What Does a Positive TB Skin Test Indicate?


Quick Answer

A positive TB test only indicates that someone has contracted this bacterial infection, according to the Centers for Disease Control and Prevention. The TB test cannot indicate whether it is inactive or active tuberculosis. To detect this, the patient needs further testing.

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What Does a Positive TB Skin Test Indicate?
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Full Answer

The tuberculin skin test involves the injection of a purified protein derivative into the arm. A skin reaction, such as a red lump or swelling, can occur approximately 48 to 72 hours after a TB test. The lump size will indicate an infection with the bacteria, states the National Institutes of Health's MedlinePlus.

However, TB skin tests are not infallible and can give either a false positive or false negative result in some cases, notes Mayo Clinic. A factor that can cause a false positive is if a person has recently received a TB vaccination. When there is a positive reaction, further testing is necessary to properly diagnose the infection. These tests can include a blood test, chest X-ray and sputum culture.

There are two forms of tuberculosis, inactive or active TB. Someone can have inactive TB for many years and may not have the disease, but there is the possibility that it can develop in the future. Treatment may be necessary to prevent the active form of TB from developing. Active tuberculosis also requires treatment because it is a contagious disease.

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