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Short answer: The tuberculin skin test (TST) and interferon-gamma release assay (IGRA) look for an immune response associated with Mycobacterium tuberculosis infection. A chest X-ray does something different: it looks for lung changes that may be consistent with pulmonary tuberculosis (TB) disease. None of these tests, used alone, can reliably tell whether a person has inactive (latent) TB infection or active TB disease.
A positive TST or IGRA requires clinical follow-up. A negative result makes infection less likely but does not rule out TB disease when symptoms or other risk factors are present. Testing choice and interpretation belong with a health care professional; U.S. Centers for Disease Control and Prevention (CDC) guidance is general and local screening rules can differ.
What each test is designed to answer
TB screening is easier to understand when the tests are separated by role. TST and IGRA answer an infection question: has the immune system shown evidence of recognizing TB-related material? Chest radiography answers an anatomic question: are there chest findings that could fit pulmonary TB disease or another condition?
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- IGRA: A blood sample is exposed in the laboratory to synthetic peptides that simulate TB antigens. The laboratory measures interferon-gamma released by white blood cells.
- Chest X-ray: A radiograph images the chest and may reveal abnormalities that prompt further evaluation for pulmonary TB.
Because the first two tests measure immune memory rather than disease activity, a positive result does not by itself establish active, contagious, or pulmonary TB.
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How the tuberculin skin test (TST) works
Injection and delayed reaction
A clinician or other trained health worker injects standardized PPD just under the skin. If the person’s immune system recognizes TB-related proteins, a delayed immune reaction can develop at the injection site. The reaction is assessed during a later visit rather than immediately after the injection.
Why the reading visit matters
The result is based on the diameter of induration, measured in millimeters, not simply on redness. A trained person must inspect and measure the site during the clinic’s specified reading window. Missing that visit can leave the test without an interpretable result and may require the clinician to choose another testing approach.
“Reliable administration and reading of the TB skin test requires standardization of procedures, training, supervision, and practice.” — Centers for Disease Control and Prevention, Clinical Testing Guidance for Tuberculosis: Tuberculin Skin Test, January 31, 2025.
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Risk-based cutoffs
There is no single universal number that defines a positive TST. CDC guidance uses different thresholds—5, 10, or 15 millimeters—depending on the person’s likelihood of TB exposure or risk of progressing to disease. The same measured induration can therefore be interpreted differently in different risk groups. A clinician combines the measurement with exposure history, medical conditions, and other risk information.
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What can affect a TST
Previous Bacille Calmette-Guérin (BCG) vaccination can cause a false-positive TST. This is one reason a clinician may favor a blood test for someone who received BCG. A TST also depends on the person returning for a properly timed reading, so follow-up reliability is part of test selection.
How an IGRA blood test works
Laboratory measurement
For an IGRA, blood is drawn and processed with controls and synthetic peptides representing TB antigens. In most people with TB infection, white blood cells recognize those antigens and release interferon-gamma. The laboratory measures that response and reports the result according to the test’s validated procedures.
Practical advantages
IGRA requires one blood-draw visit and does not require a return appointment for a skin-test reading. CDC encourages blood tests in most situations and particularly favors them for people who received BCG or who may not be able to return for a TST reading. The sample still has to be collected, handled, and processed correctly by a clinical service; an IGRA is not a home self-test.
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BCG and IGRA
CDC states that BCG vaccination does not affect TB blood-test results. That makes IGRA useful when a TST could be difficult to interpret because of BCG-related cross-reaction. The choice can also depend on laboratory access, cost, age, and the clinician’s assessment.
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What a chest X-ray contributes
Imaging is not an infection test
A chest radiograph creates an image of the lungs and surrounding chest structures. It may show abnormalities that are consistent with pulmonary TB, but an abnormal image does not definitively diagnose TB disease. Other illnesses can produce similar findings, and some people with TB may have an image that is not diagnostic.
When clinicians order it
Chest X-ray is commonly part of the evaluation after a positive TST or IGRA, and it may be ordered sooner when symptoms or the clinical history raise concern. In a person with a positive infection test and no symptoms, a normal chest radiograph can generally help rule out pulmonary TB according to CDC clinical guidance. It does not, however, answer every question about TB outside the lungs or replace microbiologic testing when disease is suspected.
TST, IGRA, and chest X-ray compared
| Comparison point | TST | IGRA | Chest X-ray |
|---|---|---|---|
| Main role | Detects an immune response associated with TB infection | Detects an immune response associated with TB infection | Looks for chest abnormalities that may indicate pulmonary disease |
| What happens | PPD is injected under the skin, followed by a trained reading | Blood draw followed by laboratory processing | Chest imaging performed as part of clinical evaluation |
| Return visit for test reading | Yes; the site must be read by a trained person within the clinic’s specified window | No return visit for a skin-test reading | Not applicable |
| Effect of BCG vaccination | BCG may cause a false-positive reaction | Not affected by BCG, according to CDC guidance | BCG does not change the imaging role |
| What it cannot establish alone | Cannot distinguish latent infection from active disease | Cannot distinguish latent infection from active disease | An abnormality does not definitively diagnose TB disease |
The tests are complementary rather than interchangeable. TST and IGRA provide evidence about infection; imaging and, when needed, bacteriologic studies help determine whether disease is present.
