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IGRAs: Should they replace the TST in the identification of latent tuberculosis?. Objectives • Describe how interferon-gamma release assays (IGRAs) work. • List three advantages and disadvantages of IGRA in comparison to tuberculin skin testing (TST).

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slide1

IGRAs: Should they replace the TST in the identification of latent tuberculosis?

  • Objectives
      • • Describe how interferon-gamma release assays (IGRAs) work.
    • • List three advantages and disadvantages of IGRA in comparison to tuberculin skin testing (TST).
    • • Identify populations where IGRA testing may be of benefit in the management of latent tuberculosis infection.
    • AllenKraut, MD, frcpc
  • Medical Director, Occupational Health WRHA WRHA T8 Forum April 12.2012
  • Conflict of Interest
    • • Received Quantiferon TB Gold in Tube Tubes from Cellestis as part of a research study.
slide2

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  • New Technologies - Blood tests
    • • Interferon Gamma Release Assays (IGRAs)
        • • White blood cells in people infected with TB release Gamma interferon
      • •  Detect specific Mycobacterium TB proteins
          • • Less likely to give false positive results
      • • Can not differentiate latent and active disease
  • Some issues with TST
      • •  Difficulty reading test.
      • • 6mm inter reader variability
      • •  Not specific for Mycobacterium Tuberculosis
        • • False +ve with BCG or Atypical Mycobacterium
    • •  Requires two visits days apart for reading
    • • Subject to boosting
    • •  Definition of positive test depends on circumstances
  • Interferon Gamma Release Assays (IGRAs)
    • • Quantiferon-TB Gold In-Tube Assay
      • •  ESAT-6, CFP - 10, TB7.7
      • •  Measure IFN- Gamma ELISA
    • • T-spot.TB Assay
      • •   ESAT-6, CFP - 10
      • • Count spots which are related to the number of cells releasing Gamma Interferon.
slide3

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IGRAs

  • IGRAs in HCP
      • • Significant discordance is found between TST and IGRA positivity rates in healthcare workers (HCWs),
        • • TST+/IGRA- - BCG vaccinations.
    • •  IGRAs seem to correlate with markers of exposure in HCWs
    • • Serial testing results limited
          • • CCDRVol36 June 2010
  • Advantages
      • •   More specific for Mycobacterium TB.
        • " Atypical mycobacteria
          • • M. koniosii. M siulgoi. and M matmum
      • •   Single patient encounter
    • •   Objective criteria for positive response Disadvantages
      • •   Requires blood draw
      • •   Requires sophisticated equipment
      • •   Elements of processing time sensitive
      • •   Results may not be readily available
      • •   ? Immunosuppressed -Tspot.TB may be better
      • •   Higher direct costs, but may have lower costs if include all required follow up and treatment
  • 6,530 healthcare workers (HCWs) screened for latent tuberculosis infection
    • AS 10 I It U > .crcou-d »iiMJI\'l <;i f
    • 25 fold increase in conversion rate using QFT vs TST Direct costs
      • • QFTTB Gold in Tube $436,096
    • • TST $78,360. Indirect costs
      • • confirmatory TSTs, additional chest radiographs, extra nurse assessments, and examinations.
  • Total costs $521,890

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    • Inlrctun Control and Hoipi|alCp«l«niiiloa 2010:11.HIS !>8S
slide4

IGRA result

♦ve

-ve

TST result

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LTBI

low risk don\'t treat. High risk treat.

-ve

High Risk Treat Low risk ??

No LTBI

    • IGRA performance in contacts and outbreak investigations
    • •  IGRAs correlate well with surrogate markers of exposure
      • in contact and outbreak settings, but not necessarily better than TST in all populations.
  • •  Correlation between IGRA results and surrogate markers of
      • exposure is better than TST in low incidence settings where BCG has been commonly used; this is not evident in high incidence countries.
    • •  Discordance between TST and IGRAs are almost always
      • found. Concordance levels seem to vary when IGRA and TST cut-off points are changed
  • Are IGRA results constant?
    • • Reversion rates are higher when baseline IFN-y levels are just above the cut-off point and when baseline results are discordant (i.e. TST-/IGRA+).
    • • Reversion rates low when baseline IFN-y levels are high and when baseline results are concordantly positive (TST+/IGRA+).
  • CTS recommendations
    • • Immunocompromised •TSTfirst test
      • • If TST -ve IGRA can be used and if +ve consider treatment
      • • Degree of benefit unknown in TST-ve IGRA+ve.
      • • T Spot .TB may be better in an immunosuppressed population
  • CTS recommendations
    • • IGRAs should not be used in the diagnosis of active TB in adults may be a supplemental aide in dx in children.
    • • Contacts-
        • • IGRAs can be used to confirm +ve TSTS
      • • IGRAS or TSTs can be used to identify +vesforTXforLTBI
    • International Guidelines
  • Clin Microbiol Infect 2011; 17: 806-814
      • •   33 guidelines and position papers from 25 countries and two supranational organizations.
      • •   The results show considerable diversity in the recommendations on IGRAs
        • •   (i) two-step approach of tuberculin skin test (TST) first, followed by IGRA either when
          • •   the TST is negative (to increase sensitivity, mainly in immunocompromised individuals).
          • •   or when the TST is positive (to increase specificity, mainly In BCG vaccinated individuals);
        • •   (ii) Either TST or IGRA, but not both;
        • •   (iii) IGRA and TST together (to increase sensitivity),
        • •   (iv) IGRA only, replacing the TST.
      • •   Overall, the use of IGRAs is increasingly recommended,
slide5

International Guidelines

  • Clin Microbiol Infect 2011; 17:806-814
      • •  Most of the current guidelines do not use objective. transparent methods to grade evidence and recommendations, and
      • •  Do not disclose conflicts of interests
      • •  future IGRA guidelines must aim to be transparent, evidence-basea. periodically updated, and free of financial conflicts and industry involvement.

Conclusions

    • IGRAs will help identify who needs treatment for LTBI
    • Exact role need to be determined
        • • Very helpful in low risk TST +ve BCG population
      • •  ? immunosuppressed population
  • •  Useful for population that is hard to follow Definition of positive reaction may have to vary depending on situation of testing
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