The Tuberculosis (TB) skin test screens for infection with TB bacteria. It identifies latent TB infection, where bacteria are present but not causing active disease symptoms. Detecting latent TB allows for preventive treatment, reducing the risk of progression to active TB disease and its spread.
Purpose and Proper Administration of the TB Skin Test
The TB skin test, also known as the Mantoux tuberculin skin test or PPD test, involves injecting a small amount of purified protein derivative (PPD) into the skin. This PPD is derived from proteins of the Mycobacterium tuberculosis bacterium. The test aims to elicit a localized immune response if the person has been previously exposed to TB bacteria.
For the test to be effective, the injection must be administered correctly as an intradermal injection. This means the PPD solution is delivered precisely into the superficial layer of the skin, between the epidermis and dermis. A proper intradermal injection will create a distinct, pale, raised blister called a wheal. The formation of this wheal confirms that the PPD has been delivered to the correct skin layer, which is essential for an accurate test result.
Immediate Physical Signs of a Deep Injection
When a TB skin test is injected too deeply, immediate physical signs may be observed. The characteristic wheal may be absent or very flat and indistinct, indicating the PPD solution was not deposited in the intended superficial skin layer.
More bleeding than typically seen with a correct intradermal injection may occur. The injection might also feel more painful or cause a stinging sensation. In some cases, a small bruise may develop around the injection site due to increased trauma to the underlying tissues.
Consequences for Test Accuracy and Interpretation
A deep injection significantly affects the accuracy and interpretation of the TB skin test. The PPD must be precisely in the superficial dermal layer for the immune system to adequately recognize it and mount a measurable reaction. If the PPD is injected too deeply, it disperses into subcutaneous tissue, where the immune response is not effectively localized.
Consequently, a deeply injected test is considered invalid. This improper placement carries a high risk of a false negative result, meaning an individual who is truly infected with TB bacteria might test negative. The immune response, if any, may be too diffuse or insufficient to trigger a visible and measurable reaction. Even if some reaction occurs, the hardening (induration) will be difficult or impossible to measure accurately, rendering the test uninterpretable.
Recommended Actions After a Suspected Deep Injection
If a TB skin test is suspected to be injected too deeply, the individual should immediately inform the healthcare provider. It is important not to rub, press, or manipulate the injection site, as this could further interfere with the test’s integrity.
A repeat test will be necessary to obtain a valid result. This subsequent test is typically administered on a different site to ensure an accurate result. While a deep injection invalidates the test, it generally does not pose a significant health risk or cause serious complications. The primary concern is the unreliability of the test result itself, which necessitates retesting to properly assess for TB infection.