II. Indications

  1. Mycobacterium tuberculosis testing
    1. Low risk patients
      1. Routine screening not recommended
      2. Screen only if symptoms suggest possible Tuberculosis
    2. High risk patients
      1. See Tuberculosis Risk Factors for Tuberculosis Screening Indications
  2. Cases in which IGRA is preferred over TST
    1. Unreliable follow-up for TST testing (homeless patients, Illicit Drug use)
    2. Patients with prior BCG vaccine
  3. Other indications
    1. May be indicated in health care settings, homeless shelters and prisons for suspected contacts
    2. Consider as pre-screening before starting immune suppressants or immune modulators (e.g. TNF agents)

III. Contraindications

  1. Children under age 5 years

IV. Mechanism

  1. Lab assay specific for prior Mycobacterium tuberculosis exposure
    1. Patient's White Blood Cells are mixed with Mycobacterium tuberculosis antigens
    2. Prior exposure is associated with interferon-gamma release
  2. Detects interferon-gamma release
    1. Originates from memory T-cells
    2. Released by cells previously sensitized to Mycobacterium tuberculosis-specific proteins

V. Types: Interferon-gamma release assays (IGRA)

  1. Quantiferon-TB Gold-In-Tube Test
    1. Detects antigens ESAT-6, CFP-10, TB7.7
    2. Measures Interferon-gamma level via EIA whole blood in 16 hours
    3. Test Sensitivity: 70-91%
    4. Test Specificity: 95-99%
  2. T-SPOT TB Test
    1. Detects antigens ESAT-6, CFP-10
    2. Measures Interferon-gamma-producing cell count via EIA
    3. Analyzes peripheral blood mononuclear cells in 8-30 hours
    4. Test Sensitivity: 84-91%
    5. Test Specificity: 95-97%

VI. Efficacy

  1. False Negative tests (lower Test Sensitivity than with TST)
    1. Immunocompromised patients (anergy)
      1. Advanced disease, malnourishment or low CD4 Count
    2. Delayed lab testing >12 hours from time of blood draw
    3. Inadequate Temperature or other mishandling of lab specimen
    4. Lab testing too soon after exposure
  2. False Positive tests (higher Test Specificity than TST)
    1. Booster effect from EST-6 and CFP-10 antigens following PPD
    2. No false positives with prior BCG Vaccination
    3. No false positives with non-tuberculosis Mycobacterium strains

VII. Disadvantages

  1. Does not distinguish latent from Active Tuberculosis (same as with TST)
  2. Sample must be tested within 12 hours of being obtained
  3. Old test was more labor intensive and expensive
    1. Newer tests (QFT-GIT, T-SPOT) are more easily run now
  4. QFT-GIT is affected by immune status
    1. T-SPOT is not limited by immune status

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