The Tuberculin Skin Test (TST), also known as the Mantoux test, is a common procedure used to check if a person has been exposed to the bacteria that cause tuberculosis (TB). A tiny amount of testing fluid is injected just under the skin of the forearm to see if the immune system reacts. You can shower immediately after a TB test, but you must take specific precautions to ensure the accuracy of the result.
Caring for the Injection Site: Hygiene and Activity
The primary goal of post-test care is to keep the injection site clean and dry while avoiding any irritation or physical alteration that could compromise the results. Since the test fluid is injected into the dermal layer, external water exposure will not wash it away or interfere with the biological reaction. Therefore, bathing or showering is completely safe, provided you handle the area with care.
When washing, use mild soap and allow the water to run gently over the test site on your forearm. Actively avoid scrubbing, rubbing, or vigorously wiping the area with a washcloth or loofah. Excessive friction can cause redness and irritation that might be mistaken for a positive immune response when the test is read.
After showering, pat the site dry with a clean, soft towel rather than rubbing it. Furthermore, do not apply any products directly to the test area, including lotions, creams, or perfumes, for the entire waiting period. Resist the temptation to cover the small puncture site with a bandage or adhesive dressing.
You should also limit activities that could lead to physical irritation or excessive moisture at the injection site. This includes avoiding vigorous exercise or heavy lifting that could cause significant sweating or friction from clothing over the forearm. The reaction the test measures is a specific delayed-type hypersensitivity, and physical trauma, such as scratching or rubbing, can create a false inflammatory response. If the site begins to itch, a cool compress or an ice cube wrapped in a thin cloth can be applied to alleviate the discomfort.
What to Expect When the Test is Read
After the tuberculin fluid is injected, the immune response is time-dependent. The test must be read by a healthcare professional between 48 and 72 hours later. Missing this window means the reaction may no longer be reliable, and the test will need to be repeated.
When the clinician examines the site, they are not looking at general redness or bruising, which are common and inconsequential reactions. They are specifically measuring a firm, raised swelling called induration. The measurement is taken across the diameter of this hardened area and is expressed in millimeters.
A positive result, indicated by an induration of a specific size, suggests exposure to the TB bacteria at some point. However, a positive TST does not mean you have active tuberculosis disease; it only indicates the presence of the bacteria, which could be a latent infection. The size of the induration considered positive varies based on individual risk factors, such as immune status or recent exposure history.
If the result is positive, further testing, such as a chest X-ray or a blood test, will be required to determine if you have a latent infection or active disease. A negative result, where the induration is below the threshold, typically means no prior exposure, though a repeat test may be recommended if recent exposure is suspected.