A tuberculosis (TB) test determines if an individual has been infected with the bacteria that cause tuberculosis. Healthcare professionals commonly use two main types: the tuberculin skin test, also known as the Mantoux test, and blood tests, referred to as interferon-gamma release assays (IGRAs). While mild, temporary reactions are common after these tests, the tests themselves do not introduce live bacteria, so they cannot cause TB disease. This article will clarify the typical reactions to expect, when to seek medical attention for unusual symptoms, and how these tests detect an immune response rather than causing infection.
Typical Reactions to a TB Test
The tuberculin skin test involves injecting a small amount of purified protein derivative (PPD) from the TB bacteria under the skin. This injection creates a small bump or wheal that usually disappears within a few hours. Individuals might observe common local reactions at the injection site, such as redness, swelling, or a firm, raised bump. Itching and tenderness are also common. These localized symptoms are not indicators of illness but a normal immune response, showing the body’s reaction to the injected proteins.
For blood tests, reactions are generally even milder. Individuals might experience minor bruising or slight soreness at the blood draw site. These typical reactions, whether from a skin test or a blood test, are usually short-lived. They tend to resolve within a few days to a week or two, as the body’s temporary response to the procedure.
When to Seek Medical Attention
While most reactions to a TB test are mild and temporary, certain signs warrant medical attention. Though rare, some individuals might experience a severe allergic reaction to the components used in the test. Symptoms can include hives, a widespread rash, difficulty breathing, or swelling of the face or throat. These symptoms indicate an urgent need for medical evaluation.
Signs of a localized infection at the injection site are concerning, as these are not typical reactions. These may include pus, increasing pain, warmth spreading around the injection area, or redness that extends significantly beyond the expected reaction site. If systemic symptoms like a persistent fever, unusual body aches, or general malaise develop after the test, consult a healthcare provider. These uncommon symptoms require professional assessment to rule out other conditions or complications.
Understanding TB Testing and Infection
A common concern is whether a TB test can transmit the disease, but this is a misconception. Both skin and blood tests do not introduce live Mycobacterium tuberculosis bacteria. Therefore, these tests cannot cause a tuberculosis infection or active TB disease.
Instead, these tests detect the body’s immune response to specific, non-living components or antigens derived from TB bacteria. For instance, the skin test uses a purified protein derivative, while blood tests measure how immune cells react to certain TB proteins. A positive test result indicates the immune system has recognized these bacterial components, suggesting a past exposure or current latent infection. Latent TB infection means bacteria are inactive, causing no symptoms and not being contagious; active TB disease involves symptoms and can be spread. The TB test serves as a screening tool for infection, and a positive result typically necessitates further diagnostic steps, such as a chest X-ray or sputum analysis, to determine if active disease is present.