Tuberculosis (TB) testing screens for infection with the bacterium Mycobacterium tuberculosis. A common concern is whether the test itself can cause general sickness. TB tests generally do not lead to systemic illness because the testing materials are inert and cannot cause infection. However, a localized immune response at the injection site is a frequent and expected occurrence after a skin test.
How the TB Test Works
There are two primary methods used for screening for TB infection. The first is the Tuberculin Skin Test (TST), also known as the Mantoux test, which involves injecting a small amount of purified protein derivative (PPD) just beneath the skin’s surface. This solution is derived from the TB bacterium but contains no live or dead bacteria, making it incapable of causing the disease.
The second test is the Interferon Gamma Release Assay (IGRA), which is a blood test. The IGRA measures the release of interferon-gamma by white blood cells after exposure to specific TB antigens in a laboratory setting. Unlike the skin test, the IGRA requires a standard blood draw and is not subject to interference from prior BCG vaccination.
Expected Localized Reactions
Following a Tuberculin Skin Test, minor side effects are common and are an expected part of the process. The most frequent reactions are localized to the injection site and may include mild pain, discomfort, or itching. Patients often notice redness or a rash appearing at the site within 12 hours of the injection.
The immune system’s response to the PPD material typically causes a palpable, raised, hardened area, known as induration, which the healthcare worker measures 48 to 72 hours later to determine the result. This localized swelling is the body’s delayed-type hypersensitivity response to the injected protein. These site-specific symptoms are temporary and are not an indication of a systemic infection or generalized sickness.
Stronger, but still localized, reactions may occasionally occur in highly sensitized individuals, sometimes resulting in blistering or scabbing. The effects are generally confined to the area of the injection. These localized symptoms typically disappear within one to two weeks without intervention.
The Difference Between Local Reaction and Systemic Sickness
Anxiety regarding systemic sickness, such as developing a fever, chills, or body aches, stems from a misunderstanding of the test material. The Tuberculin Skin Test utilizes purified protein derivative (PPD), a fraction of protein taken from the Mycobacterium tuberculosis complex. Since this material is a protein extract and not a live organism, it is impossible to contract tuberculosis disease from the test itself. The test simply detects if the immune system has previously developed an awareness of these proteins.
If a person experiences generalized symptoms associated with sickness, such as nausea, headache, or malaise, these are generally not direct side effects of the TST. The test material is designed to elicit a localized, delayed-type hypersensitivity reaction, which is a cellular immune response confined to the skin. Therefore, the onset of flu-like symptoms following the test is most often coincidental, likely caused by a concurrent viral infection or another underlying condition. Historically, earlier forms of tuberculin testing caused severe systemic reactions, but the modern Mantoux test was developed to avoid this outcome by local application.
The TB blood test, or IGRA, involves a standard blood draw and carries no risk of localized reactions beyond minor bruising or discomfort at the venipuncture site. The test is not injecting a virus or live bacteria, meaning it cannot trigger the typical systemic symptoms of a contagious illness. Systemic reactions, if they occur, are extremely rare and are usually confined to a severe allergic response, not a general infection.
Recognizing Severe Adverse Events
While the TB test is generally safe, severe adverse reactions are extremely rare and require immediate medical attention. These reactions are typically allergic, such as anaphylaxis, which can be life-threatening. Warning signs include a rapid heart rate, severe hives or rash that spreads beyond the injection site, or swelling of the face, tongue, or throat.
Fainting or presyncope may occur immediately following the injection, though this is often related to the injection process rather than the PPD itself. Severe blistering or ulceration at the injection site should be reported to a healthcare provider promptly.