QuantiFERON-TB Gold (Interferon-gamma release assay (IGRA)

Lung

Report in 72Hrs

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No Fasting Required

Details

Detects latent or active tuberculosis (TB) infection

₹999₹2,400

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QuantiFERON-TB Gold (Interferon-gamma Release Assay) - Comprehensive Test Guide

Section 1: Why is it done?
- Test Purpose: Detects Mycobacterium tuberculosis infection by measuring interferon-gamma (IFN-γ) release from immune cells in response to TB-specific antigens
- Screening for Active TB: Identifies patients with clinical symptoms suggestive of active tuberculosis disease
- Detection of Latent TB Infection (LTBI): Identifies people with latent TB who are asymptomatic but harbor viable TB organisms
- Contact Investigation: Testing individuals exposed to confirmed TB cases to determine infection status
- Pre-employment or Immigration Screening: Routine screening for occupational or immigration purposes, particularly in high-risk settings
- Pre-treatment Assessment: Testing before initiating immunosuppressive therapy (biologics, TNF-inhibitors) or in patients with HIV/AIDS
- Superior to TST: Preferred over Tuberculin Skin Test (Mantoux test) due to better specificity and avoidance of BCG vaccination cross-reactivity
Section 2: Normal Range
- Test Values and Interpretation:
- IFN-γ Concentration Measured In: International Units per milliliter (IU/mL)
- Negative Result: IFN-γ ≤ 0.35 IU/mL in TB antigen-stimulated sample • Indicates NO TB infection (no TB exposure or very early infection before immune response develops)
- Positive Result: IFN-γ > 0.35 IU/mL in TB antigen-stimulated sample • Indicates TB infection (latent or active TB disease)
- Indeterminate Result: High background IFN-γ in nil/control sample or low response to mitogen • Suggests possible immunocompromise or technical issue; retest recommended
- Reference Range Components: Test measures IFN-γ in three components: • TB antigen tube (ESAT-6, CFP-10, TB7.7) • Nil/Negative control • Mitogen/Positive control (phytohemagglutinin) Result = TB antigen IFN-γ - Nil control IFN-γ
Section 3: Interpretation
- Positive Result Interpretation: Indicates TB infection (cannot differentiate between latent and active TB) • Clinical context, symptoms, and imaging studies essential to distinguish latent LTBI from active TB disease • Patient requires further evaluation including chest X-ray and clinical assessment • Most positive patients (85-90%) have latent TB; only minority progress to active disease
- Negative Result Interpretation: No TB infection detected; very unlikely to have TB disease • High negative predictive value (NPV: 98-99%) • Exception: May be falsely negative in early infection (<2-8 weeks), severe immunosuppression (CD4<50 in AIDS), or overwhelming active TB • HIV-negative patients with negative IGRA very unlikely to have active TB
- Indeterminate Result Interpretation: Cannot classify as positive or negative; requires retest • May indicate impaired cell-mediated immunity • Common in HIV/AIDS patients with CD4<100, during acute infections, or on immunosuppressive medications • Technical issues or improper sample handling can also cause indeterminate results
- Factors Affecting Interpretation: Immunosuppression: CD4 count <200 in HIV, use of TNF-α inhibitors Duration of infection: Early TB (<2 weeks) may test negative Severely ill patients: May have false-negative results Recent TB vaccination: Does NOT cause false positives (unlike TST) Non-tuberculous mycobacteria (NTM): May cause positive results Prior TB disease: Remains positive lifelong after TB exposure
- Clinical Significance of Result Patterns: High IFN-γ levels: Suggest recent infection or high bacterial burden Low-positive IFN-γ: May indicate early infection or lower immune response Converting from negative to positive: Indicates recent TB exposure Converting from positive to negative: Rare; suggests either initial false-positive or severe immunosuppression
- Sensitivity and Specificity: Overall sensitivity: 90-95% for active TB, 90% for LTBI Overall specificity: 96-98% (better than TST, not affected by BCG) Performance varies with immune status and severity of disease
Section 4: Associated Organs
- Primary Organ System: Immune system and lymphoid tissue Primarily T-lymphocytes (CD4+ T cells) in response to M. tuberculosis antigens
- Organs Involved in TB Infection: Lungs: Most common site (85% of active TB) - pulmonary tuberculosis Lymph nodes: Mediastinal and hilar lymphadenopathy Pleura: Pleural effusion and tuberculous pleuritis Spine: Pott's disease (TB spondylitis) Brain and meninges: TB meningitis Liver and spleen: Hepatic and splenic TB Kidneys and urinary tract: Urogenital TB Bones and joints: TB osteomyelitis and arthritis
- Diseases Diagnosed by IGRA: Latent TB Infection (LTBI): Asymptomatic TB exposure without active disease Active Tuberculosis (PTB): Progressive pulmonary TB with transmission risk Extrapulmonary TB: TB affecting organs beyond the lungs Miliary TB: Disseminated TB with widespread organ involvement TB in HIV/AIDS patients: Complicated by severe immunosuppression
