Interferon-Gamma Release Assay (IGRA) Testing: Principles, Methods, and Clinical Applications

Interferon-gamma release assay (IGRA) testing is an advanced immunodiagnostic method used to detect latent tuberculosis infection and other intracellular pathogen infections by measuring the cell-mediated immune response of T cells. This comprehensive guide covers the technical principles, testing samples, methodologies, and clinical applications of interferon release testing. Understanding IGRA testing helps healthcare professionals make informed diagnostic decisions when screening high-risk populations for tuberculosis infection.

Technical Overview

Interferon-gamma release assay (IGRA) testing is an in vitro diagnostic method that measures the immune response of T lymphocytes to specific microbial antigens. The most widely recognized application of this technology is the detection of Mycobacterium tuberculosis infection, including both latent tuberculosis infection (LTBI) and active tuberculosis disease. The assay is based on the principle that T cells previously sensitized to tuberculosis antigens will release interferon-gamma (IFN-γ) when re-exposed to those same antigens during in vitro incubation.

Unlike the traditional tuberculin skin test (TST), IGRAs utilize highly specific antigens—early secreted antigenic target 6 (ESAT-6), culture filtrate protein 10 (CFP-10), and TB7.7—which are encoded by genes located in the region of difference 1 (RD1) of the Mycobacterium tuberculosis genome. These antigens are absent from Mycobacterium bovis BCG strains and most non-tuberculous mycobacteria, resulting in significantly improved specificity compared to the TST. This characteristic makes IGRAs particularly valuable for testing individuals who have received BCG vaccination, which is common in many countries worldwide.

The test requires live, functional T cells capable of responding to antigen stimulation, which means proper blood sample collection, transport, and timely processing are critical to obtaining accurate results. The assay is performed in regulated clinical laboratories following established diagnostic standards and quality management systems compliant with ISO 15189 medical laboratory requirements.

Testing Samples

Whole Blood Samples

Venous whole blood collected in lithium heparin anticoagulant tubes is the standard specimen type for interferon-gamma release assays. Blood must be collected using specialized collection tubes or transferred to stimulation tubes within a specified time window, typically within 8 to 16 hours of collection depending on the assay platform and validated transport conditions. The blood should be maintained at room temperature (approximately 17-25°C) during transport, and samples must never be refrigerated or frozen before stimulation, as temperature extremes can compromise lymphocyte viability.

Special Collection Tubes

Some IGRA platforms require dedicated blood collection tubes pre-coated with antigens, including a Nil control tube, a TB antigen tube, and a mitogen-positive control tube. These tubes must be filled to the indicated volume to ensure the correct blood-to-antigen ratio and must be gently but thoroughly mixed immediately after collection to ensure adequate contact between blood and tube contents before incubation.

Peripheral Blood Mononuclear Cells (PBMCs)

For research applications or specialized testing platforms such as ELISpot-based assays, peripheral blood mononuclear cells are isolated from whole blood using density gradient centrifugation. PBMC-based methods allow for precise cell counting and normalization of results, offering greater sensitivity in certain clinical contexts, including testing of immunocompromised patients where cell counts may be reduced.

Alternative Sample Types

In specific research and clinical settings, IGRA technology has been adapted for use with body fluid specimens such as pleural fluid, cerebrospinal fluid, bronchoalveolar lavage fluid, and ascites. These applications aim to detect local immune responses at the site of suspected extrapulmonary tuberculosis and are typically performed in specialized laboratories with validated protocols.

Testing Items

Interferon-gamma release assay testing typically includes the following analytical components and reported parameters:

• Nil control value: The interferon-gamma concentration measured in the unstimulated control, representing the background IFN-γ level in the patient's blood without antigen stimulation
• TB antigen response: The interferon-gamma concentration produced in response to ESAT-6, CFP-10, and TB7.7 antigen stimulation
• Mitogen response: The interferon-gamma concentration produced in response to phytohemagglutinin or other mitogens, serving as a positive control to verify T cell functionality and test validity
• Quantitative IFN-γ values: Measured concentrations in IU/mL or equivalent units for each tube or antigen well
• Calculated results: Antigen-stimulated value minus Nil value, interpreted against validated assay-specific cutoff thresholds
• Spot-forming units: For ELISpot platforms, the number of interferon-gamma-secreting cells counted per defined cell input
• Final interpretation: Positive, negative, or indeterminate result issued according to manufacturer specifications and laboratory validation criteria

Indeterminate results may occur when the nil value is elevated or the mitogen response is inadequate, indicating either nonspecific IFN-γ production or insufficient lymphocyte activity, commonly observed in immunosuppressed patients. Laboratory reports should clearly communicate indeterminate outcomes and recommend repeat testing or alternative diagnostic approaches when appropriate.

Testing Methods

Enzyme-Linked Immunosorbent Assay (ELISA)

ELISA-based IGRA platforms measure the concentration of interferon-gamma released into plasma after whole blood stimulation. The stimulated plasma samples are transferred to antibody-coated microplate wells, where IFN-γ is captured by immobilized antibodies and detected through enzyme-conjugated secondary antibodies and colorimetric substrates. Optical density readings from a microplate spectrophotometer are converted to quantitative concentrations using a standard curve generated from recombinant IFN-γ calibrators. This format provides quantitative results and supports batch processing in high-throughput clinical laboratories.

