TB Blood Test vs. PPD Skin Test: What’s the Difference?
Compare the one-visit IGRA blood test with the PPD skin test, including BCG vaccination, timing, interpretation, and follow-up.
Medically reviewed by Rhonda Collins, FNP-C | Mito Health on August 28, 2026.
A TB blood test, also called an interferon-gamma release assay or IGRA, needs one blood-draw visit. A tuberculin skin test, also called a TST or PPD, places testing material under the skin and requires you to return so the reaction can be read. Both test for tuberculosis infection, but they differ in workflow, how BCG vaccination affects results, and when each option may make sense. The CDC’s clinical testing guidance describes both methods and their visit requirements.
TB blood test vs. skin test at a glance
| | TB blood test (IGRA) | TB skin test (TST or PPD) | | --- | --- | --- | | How it works | Measures an immune response in a blood sample | Measures the skin’s reaction to injected testing material | | Visits | One blood-draw visit | Placement visit plus a return visit for reading | | BCG vaccination | BCG does not cause a positive result | BCG can affect the reaction | | Result | Positive, negative, or sometimes indeterminate | Positive or negative based on the measured reaction and risk context | | Important limit | Does not diagnose active TB disease | Does not diagnose active TB disease |
The practical difference is often the workflow. An IGRA avoids the return visit needed to read a skin test. A skin test remains an accepted alternative in some circumstances, including when a blood test is unavailable, too costly, or otherwise too burdensome.
How does a TB blood test work?
An IGRA measures how immune cells in your blood respond to proteins associated with Mycobacterium tuberculosis, the bacteria that cause TB. QuantiFERON-TB Gold Plus is one FDA-approved IGRA available in the United States, according to the CDC’s overview of recommended TB tests.
The result indicates whether the immune response supports TB infection. It does not show whether an infection is inactive or has progressed to active TB disease. That distinction requires medical evaluation and may involve a symptom review, chest imaging, and other testing.
How does a PPD skin test work?
A trained professional injects a small amount of tuberculin testing material under the skin of the forearm. You then return so the site can be examined and the firm swelling, if any, can be measured. The result is interpreted with your likelihood of infection and risk of progression.
Because the reaction must be read within the required window, missing the return visit can mean the process has to be repeated. Some baseline screening programs also use a two-step skin-test process. The CDC’s skin-test guidance explains the reading window and when two-step testing is used.
Which TB test needs fewer visits?
The blood test needs one collection visit. The PPD skin test needs one visit for placement and a second visit for reading. This makes the blood test useful when returning for a reading would be difficult.
One visit does not mean the blood test is automatically the right choice for every person or every requirement. Confirm what your school, employer, clinical program, or jurisdiction accepts before ordering.
How does BCG vaccination affect the choice?
BCG vaccination can cause a false-positive skin-test reaction. It does not cause a positive TB blood-test result. For this reason, U.S. guidance generally prefers an IGRA for people who received BCG.
BCG is used in many countries where TB is common, so people born or vaccinated outside the United States may not know whether it appears in their records. If you have a BCG history, include it when discussing the reason for testing.
What do positive, negative, and indeterminate results mean?
A positive blood or skin test usually supports TB infection. Neither test can tell whether the infection is inactive or active. A positive result needs medical evaluation to rule out active TB disease.
A negative result makes infection less likely, but it cannot exclude active TB when symptoms are present. Testing soon after exposure can also be too early for an immune response to be detectable. Your exposure timing and risk can determine whether follow-up testing is appropriate.
An IGRA can return an indeterminate result when its controls do not provide enough information to classify the result. A clinician may recommend repeating the blood test or using another TB test. The CDC also emphasizes that a positive result or symptoms require evaluation for TB disease.
Who may prefer each testing route?
A TB blood test may be useful when:
- You received the BCG vaccine.
- You may have difficulty returning for a skin-test reading.
- Your school or workplace accepts an IGRA and you prefer a one-visit collection.
- A clinician or public-health program directs you to blood testing.
A PPD skin test may remain a reasonable option when:
- A blood test is unavailable, too costly, or too burdensome.
- Your institution specifically requires or provides skin testing.
- A clinician selects it based on age, history, or the testing setting.
Neither option is a universal replacement for the other. The best fit depends on the person, the reason for testing, and the applicable requirement.
When do symptoms or positive results need medical evaluation?
Do not use either screening test as the only response to symptoms that could indicate active TB, such as a persistent cough, chest pain, coughing up blood, fever, night sweats, or unexplained weight loss. Seek prompt medical evaluation. Active TB assessment should not wait for a screening result.
A positive blood or skin result also needs follow-up. Testing for infection is only one part of the evaluation used to distinguish inactive TB infection from active disease.
Choosing a QuantiFERON-TB Gold Plus blood test
If an IGRA fits your requirement or clinical direction, Mito offers the exact Tuberculosis Blood Test (QuantiFERON-TB Gold Plus). Check the accepted test type and required paperwork before you buy.
If you are testing for a program or job, read what to verify before ordering QuantiFERON-TB Gold for school or work.
This article is for educational purposes and does not replace medical advice, diagnosis, or treatment.