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Quantiferon-Gold implementation: Beth Israel Deaconess Medical Center/MA State TB Lab collaboration


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Title: Quantiferon-Gold implementation: Beth Israel Deaconess Medical Center/MA State TB Lab collaboration

IGRAs for Diagnosis of Tuberculosis 2010
Nira Pollock, M.D., Ph.D. Division of Infectious
Diseases Beth Israel Deaconess Medical
Center Boston, MA May 1, 2010
Problems with the PPD
  • False positives
  • Recent BCG vaccine
  • non-TB mycobacteria (NTM)
  • False negatives
  • 25-30 patients with active pulmonary TB
    initially negative
  • Newborn/elderly, immunosuppression, renal
    failure, acute non-TB infection, etc
  • unable to distinguish active disease from past

Interferon-gamma Release Assays (IGRAs) basic
  • Expose T cells (isolated, or within whole blood)
  • TB antigens (in peptide form), vs
  • positive control antigen (mitogen, e.g.
    phytohemagglutinin A), vs
  • negative control (e.g. saline)
  • Incubate overnight
  • T cells (both CD8 and CD4) previously sensitized
    to these TB antigens in vivo release IFN-?
  • Mitogen stimulates cells non-specifically to
    release IFN-?? as control for general T-cell
  • Saline control defines level of background
    (should be low)
  • Quantify amount of IFN-??produced under each

IGRAs basic concepts, cont.
  • Theory overnight incubation detects sensitized
    effector T cells, i.e. already activated in
    vivo (longer incubation could activate resting
    central memory T cells also)
  • ? Primarily CD4 (Mack et al, 2009)
  • Like PPD, IGRAs are unable to distinguish between
    LTBI and active disease

Mack et al, TBNET consensus statement Eur Respir
J 2009
Quantiferon-TB-Gold (Cellestis,
Inc.)FDA-approved May 2005 for detection of LTBI
and TB disease
must incubate cells with antigen within 12
hours of collection
Quantiferon-TB Gold
  • Peptide antigens used in assay simulate two
    proteins specific to Mycobacterium tuberculosis
    complex (MTBC M. tuberculosis, M. bovis, M.
    africanum, M. microti, M. canettii)
  • ESAT-6, CFP-10 (genes coding for both are found
    within MTBC RD1 region, which is deleted in M.
    bovis BCG strain)
  • Eliminates false-positives due to BCG vaccination
    and to almost all NTM (exceptions M.
    kansasii, M. marinum, M. szulgai)

3rd generation QFT-Gold In Tube (IT)
  • FDA-approved October 2007
    longer making prior version!
  • Specimen collection draw whole blood directly
    into three proprietary 1 mL blood collection
  • 1) TB-specific Ag (dried onto wall of tube)
  • 2) Nil (-) control
  • 3) Mitogen () control (dried onto wall of tube).
  • TB-specific peptide antigens ESAT-6, CFP-10,
    TB7.7. Goal of adding extra antigen increase
    sensitivity. (Like ESAT-6/CFP-10, TB 7.7 is not
    present in BCG strains and most NTM.)

QFT-Gold IT (continued)
  • Must not under or over-fill tube. Shake 10x
    vigorously after draw. Keep at room temp.
  • Must put at 37ºC within 16h of collection.
  • Incubate upright at 37ºC for 16-24h
  • Tubes can then be held at 2-27ºC for up to 3 days
    prior to centrifugation (so can ship at room
  • Centrifuge 15 to separate plasma from cells,
    remove gt150 ?L plasma to assay (can store spun
    tube or plasma at 4ºC for 28 days).
  • Quantify IFN-? in plasma by ELISA, as for QFT-G
  • IT test format allows o/n incubation at site of
    draw (e.g. hospital or clinic), vs central
    testing center
  • QFT-G IT is being done at the Hinton State Lab
    (contact them to obtain tubes and arrange
    submission) also offered at e.g. Quest

(No Transcript)
QFT-G IT, continued
  • Quantification of IFN-? ELISA, as for QFT-G
  • Results readout positive, negative, or
  • Ideally, lab should report absolute value result
    in IU/mL, so that clinician can evaluate how
    close absolute value is to the cutoff
  • Lab should also report reason for indeterminate
  • Low mitogen response insufficient or
    dysfunctional lymphocytes, reduced lymphocyte
    activity due to prolonged specimen transport,
    improper specimen handling
  • High background in nil control heterophile
    antibodies (interfering human anti-mouse
    antibodies), intrinsic IFN-gamma secretion (?
    recent vaccination, ? just true for some
    people--1-2 of population per Cellestis website)

