New Technology in the Diagnosis of Tuberculosis

1
New Technology in the Diagnosis of Tuberculosis

• Gerry Guibert, Laboratory Director
• Monterey County Health Department
• July 27, 2008

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Role of the Public Health Laboratory

• The only BSL 3 laboratory in the county with a
  strong partnership with local hospitals in
  diagnosis of active TB.
• The only laboratory in the county using cell
  mediated blood test to detect LTBI.

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Re-emergence of TB and the quest for improved
diagnostics
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CDC guidelines for laboratory diagnosis of active
TB

• Process specimens and report fluorochrome smear
  within 24 hours of receipt
• Detect AFB by culture within 14-21 days.
• Use rapid identification systems to identify TB
  within 24 hours of detection in culture.
• Report drug susceptibility within 30 days of
  receipt.

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The Quantum LeapNucleic Acid Amplification

• NAA allows direct identification of TB within 24
  hours of specimen receipt.
• Well validated for respiratory specimens.
• Laboratories should only perform the test at the
  request of the physician in situations where
  there is a high suspicion of TB.
• Not a replacement for smear and culture
• Several specimens may need to be tested to
  achieve optimum sensitivity
• Laboratories should provide information on
  performance parameters (sensitivity, specificity
  and predictive value)

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Performance of NAA compared to culture
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A 100 year update to diagnosing latent TB
infection

• Tuberculin Skin Test
• In vivo (delayed hypersensitivity reaction)
• Undefined protein antigen (PPD)
• Two visits (Intradermal injection and reading at
  72 hrs).
• Inherent variation in administering and reading
  TST
• 22-30

• Interferon Gamma Release Assay (QuantiFERON)
• In vitro cytokine based immunoassay
• Defined synthetic peptide antigen
• One visit (requires phlebotomy and processing
  within 16 hours)
• In contrast to the TST, the performance and
  reading of the test is standardized
• 70 plus cost of phlebotomy

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Interferon Gamma Release Assays (IGRA)
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Species specificity of synthetic peptides
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Performance Comparison TST vs. QFT
Ref Menzies et. al. 2007. Meta-analysis New
Tests for the Diagnosis of Latent Tuberculosis
Infection Areas of Uncertainty and
Recommendations for Research. Ann Intern Med
146340-354
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Longitudinal study TST vs. QFT

• TST and QFT (In-Tube) compared in a study of
  exposed close contacts of active TB cases
  followed for 2 years
• 47 different culture positive source cases with a
  total of 601 contacts (contacts defined as having
  had 40 or more hours in closed rooms).
• 66/601 (11) contacts had positive QFT 243
  (40.4) had positive TST
• INH was only offered to QFT positive contacts 41
  declined treatment.
• Ref Diel et. al. 2008. Predictive value of a
  whole-blood IFN-? assay for the
  development of active TB disease. Am. J. Respir
  Crit Care Med.

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Predictive value (risk of progression to active
disease)

• Six of the 41 (14.6) QFT positive contacts who
  declined treatment progressed to active disease
• Five of 219 (2.3) TST positive contacts and
  untreated progressed to active disease
• None of the 181 subjects who were TST positive
  but QFT negative (and not given INH prophylaxis)
  progressed to active TB disease (after 2 years)
• Of the contacts who progressed to active disease
  5 (83) had positive TST and 6 (100) had
  positive QFT

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Which test costs more?
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How to access QFT testing

• Refer patient to PHL for testing. Appointments
  are recommended.
• or
• Collect blood in specialized tubes (hospitals)
  and deliver to PHL within 16 hours (if an
  incubator is available, incubated blood can be
  held up to 72 hours).
• or
• If a both an incubator and a centrifuge is
  available, plasma can be separated by
  centrifugation (after the incubation step) and
  stored in a refrigerator for up to 4 weeks.