7. Once the result of a positive skin test is confirmed, treatment should begin.
  8. If the result is negative but the person has symptoms, testing should be done again.

 

Conclusion

Tuberculosis can be detected with the help of two kinds of tests. One is the TB skin test and the other is the TB blood test. The result doesn’t state if the diseased person has a latent infection, or has active TB.

Further testing is required to indicate that. Lab tests, chest X-rays, physical examination, and medical history are other tests that help determine TB disease.

Tuberculosis is curable. One should take all the medicines as advised by the doctor and stick to the doctor’s recommendations in order to recover fully.


References

  1. https://www.sciencedirect.com/science/article/pii/S2213260016304362
  2. https://ajph.aphapublications.org/doi/abs/10.2105/AJPH.88.5.792
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