What Is the IGRA Test?

The IGRA (interferon-gamma release assay) is a modern blood test used to detect TB infection. It works by measuring the immune system's response to specific proteins found in Mycobacterium tuberculosis — the bacterium that causes TB. When TB bacteria are present in the body, the immune system's T-cells become sensitized and release a protein called interferon-gamma when exposed to TB antigens.

The IGRA test measures the amount of interferon-gamma released by T-cells in a blood sample when exposed to TB-specific antigens. A positive result indicates that the immune system has been exposed to TB bacteria, suggesting TB infection.

QuantiFERON-TB Gold Plus: The Most Common IGRA Test

The most widely used IGRA test for immigration purposes is the QuantiFERON-TB Gold Plus (QFT-Plus), manufactured by QIAGEN. This fourth-generation IGRA test uses two TB-specific antigens — ESAT-6 and CFP-10 — to stimulate T-cells in the blood sample.

The QuantiFERON-TB Gold Plus is FDA-approved and is the IGRA test recommended by the CDC for immigration medical examinations. It has high sensitivity (ability to detect true TB infection) and high specificity (ability to correctly identify people without TB infection).

How the IGRA Test Is Performed

The IGRA test requires a simple blood draw. Here is what to expect:

• A small amount of blood (approximately 4 mL total) is drawn from a vein in your arm
• The blood is collected into special tubes containing TB antigens and control substances
• The tubes are sent to a laboratory for processing
• The laboratory measures the amount of interferon-gamma released by your T-cells
• Results are reported as positive, negative, or indeterminate
• Results are typically available within 2 to 3 business days

Advantages of the IGRA Over the TB Skin Test

The IGRA blood test has several important advantages over the traditional TB skin test (TST/Mantoux) for immigration purposes. Most significantly, the IGRA is not affected by prior BCG vaccination — a critical advantage for immigration applicants from countries where BCG is routinely administered.

The IGRA also requires only one patient visit (no return visit to read a skin reaction), eliminates reader variability, and provides an objective laboratory measurement. These advantages make the IGRA the preferred TB screening method for immigration medical examinations.

Interpreting IGRA Results

A negative IGRA result means that TB infection is unlikely. In the context of the immigration medical exam, a negative IGRA with no symptoms of TB disease means no further TB evaluation is needed.

A positive IGRA result means that TB infection is likely. Further evaluation — including a chest X-ray and clinical assessment — is required to determine whether the infection is latent or active. An indeterminate result means the test could not be interpreted and needs to be repeated.