Enzyme-Linked Immunospot Assay (ELISpot)

ELISpot-based methods detect individual interferon-gamma-secreting T cells at the single-cell level. Isolated PBMCs are incubated on membranes coated with anti-IFN-γ antibodies. Cytokine secreted by each activated T cell is captured locally, and subsequent development steps produce visible spots, each representing one antigen-responsive cell. The number of spot-forming units is counted using automated imaging systems or manual microscopy. ELISpot offers enhanced analytical sensitivity due to local cytokine capture and requires fewer responding cells to generate a positive signal, making it valuable for testing immunocompromised populations.

Chemiluminescence Immunoassay (CLIA)

Automated chemiluminescent IGRA platforms utilize acridinium ester or alkaline phosphatase-labeled detection antibodies and chemiluminescent substrates to quantify interferon-gamma. These fully automated systems offer reduced hands-on time, improved reproducibility, random access capability, and shorter turnaround times compared to manual ELISA methods. Automation minimizes operator-dependent variability and supports standardized workflow integration in high-volume laboratories.

Flow Cytometry-Based Intracellular Cytokine Staining

Research and reference laboratories may employ flow cytometric methods in which intracellular IFN-γ accumulation is detected in CD4+ and CD8+ T cell subsets after antigen stimulation with secretion inhibitors such as brefeldin A. This approach permits simultaneous identification of responding cell phenotypes and polyfunctional cytokine profiles, providing deeper immunological insight for research studies, vaccine evaluation, and host-response characterization.

Quality Control and Method Validation

Each analytical run must include validated controls, including the nil and mitogen controls inherent to the assay design, as well as positive and negative reference samples to monitor assay performance. Laboratories should participate in external quality assessment schemes and proficiency testing programs. Method verification studies must document analytical sensitivity, specificity, precision, and inter-laboratory agreement before clinical implementation. Accredited third-party laboratories and hospital clinical laboratories performing IGRAs operate under quality management systems aligned with ISO 15189 and applicable national medical device and in vitro diagnostic regulations.

Applications

Interferon-gamma release assay testing serves numerous clinical, occupational, and public health purposes:

• Latent tuberculosis infection screening prior to biological therapy and immunosuppressive treatment, including anti-TNF therapy, chemotherapy, and transplantation protocols
• Pre-employment and periodic occupational health screening for healthcare workers, laboratory personnel, and other high-exposure professions
• Contact investigation of individuals exposed to active pulmonary tuberculosis cases
• Immigration medical examinations and pre-admission screening requirements in various countries
• Ancillary diagnostic support for active tuberculosis, particularly in conjunction with microbiological, molecular, and radiological findings in cases of extrapulmonary tuberculosis
• Screening of HIV-infected individuals and other immunocompromised patients at elevated risk of tuberculosis reactivation
• Pediatric tuberculosis infection evaluation where BCG vaccination status complicates tuberculin skin test interpretation
• Monitoring and decision support in tuberculosis preventive therapy programs
• Research applications including tuberculosis vaccine immunogenicity trials and host immune response studies

Beyond tuberculosis, interferon-gamma release assay principles have been extended to the diagnosis of other infections, such as the interferon-gamma release assay for detecting prior sensitization to certain intracellular pathogens, and immune functional assessment in immunodeficiency evaluation, demonstrating the broader versatility of the underlying cytokine-release methodology.

FAQ

What is the difference between IGRA and the tuberculin skin test?

IGRA is an in vitro blood test requiring a single patient visit, while the tuberculin skin test requires intradermal injection and a return visit for reading. IGRAs use antigens absent from BCG vaccine and most environmental mycobacteria, offering higher specificity in BCG-vaccinated populations and eliminating reader subjectivity. The TST remains less expensive and does not require phlebotomy or laboratory infrastructure, so the choice depends on clinical context, resources, and patient factors.

Can IGRA distinguish latent from active tuberculosis?

No. IGRAs detect immune sensitization to tuberculosis antigens but cannot differentiate latent infection from active disease. Active tuberculosis diagnosis requires microbiological culture, molecular testing such as nucleic acid amplification, smear microscopy, and clinical-radiological evaluation. IGRA results must always be interpreted alongside clinical assessment.

What causes an indeterminate IGRA result?

Indeterminate results typically arise from an elevated nil value suggesting nonspecific immune activation, or a poor mitogen response indicating inadequate T cell function. Common contributing factors include immunosuppression, HIV infection, lymphopenia, very young or advanced age, improper sample handling, or delayed processing beyond the validated window.

How soon can IGRA detect infection after exposure?

Seroconversion to a positive IGRA generally requires several weeks following infection, similar to the development of tuberculin skin test conversion. Testing immediately after a known exposure may yield a false-negative result; guidelines often recommend testing at least 8 weeks post-exposure unless earlier evaluation is clinically warranted.

Does prior BCG vaccination affect IGRA results?

No. Because the ESAT-6, CFP-10, and TB7.7 antigens used in IGRAs are absent from BCG vaccine strains, prior BCG vaccination does not cause false-positive results. This represents a major advantage over the tuberculin skin test in BCG-vaccinated populations.

How should blood samples for IGRA testing be handled?

Whole blood samples must be collected into appropriate anticoagulant or pre-coated stimulation tubes, mixed gently, maintained at room temperature, and processed within the validated time frame specified by the assay manufacturer, typically within 8 to 16 hours. Refrigeration, freezing, or exposure to temperature extremes must be avoided to preserve lymphocyte viability and ensure valid results.

Can IGRA be used to monitor treatment response?

IGRA is not recommended for monitoring treatment response or confirming cure, as test reversion or persistence does not reliably correlate with microbiological outcomes. Quantitative IGRA values can fluctuate due to biological and analytical variability. Its primary role is initial infection detection to guide preventive therapy decisions.