Tuberculin skin tests (TSTs) and interferon-gamma release assays (IGRAs) detect immune evidence of tuberculosis infection. A chest X-ray does something different: it looks for lung abnormalities that could indicate pulmonary TB disease. No one of these tests, by itself, proves that a person has active TB disease. A positive TST or IGRA starts a clinical evaluation, while symptoms can require evaluation even when an infection test is negative.
This guide follows current general U.S. guidance from the Centers for Disease Control and Prevention (CDC), including pages dated January 31, 2025, and June 17, 2024. A clinician or public-health service must choose the test, perform it, and interpret the result; local screening rules can differ.
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The three tests have different jobs
| Test | What it measures or shows | What happens | Main limitation |
|---|---|---|---|
| TST (Mantoux skin test) | An immune reaction to purified protein derivative (PPD) associated with TB infection | PPD is injected just under the skin, then a trained reader measures the reaction at a return visit | It does not distinguish latent (inactive) infection from active disease |
| IGRA (TB blood test) | Interferon-gamma released by white blood cells exposed to TB-related synthetic peptides | Blood is drawn, mixed with controls and TB antigens, and processed by a laboratory | It does not distinguish latent infection from active disease |
| Chest X-ray | Chest abnormalities that may be consistent with pulmonary TB | A radiograph is taken as part of a clinical evaluation | An abnormal image is not definitive proof of TB, and the image is not an infection test |
The TST and IGRA answer the question, “Is there evidence of TB infection?” The chest radiograph helps answer a later question: “Could disease be affecting the lungs?” That distinction determines what happens after screening.
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Injection and delayed reaction
A health professional injects a standardized amount of purified protein derivative (PPD) into the skin, usually creating a small raised area. If the immune system recognizes TB-related proteins, a delayed immune response can develop over the next couple of days. The injection itself is not a test for active disease and does not contain live TB bacteria.
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Why the return visit matters
The person must return within the clinic’s specified reading window so a trained health worker can examine the site. The reader measures induration—the firm, raised swelling—in millimeters. Redness alone is not the described measurement. Missing the reading appointment can make the test unusable and may require a new test.
CDC guidance uses risk-based cutoffs of 5, 10, or 15 millimeters. There is no single positive size for everyone: the applicable threshold depends on the person’s likelihood of TB exposure or risk of progression to disease. A clinician therefore records both the millimeter measurement and the person’s risk category before calling the result positive or negative.
What can affect a TST
Prior Bacille Calmette-Guérin (BCG) vaccination can cause a false-positive TST because the vaccine may produce an immune reaction to some of the proteins used in the test. This is one reason CDC generally prefers an IGRA for people who received BCG. A TST should be administered and read using standardized procedures, training, supervision, and practice; as CDC states, “Reliable administration and reading of the TB skin test requires standardization of procedures, training, supervision, and practice.”
How an IGRA blood test works
Immune cells and TB antigens
For an IGRA, a laboratory exposes a blood sample to synthetic peptides that simulate TB antigens, along with control samples. In most people with TB infection, white blood cells recognize the antigens and release interferon-gamma. The laboratory measures that immune signal and reports the result to the ordering clinician.
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Why clinicians may choose IGRA
An IGRA requires one blood draw and laboratory processing but does not require a second visit to have a skin reaction read. CDC encourages blood testing in most situations and particularly favors it when a person received BCG or may not return reliably for a TST reading. The CDC’s latent-TB overview identifies two FDA-approved TB blood tests commercially available in the United States; the choice of assay and laboratory is a clinical decision.
Because the blood test measures immune memory rather than live bacteria, a positive IGRA indicates TB infection but does not show whether the infection is latent or whether TB disease is active in the lungs or elsewhere.
What a chest X-ray contributes
Looking for pulmonary abnormalities
A chest radiograph produces an image of the lungs and surrounding chest structures. Clinicians may order it after a positive TST or IGRA, or earlier when symptoms or other findings raise concern. The image can show abnormalities that are compatible with pulmonary TB and can help separate a positive infection test with no evidence of lung disease from a possible disease process.
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Why an X-ray cannot diagnose TB alone
Many conditions can produce shadows, cavities, scarring, or other changes on a radiograph, and TB can occasionally be present without a characteristic image. An abnormal X-ray therefore does not confirm TB disease. When a person has a positive infection test and no symptoms, CDC guidance says a normal chest radiograph can generally help rule out pulmonary TB; the clinician still considers the person’s history and risks.
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TST, IGRA, and X-ray compared in practice
| Practical question | TST | IGRA | Chest X-ray |
|---|---|---|---|
| Primary role | Detect immune evidence of TB infection | Detect immune evidence of TB infection | Look for chest findings that may indicate pulmonary disease |
| Patient procedure | PPD injection under the skin | Blood draw | Radiographic imaging |
| Return visit needed for the test itself? | Yes, for measurement by a trained reader | No reading visit, although follow-up care may still be needed | No separate infection-test reading; timing is determined by the clinical evaluation |
| Effect of BCG vaccination | May produce a false-positive reaction | Not affected by BCG, according to CDC guidance | BCG does not change the imaging role |
| Can it establish active TB by itself? | No | No | No |
Which test might a clinician select?
When an IGRA is often practical
- The person received BCG vaccination, making a false-positive TST more likely.
- The person may not be able to return for a TST reading.
- A blood draw and laboratory processing are readily available.
When a TST remains useful
- IGRA is unavailable, too costly, or otherwise burdensome.
