The Two Types of TB Tests

There are two main methods for testing TB infection: the tuberculin skin test (TST), also called the Mantoux test or PPD test, and the interferon-gamma release assay (IGRA), a blood test. Both tests detect the immune system's response to TB bacteria, but they work differently and have different advantages and limitations.

For immigration medical examinations, the CDC technical instructions now require the IGRA blood test as the preferred method. The TB skin test is no longer the standard for immigration purposes, though it may still be used in limited circumstances as determined by the civil surgeon.

How the TB Skin Test (TST) Works

The tuberculin skin test involves injecting a small amount of tuberculin purified protein derivative (PPD) just under the skin of the forearm. The patient must return 48 to 72 hours later so a healthcare provider can measure the size of any skin reaction (induration) at the injection site.

The TST requires two separate visits and is subject to reader variability. Importantly, prior BCG vaccination — common in many countries outside the United States — can cause a false-positive TST result, making it less reliable for immigration applicants from countries where BCG is routinely administered.

How the IGRA Blood Test Works

The IGRA blood test (QuantiFERON-TB Gold Plus) requires only a single blood draw. The blood sample is sent to a laboratory, where it is exposed to TB antigens. If TB infection is present, the immune system's T-cells release interferon-gamma, which is measured to determine a positive or negative result.

The IGRA test is not affected by prior BCG vaccination, making it significantly more specific than the TST for immigration applicants who received BCG as children. Results are typically available within 2 to 3 business days.

Why USCIS Prefers the IGRA for Immigration

The CDC updated its technical instructions for immigration medical examinations to require the IGRA blood test as the preferred TB screening method for several reasons: it requires only one patient visit, it is not affected by prior BCG vaccination, it has higher specificity than the TST, and it eliminates the subjectivity of skin test reading. For immigration applicants — many of whom received BCG vaccination in their home countries — the IGRA provides more accurate results.

• Only one visit required (no return visit to read results)
• Not affected by prior BCG vaccination
• Higher specificity — fewer false positives
• Objective laboratory measurement (no reader variability)
• Results available in 2 to 3 business days

What to Expect at Our Clinic

At our clinic, all immigration TB screening is performed using the IGRA blood test (QuantiFERON-TB Gold Plus). A small blood sample is drawn at your appointment and sent to our laboratory partner. You do not need to return for a skin test reading. If your IGRA result is positive, our civil surgeon will review the result with you and determine whether a chest X-ray is indicated. We will guide you through every step of the process.