A tuberculosis (TB) test, also known as a Mantoux test or tuberculin skin test (TST), diagnoses infection with Mycobacterium tuberculosis. This test involves introducing a small solution into the skin. While it indicates exposure, it does not distinguish between latent or active infection, requiring further evaluation if positive. Accurate administration is important for reliable outcomes.
The Specific Needle Used
The tuberculin skin test uses a very fine and short needle for accurate solution delivery. Healthcare professionals typically use a disposable 27-gauge needle; higher gauge numbers indicate a thinner needle. These needles are generally 1/4 to 1/2 inch long.
The needle attaches to a specialized tuberculin syringe, designed for administering small, precise fluid volumes. These syringes commonly hold 0.5 mL to 1 mL and have clear, distinct markings to measure the 0.1 mL of tuberculin purified protein derivative (PPD) solution. This combination allows for the controlled injection needed for this diagnostic procedure.
Why Needle Size Matters for TB Tests
Needle size matters because the TB test requires an intradermal injection, delivering the solution precisely into the dermis, the skin’s upper layer. This thin layer requires a small, short needle to keep the solution within this intended space. Injecting too deeply, such as into subcutaneous tissue or muscle, can lead to inaccurate results.
Proper intradermal injection forms a small, pale, raised bump known as a “wheal” or “bleb” at the site. This wheal, typically 6 to 10 mm in diameter, confirms correct placement within the dermis. Its formation allows the immune response to occur in the correct skin layer for test interpretation. Without a properly formed wheal, the test may yield a false negative result, potentially delaying necessary medical evaluation.
Administering the TB Test
Administering the TB test requires careful technique. The injection site is typically the inner surface of the forearm, about 2 to 4 inches below the elbow, in an area free from scars, sores, or prominent veins. Cleaning the site with an alcohol swab and allowing it to dry prepares the skin for injection.
The needle is inserted slowly, with the bevel (the slanted tip) facing upward, at a shallow angle of 5 to 15 degrees to the skin. This low angle helps ensure the needle penetrates only the topmost layers, allowing the entire bevel to be covered but remaining visible just beneath the skin’s surface. A slow and steady injection of the 0.1 mL of tuberculin solution then creates the characteristic wheal. Proper technique, combined with the specialized needle and syringe, ensures the accuracy and reliability of the TB skin test.