How to Take a TB Test: TST vs IGRA Procedures and Results
Tuberculosis testing refers to two common screening methods: the tuberculin skin test and the blood-based interferon gamma release assay. Both aim to detect immune response to Mycobacterium tuberculosis. This overview explains why each test is used, who typically gets screened, how each procedure works step by step, how results are reported, and what workplaces and clinics usually record.
Why tuberculosis screening is ordered
Screening looks for evidence of infection rather than active disease. Employers, healthcare settings, and public health programs use screening to identify people who may need further evaluation or preventive treatment. Screening is common for new employees in hospitals, people with recent exposure to a known case, and travelers from areas with higher tuberculosis rates. The goal is to find latent infection before it progresses to contagious illness.
Main test types and what they measure
Two tests are used most often. The skin method places a small amount of purified protein under the skin and measures local immune response. The blood method measures immune signaling after blood is mixed with specific tuberculosis antigens in a laboratory. Both detect immune memory to tuberculosis bacteria, not the bacteria itself.
| Feature | Tuberculin skin test | Blood-based assay |
|---|---|---|
| Sample | Skin injection | Venous blood draw |
| Where results appear | Induration measured on arm at 48–72 hours | Laboratory report, usually 24–72 hours |
| Typical advantages | Low-cost, long clinical history | Single visit, less cross-reaction with prior vaccine |
| Common limitations | Requires return visit; prior vaccine can affect | Needs lab infrastructure; possible indeterminate results |
Who should be tested and eligibility considerations
Testing is recommended for people with known exposure, those entering high-risk jobs, people with medical conditions that raise progression risk, and those arriving from areas with higher prevalence. Occupational health programs may require baseline testing for new hires and periodic screening for staff with ongoing exposure. Testing choices consider age, vaccination history, immune status, and the reason for screening.
Step-by-step: tuberculin skin test (TST)
The skin method begins with a trained clinician injecting a small volume just under the skin of the forearm. The injection produces a pale raised area if placed correctly. The person returns in 48 to 72 hours for measurement. A clinician or qualified reader measures the width of any firm swelling, not redness, and records the measurement in millimeters. The record typically includes date of placement, date of reading, site, and millimeter measurement.
Step-by-step: blood-based assay (IGRA)
The blood method requires a standard venous blood draw at a clinic. The sample is sent to a laboratory that mixes the blood with specific tuberculosis antigens and measures released immune signaling. Results are reported as positive, negative, or indeterminate, and the laboratory report includes assay name and date. No return visit for reading is needed, but some clinics ask for follow-up if the result is indeterminate.
Pre-test preparation and practical contraindications
No special fasting or diet is required for either test. For the skin method, avoid placing the injection over a scar or rash on the chosen arm; a different site or method may be needed if the skin is broken. People with a history of severe reaction to a prior skin test or with conditions that affect the immune response often have the blood test instead. Recent live-virus vaccination or recent infection can affect immune responses; scheduling and test choice may be adjusted for timing considerations.
How results are read and reported
For the skin method, clinicians measure the width of induration in millimeters. Thresholds for a positive reading vary by risk group and clinical guidelines. For the blood method, the lab report gives an interpretation and numerical value when available. An indeterminate blood result means the test could not provide a clear answer; a repeat test or alternative method is often considered. Reports used for occupational records usually list the test type, date, and interpretation.
Follow-up actions after positive or indeterminate results
A positive screening test leads to evaluation for active disease before preventive steps. That evaluation often includes symptom review, a chest X-ray, and, if symptoms or imaging suggest active disease, microbiologic tests on respiratory samples. For an indeterminate blood test, clinicians may repeat the blood assay, use the skin method, or review immune status. Workplace programs commonly require documented evaluation and treatment plans where indicated.
Timing, documentation, and workplace requirements
Timing matters for accurate interpretation. The skin test must be read within the recommended 48–72 hour window. A baseline blood assay can be done at a single visit. Employers and clinics typically document the test type, dates, site of skin placement, measurement in millimeters for skin testing, laboratory name for blood testing, and interpretation. Serial testing programs note conversion versus boosting and may prefer the blood method when frequent retesting is expected.
Trade-offs and practical considerations
Both methods detect immune response, not active bacteria. Prior vaccination with BCG can cause false-positive skin results in some people but less often affects the blood assay. Immunosuppression and recent exposure can cause false-negative results for either method. The skin test needs a return visit, which can be a scheduling barrier. The blood assay needs timely lab processing and may return indeterminate results, especially in people with weakened immune systems. Clinics balance access, turnaround time, and the specifics of the population when choosing a default test.
How long does a TB test take?
What is IGRA test turnaround time?
Where are TB screening clinic locations?
Screening choices come down to context: who is being tested, why they are being tested, and what the clinic or employer can support in terms of timing and documentation. The skin test remains common where low cost and long history matter. The blood assay is often chosen to avoid repeat visits and to reduce effects from prior vaccination. Both require careful recording and follow-up when results are positive or unclear.
This article provides general information only and is not medical advice, diagnosis, or treatment. Health decisions should be made with qualified medical professionals who understand individual medical history and circumstances.