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How clinicians choose between TST and IGRA
When IGRA is often preferred
- The person received BCG vaccination, because BCG can produce a false-positive TST but does not affect IGRA results.
- The person may not return for a skin-test reading.
- A blood-draw visit and laboratory processing are practical in the local health system.
- The clinician wants to follow CDC’s general preference for blood testing in most circumstances.
When TST remains appropriate
- IGRA is unavailable, too costly, or logistically burdensome.
- The clinic can administer and read the test using standardized procedures.
- The patient is a child younger than five; CDC guidance recommends TST for this age group.
These are general U.S. recommendations, not a self-selection checklist. A clinician also considers age, immune status, prior results, exposure setting, likelihood of returning, and local requirements.
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What happens after a positive TST or IGRA
A positive result means infection is likely
A positive TST or IGRA usually indicates TB infection. It does not reveal whether the infection is inactive (latent) or whether active TB disease is developing. The next step is a medical evaluation, not an assumption that disease is present or absent.
The evaluation for TB disease
CDC describes a complete evaluation as including medical history, physical examination, an infection test, chest radiograph, and bacteriologic examination when indicated. Bacteriologic work may include sputum studies. Culture is the microbiologic gold standard for diagnosing TB disease. The clinician may also assess exposure timing, previous treatment, immune-suppressing conditions, and symptoms.
Why follow-up matters even without symptoms
Someone with a positive infection test and no evidence of disease may have latent TB infection. Latent infection is not the same as active disease, but treatment can prevent progression. CDC states that treatment for latent TB infection is 90% effective at preventing development of TB disease. That figure describes treatment effectiveness, not the accuracy of TST or IGRA.
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Symptoms can change the priority
A negative TST or IGRA makes TB infection less likely, but it does not exclude TB disease in every circumstance. If a person has symptoms or a concerning clinical history, CDC advises clinicians not to wait for the infection-test result before beginning other diagnostic testing.
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Symptoms require professional assessment
Persistent cough, coughing blood, fever, night sweats, unexplained weight loss, chest pain, or unusual fatigue can have many causes. Their presence does not prove TB, but it does mean a negative screening result should not be treated as a complete rule-out. The clinician may proceed with examination, chest imaging, and sputum or other bacteriologic studies while the TST or IGRA is pending or after a negative result.
People with a previous positive test
People with a documented previous positive TST or IGRA, or a history of treatment for TB infection, generally should not simply repeat the same screening test. CDC notes that most people remain positive and that additional testing probably will not contribute to care. Instead, the clinician reviews the prior documentation, symptoms, treatment history, and any new exposure or disease concern.
A practical decision path for a screening appointment
- Explain your history. Tell the clinician about known TB exposure, prior positive tests, previous treatment, BCG vaccination, immune-suppressing conditions, and current symptoms.
- Confirm the purpose of testing. Employment, school, immigration, contact investigation, and symptom evaluation can have different requirements.
- Choose the available infection test with the clinician. Discuss whether IGRA or TST is better suited to your age, BCG history, ability to return, cost, and local access.
- Complete the required follow-up. Return for a TST reading when instructed, or confirm that an IGRA sample was collected and processed by the laboratory.
- Follow through on a positive or concerning result. Expect a history and examination and, when appropriate, chest radiography and bacteriologic testing.
- Report symptoms promptly. Do not rely on a negative screening result to dismiss symptoms that could indicate TB disease.
Common misunderstandings and how to avoid them
- “A positive skin test proves active TB.” It shows an immune response associated with infection; it cannot separate latent infection from active disease.
- “Redness is the TST result.” The described measurement is induration in millimeters, interpreted in relation to risk.
- “A chest X-ray detects TB infection.” It looks for chest abnormalities and cannot replace an infection test.
- “A normal X-ray proves there is no TB anywhere in the body.” It mainly informs assessment of pulmonary disease and does not settle every form of TB.
- “A negative test always rules out TB.” Symptoms and clinical judgment can require additional testing despite a negative infection screen.
- “I can repeat a test after every screening request.” Repeating TST or IGRA after a documented positive result usually adds little and should be discussed with a clinician.
What these tests can—and cannot—tell you
TST and IGRA are tools for identifying immune evidence of TB infection. Chest X-ray is a tool for looking for pulmonary abnormalities. The diagnosis of active TB is a clinical and microbiologic determination that may require several pieces of evidence. No consumer product can substitute for trained TST administration and reading, laboratory IGRA processing, or clinical radiography.
If you are being screened for a program or workplace, follow that program’s instructions and bring records of any previous TB testing or treatment. If you have symptoms suggestive of TB, contact a health care provider promptly and explain the symptoms even if a recent screening test was negative.
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