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What Does a Positive Tuberculosis Skin Test Look Like?

A positive tuberculosis (TB) skin test is a raised reddish welt on the forearm after a TB serum test. Whether the skin reaction is considered positive for diagnostic purposes depends on a number of factors including medical condition, prior exposure to TB, age or immigrant status.

  1. Features

    • Purified protein derivative (PPD) is injected just below the skin.

      A tuberculosis skin test features a health-care professional injecting a small amount of purified protein derivative (PPD) under the skin on your forearm. After the PPD has been injected, you wait 48 to 72 hours and then return to have a health-care professional look at the skin's reaction that may include a raised, reddish circle at the injection site.

    Size

    • A health-care professional will 'read' the reaction.

      According to the Centers for Disease Control and Prevention (CDC), a reaction of more than 5 mm is a positive reaction if you are in a high risk category. A reaction of between 5mm and 10mm is considered positive for some immigrants, drug users, children and other risk categories. A reaction of more than 15mm is considered positive in any person, even if there is not a known risk factor for TB.

    Significance

    • If the health- care professional determines that you have a positive skin test, she will recommend that you obtain a chest X-ray to determine whether you have latent or active tuberculosis.

    Misconceptions

    • Most people who have a positive TB test do not actually have tuberculosis. A positive skin test generally is an indication of latent TB, meaning you have been exposed to the virus, but have not developed the disease.

    Warning

    • If you have a positive tuberculosis skin test, you will be given a preventative treatment plan to ensure that you do not get the active disease. If you have active TB, you will receive treatment and should be careful not to expose those around you. You cannot expose others if you only have latent TB.

    Expert Insight

    • For more detailed information on risk factors and needed treatment, check the CDC website. Most local public health departments will provide testing and treatment options at low or no cost.

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