The Tuberculin Skin Test (TST), often called the PPD test, screens for latent tuberculosis (TB) infection. It determines if the immune system has encountered the Mycobacterium tuberculosis bacteria, but it does not diagnose active TB disease. The test involves injecting a small amount of purified protein derivative (PPD) solution. The immediate physical reaction at the injection site is crucial for confirming the test’s technical success.
The Expected Immediate Result
The TST is performed using a precise method known as an intradermal injection, meaning the PPD solution must be delivered into the dermis, the layer of skin just beneath the surface epidermis. This shallow placement is a requirement for the test to be reliable. The intended and immediate result of a successful intradermal injection is the formation of a distinct, pale, raised area called a wheal.
This wheal is the “bubble” you are referring to, and its appearance confirms that the 0.1 milliliter dose of PPD solution was correctly placed within the dermis. A properly formed wheal should measure approximately 6 to 10 millimeters in diameter and signifies the correct dosage has been delivered to the appropriate skin layer. Although the wheal is visible immediately, it is temporary and will be absorbed by the body within a few hours. The presence of this initial wheal is strictly a marker of correct administration technique, not an indication of a positive or negative result.
Causes for the Absence of the Wheal
The absence of the expected wheal indicates a technical failure in the administration of the PPD solution. The most common reason for this failure is that the injection was administered too deeply. Instead of placing the solution into the dermis, the needle may have gone into the subcutaneous tissue below, preventing the characteristic raised blister from forming.
This error often occurs if the needle insertion angle is too steep, rather than the shallow, almost parallel angle required for an intradermal injection. When the solution is delivered to the deeper tissue, it disperses differently, and the test becomes unreliable because the PPD material is not in the cellular layer necessary to trigger the delayed hypersensitivity reaction.
Another cause for the missing wheal is the accidental leakage or reflux of the PPD solution back out of the injection site. This can happen if the needle is removed too quickly or if pressure is applied immediately after the injection.
When Does the Test Need to Be Redone?
If the immediate wheal does not form, the TST is considered invalid and must be repeated. The entire purpose of the wheal is to validate that the precise dose of PPD was delivered to the correct skin depth. Without this initial confirmation, the healthcare provider cannot be certain that the test was administered in a manner that will produce a reliable final result.
A failed injection cannot be trusted to yield a dependable measurement of induration, which is the actual biological reaction read 48 to 72 hours later. The test must be immediately readministered at a new site, typically on the opposite forearm or at least 5 centimeters away from the original injection location. This necessary retesting ensures that the patient receives a technically correct procedure, which is the only way to obtain a trustworthy result for screening latent TB infection.