- Potential Complications of Positive Results: Untreated LTBI: Risk of progression to active TB (5-10% lifetime risk) Active TB: Life-threatening if untreated; 45% mortality in untreated patients Drug-resistant TB: MDR-TB or XDR-TB complicates treatment Disseminated TB: Miliary TB with multi-organ involvement Treatment-related hepatotoxicity: From anti-TB medications Immune reconstitution inflammatory syndrome (IRIS): In HIV patients starting ART
- High-Risk Populations for TB Infection: HIV/AIDS patients: 20-37% risk of active TB Immunosuppressed patients (TNF-α inhibitors): 5-10% risk Close TB contacts: Significantly increased transmission risk Healthcare workers: Occupational exposure risk Prisoners and homeless populations: High prevalence areas
Section 5: Follow-up Tests
- Recommended Follow-up Tests for Positive IGRA: Chest X-ray (CXR): Evaluate for active TB findings (infiltrates, cavitations, nodules) Sputum smear microscopy (AFB): Three consecutive sputum samples to detect acid-fast bacilli Sputum culture: Gold standard for TB diagnosis and drug susceptibility testing TB nucleic acid amplification test (NAAT): PCR/GeneXpert MTB/RIF for rapid TB detection
- Tests for Symptomatic/Suspected Active TB: CT chest: For complicated TB or when CXR findings uncertain Bronchoscopy and bronchoalveolar lavage (BAL): If sputum-negative, highly suspicious for TB Lymph node biopsy: For suspected extrapulmonary TB Lumbar puncture/CSF analysis: When TB meningitis suspected Abdominal imaging (CT/ultrasound): For suspected abdominal TB
- Associated Laboratory Tests: Complete Blood Count (CBC): Baseline assessment before TB treatment Liver function tests (LFTs): AST, ALT, bilirubin to establish baseline for hepatotoxic anti-TB drugs Serum creatinine/GFR: Baseline renal function for drug dosing HIV test: Mandatory for all TB patients to determine immune status TB drug susceptibility testing (DST): For confirmed TB to guide therapy
- Monitoring During Treatment: Monthly sputum AFB smear: During intensive phase of therapy Repeat CXR: At end of intensive phase (2 months) and completion of therapy Monthly LFTs: To monitor for hepatotoxicity from isoniazid, rifampicin, pyrazinamide Clinical assessment: Monthly during treatment to assess treatment response and adherence
- Recommended Follow-up for LTBI Treatment: Baseline LFTs before starting isoniazid or rifampicin monotherapy Baseline vision check: If ethambutol used Monthly clinical assessment: For symptoms of TB development or drug adverse effects Post-treatment follow-up: Clinical monitoring for 5 years for TB disease development
- Retesting Recommendations: Repeat IGRA: If initial result indeterminate; retest after 1-3 months Serial IGRAs: In contacts or health workers to detect seroconversion IGRA after treatment: NOT recommended as TB antibodies persist post-treatment Two-step IGRA: For immunosuppressed patients with negative initial test
- Additional Investigations for Risk Stratification: CD4 count and HIV viral load: In HIV-positive patients Immune reconstitution markers: In HIV patients starting ART Drug-resistance molecular testing: Xpert MTB/RIF Ultra with resistance genes Contact tracing investigation: Epidemiologic assessment of TB source
Section 6: Fasting Required?
- Fasting Requirement: NO - Fasting is NOT required for QuantiFERON-TB Gold testing
- Specimen Type: Whole blood (4 mL) collected in specialized QuantiFERON collection tubes Tubes contain specific TB and control antigens for direct stimulation
- No Special Medications to Avoid: Most routine medications do not interfere with IGRA results Continue all chronic medications unless specifically instructed otherwise NOTE: Immunosuppressive medications may affect test sensitivity
- Pre-test Patient Preparation: Patient can eat and drink normally before the test No specific time of day required (though morning collection preferred) Comfortable clothing to allow easy arm access for venipuncture Report any acute illness or fever prior to testing
- Critical Specimen Handling Requirements: Blood MUST be incubated within 2-4 hours of collection (critical) Improper timing can result in false-negative or indeterminate results Tubes must be inverted gently 8-10 times immediately after collection Do NOT refrigerate or freeze the specimen Keep at room temperature (18-24°C) until processing
- Factors That May Compromise Test Quality: Recent live vaccinations (MMR, varicella): May temporarily suppress immune response; wait 4 weeks Active infection/fever: Defer testing until acute illness resolves Severe immunosuppression: CD4<50 in HIV may produce indeterminate results Stress or poor sleep: May transiently affect immune response Improper blood collection tube (wrong tube or contaminated): Invalid results
- Optimal Timing for Test Collection: Any time of day acceptable (morning preferred for optimal lab processing) Avoid testing immediately after acute stress or illness Minimum 2-4 weeks after live attenuated vaccine administration May be done concurrently with other blood tests
- Patient Education Points: Inform healthcare provider of all medications, supplements, and recent vaccinations Report any current acute infections or fever Results typically available in 24 hours Positive result does NOT mean active TB disease; further testing required Negative result does NOT completely rule out TB in symptomatic patients

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