- The clinician is following guidance that recommends TST for children younger than five.
- The setting has established staff and procedures for reliable placement and reading.
These are selection considerations, not a self-screening decision tree. Age, medical history, exposure, vaccination history, local rules, and the ability to complete follow-up all influence the choice.
What happens during testing
If the clinician orders a TST
- Placement: A trained professional injects PPD under the skin.
- Scheduling: The clinic gives a specific time window for the reading visit, usually a short interval of a few days.
- Reading: The trained reader palpates the site and measures induration in millimeters, documenting the measurement rather than simply recording redness.
- Risk-based interpretation: The clinician applies the CDC 5-, 10-, or 15-millimeter threshold that matches the person’s TB risk.
- Follow-up: A positive interpretation leads to evaluation for disease; a negative interpretation is considered alongside symptoms and other risks.
If the clinician orders an IGRA
- Blood draw: A laboratory or health-care professional collects the required blood sample.
- Laboratory processing: The sample is exposed to TB-related peptides and controls, and interferon-gamma response is measured.
- Clinical review: The ordering clinician interprets the laboratory report in the context of exposure, symptoms, age, immune status, and BCG history.
- Further evaluation when indicated: A positive result does not end the process; the clinician assesses whether disease could be active.
If a chest X-ray is ordered
- Imaging: A radiology service obtains the chest radiograph.
- Interpretation: A qualified clinician or radiologist reviews it for abnormalities that could fit pulmonary TB or another condition.
- Correlation: The image is interpreted with symptoms, examination findings, infection-test results, and exposure history.
- Additional testing: If disease remains possible, the clinician may obtain respiratory specimens for bacteriologic testing rather than relying on the image alone.
What a positive TST or IGRA means
A positive TST or IGRA usually means TB infection. It does not identify whether the infection is latent (inactive) or whether TB disease is active and potentially contagious. The next step is a medical evaluation, not an automatic diagnosis of active TB.
CDC describes a complete evaluation for TB disease as including medical history, physical examination, an infection test, chest radiograph, and bacteriologic examination when indicated. Bacteriologic work can include sputum studies; culture is the microbiologic gold standard for diagnosing TB disease. The clinician decides which specimens and tests are appropriate.
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsIf the evaluation finds infection without evidence of active disease, the clinician may discuss treatment for latent TB infection. CDC’s 2025 TST guidance says such treatment is 90% effective at preventing development of TB disease. That figure describes treatment effectiveness, not the accuracy of a TST or IGRA.
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Why a negative test does not always end the evaluation
A negative TST or IGRA makes TB infection less likely, but it cannot exclude TB disease in every circumstance. Symptoms, recent exposure, immune suppression, timing of exposure, and other clinical factors can change how much confidence a clinician places in a negative result.
CDC advises that when a person has symptoms suggestive of TB, clinicians should not wait for the TST or IGRA result before beginning other diagnostic testing. A chest radiograph, examination, and bacteriologic studies may be needed while the infection-test result is pending or even after it is negative. Seek prompt medical care for concerning symptoms rather than treating a negative screening result as a clearance certificate.
Symptoms and findings that change the pathway
The screening pathway is different for a person with no symptoms and no concerning findings than for someone who is ill. A clinician may ask about persistent cough, coughing blood, fever, night sweats, unexplained weight loss, fatigue, known exposure, travel or residence in areas with high TB incidence, and immune conditions. The specific questions and tests depend on the patient’s circumstances.
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Symptoms do not prove TB, and their absence does not make a positive infection test active disease. They determine how urgently the clinician must look beyond the initial screening test.
Special situations to discuss before testing
BCG vaccination
Tell the clinician if you received BCG. CDC says BCG can cause a false-positive TST, while TB blood tests are not affected by BCG. This history often makes IGRA preferable, but availability, age, and other clinical factors still matter.
Children younger than five
CDC guidance recommends TST for children younger than five. A pediatric clinician should account for the child’s age, exposure, ability to return for reading, and any illness or immune concern.
A previous positive test or treatment
People with a documented previous positive TB test or prior treatment generally should not simply repeat a TST or IGRA. CDC notes that most people remain positive and that another test is unlikely to add useful information. Bring prior records so the clinician can decide whether examination, imaging, or other follow-up is needed.
What these tests cannot tell you
- A positive TST or IGRA cannot tell you whether TB is latent or active.
- A negative TST or IGRA cannot guarantee that a symptomatic person does not have TB disease.
- A chest X-ray cannot prove TB by itself and cannot replace microbiologic testing when disease is suspected.
- None of these tests is a consumer self-test that should be bought, administered, or interpreted without clinical oversight.
How to prepare for a screening appointment
- Bring records of prior TST or IGRA results and any TB treatment.
- Tell the clinician about BCG vaccination, known exposure, symptoms, pregnancy, immune-suppressing conditions, and medicines.
- For a TST, plan for the required return visit before leaving the clinic.
- Ask when and how the IGRA laboratory result or X-ray report will be reviewed.
- If symptoms are present, describe when they started and do not wait for a routine screening result before seeking advice.
The practical bottom line
TST and IGRA are infection tests; chest X-ray is an imaging component of the disease evaluation. A positive infection test requires clinical follow-up to determine whether infection is latent or disease is active, while symptoms can warrant imaging and bacteriologic testing even after a negative screen. The right test depends on age, BCG history, exposure and medical risk, laboratory access, and the ability to complete follow-up.
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