A tuberculosis (TB) test is a screening tool used to determine if a person has been infected with the bacteria that causes tuberculosis, Mycobacterium tuberculosis. This bacteria can exist in a latent (inactive) state without causing symptoms or in an active state that causes illness and is contagious. Understanding the options and logistics of the testing process simplifies the steps required to fulfill employment, travel, or health screening obligations.
Determining Test Requirements and Method
The need for a TB test often arises from a requirement set by an institution, such as an employer, school, or volunteer organization. For instance, people who have been in close contact with an individual diagnosed with active TB, or those who work in high-risk settings like hospitals or correctional facilities, require screening. The specific test method is typically dictated by the entity requiring the results or recommended by a healthcare provider based on the patient’s medical history.
There are two primary methods for screening for TB infection: the Tuberculin Skin Test (TST), also known as the Mantoux or PPD test, and the blood test, called an Interferon-Gamma Release Assay (IGRA). The TST involves injecting a small amount of purified protein derivative (PPD) under the skin and measuring the body’s immune response days later. The IGRA is a single blood draw that measures the release of interferon-gamma by white blood cells in response to TB-specific antigens.
The IGRA is often the preferred method for individuals who have received the Bacille Calmette-Guérin (BCG) vaccine. This is because the BCG vaccine can cause a false-positive result on the TST, but it does not affect the accuracy of the IGRA. Consulting with the entity requiring the test about their preferred or accepted method can prevent the need for retesting later.
Locating and Scheduling the Appointment
Once the required test type is determined, the next step is finding a convenient location and setting the appointment. Many facilities offer TB testing, including primary care physician offices, local health departments, and occupational health clinics. Retail health clinics and urgent care centers also frequently provide the service, often with flexible hours.
When contacting a facility to schedule, have certain information ready to ensure a smooth process. This includes the reason for the test, any required form from the requesting organization, and your insurance details. Also, confirm if they offer the specific test type you need, as not all locations administer both the TST and the IGRA.
The scheduling logistics will differ depending on the chosen method. An IGRA is a single appointment for a blood draw, similar to any routine lab work, and the sample is sent off for analysis. In contrast, the TST requires a two-part visit: the initial appointment for the injection and a mandatory follow-up appointment for the reading.
Follow-up Readings
The Tuberculin Skin Test (TST) involves a requirement that must be factored into the initial scheduling. The purified protein derivative (PPD) solution injected during the first visit must be assessed by a trained healthcare professional between 48 and 72 hours later. This window is not flexible because the body’s immune reaction, which is the basis of the test, is only reliably measurable during this specific timeframe.
The follow-up visit is necessary for the clinician to measure the size of any induration, which is the firm, raised swelling at the injection site. If a patient misses the 72-hour deadline, the test is considered invalid, and the entire procedure must be repeated. Therefore, when scheduling the initial injection, patients must confirm their availability for the reading exactly two to three days later.
This is why some facilities or employers may prefer the IGRA blood test, which avoids the need for this mandatory return visit. Planning for the reading should be the first consideration after finding a testing location.
What to Expect During the Testing Process
The physical experience of receiving a TST is quick and involves an intradermal injection, typically on the inner forearm. The healthcare provider injects a small volume of the PPD solution, creating a slight, pale, raised bump called a wheal that quickly disappears. Patients are advised not to scratch, rub, or cover the injection site with a bandage, as this could interfere with the test result.
For the IGRA blood test, the procedure is identical to a standard venipuncture, where blood is drawn from a vein in the arm. The collected blood is then placed into specialized tubes that contain the TB-specific antigens to stimulate the immune response in the laboratory. There is no special aftercare required for the IGRA, and the patient may immediately resume normal activities.
The timeline for receiving results differs between the two tests. For the TST, the result is considered immediate upon the completion of the reading during the second appointment. IGRA results require laboratory processing and typically take a few days to become available. Regardless of the method, a positive result indicates the presence of TB bacteria, which will prompt the healthcare provider to order further evaluations, such as a chest X-ray.