QFT-G IT results interpretation
Note for QFT-G this value was gt50 seems that
new cutoff would generate more positives
Note for QFT-G nil cutoff was 0.7 IU/mL seems
that new cutoff of 8.0 would generate a lot fewer
T-Spot.TB (Oxford Immunotec Elispot
technology)FDA-approved July 2008
  • in vitro diagnostic test based on an
    enzyme-linked immunospot (ELISPOT) method
  • enumerates M.tuberculosis-sensitized effector T
    cells responding to stimulation with a
    combination of peptides simulating ESAT-6 and
    CFP10 antigens, by capturing interferon-gamma
    (IFN-?) in the vicinity of T cells from which it
    was secreted

Each spotone reactive effector T-cell
TSpot.TB results interpretation
  • Positive (ESAT-6-Nil) and/or (CFP-10-Nil) are gt
    8 spots.
  • (note this cutoff used to be 6 spots)
  • Negative both (ESAT-6-Nil) and (CFP-10-Nil) are
    lt 4 spots.
  • (includes values less than zero).
  • Borderline (equivocal) highest (TB antigen-Nil)
    spot count is 5, 6 or 7 spots
  • Collect a new specimen and retest
  • Indeterminate
  • nil control count is gt10 spots, OR
  • mitogen control count is lt20 spots and (TB
    Ag-nil) counts are lt4 spots

Doing T-Spot in MA
  • Oxford Immunotec has a testing facility in
    Marlborough (since July 2009) CLIA/CAP
  • Specimens (blood only) must be shipped at room
    temp day of draw and have 32 hours to reach
    testing center (package insert says 8 hours, but
    Oxford has validated longer time frame) contact
    Oxford for details (tubes, shipping)

Assessing the accuracy of IGRAs
  • General principles used to date
  • Sensitivity approximated by measuring
    proportion of positive tests in patients with
    culture-confirmed active TB
  • Specificity approximated by measuring proportion
    of negative tests in patients with low risk for
    TB infection
  • Problem no confirmatory test exists for
    diagnosis of LTBI or culture-negative TB disease
    (no gold standard!)

QFT-G IT package insert (Jan 2009)
  • Sensitivity in culture-confirmed active TB (all
    with lt8days treatment prior to testing)
  • Japanese study (n92) QFT-G IT 93.5, QFT-G
  • Australian study (n27) QFT-G IT 88.9, QFT-G
  • US study (n44) QFT-G IT 84.1, QFT-G 77.3
  • Overall sensitivity QFT-G IT 89, QFT-G 81
  • Specificity in subjects at low reported risk for
    TB infection (US study subjects had no reported
    TB risk factors, and none had BCG history)
  • Overall specificity QFT-G IT 99.2, QFT-G
    99.8, TST 99.1

QFT-G IT package insert (Jan 2009)
  • Cautions that the performance of the USA format
    of QFT-G IT has not been extensively evaluated
  • Individuals who have impaired or altered immune
    function such as HIV infection/AIDS, s/p
    transplantation managed with immunosuppressive
    treatment, patients on immunosuppressive drugs
    (e.g. corticosteroids, methotrexate,
    azathioprine, cancer chemotherapy)
  • Patients with the following clinical conditions
    diabetes, silicosis, chronic renal failure,
    hematological disorders (e.g., leukemia and
    lymphomas), and other specific malignancies
    (e.g., carcinoma of the head or neck and lung).
  • Individuals younger than age 17 years
  • Pregnant women

Review of TSpot.TB FDA approval document/PI (July
  • Sensitivity in culture-confirmed active disease
  • 95.6 using gt6 spots, 90.7 using gt8
  • Specificity (used individuals with no TB risk
    factors and negative TST) (n306)
  • 97.1 using gt6 spots, 99.0 using gt8 spots
    (i.e. if equivocal (5,6, or 7 spots) are
    counted as negative).
  • Equivocal or borderline result (TB Ag-nil
    5,6, or 7 spots) represents the area of overlap
    between results obtained for culture-confirmed
    positive samples and low risk TB negative samples
  • Note Oxford Immunotec website (4/12/10) quotes
    95.6 sensitivity and 97.1 specificity, but
    these are for gt6 spot cutoff, whereas current
    version uses gt8 spot cutoff and equivocal range.

TSpot.TB FDA approval/PI (July 2008) clinical
  • Goal include subjects from all major risk
    groups indicated for TB screening by CDC
    guidelines (including those with potential for
    false positive/negative TST)
  • TSpot.TB vs TST evaluated in typical candidates
    for routine LTBI screening, with various risk of
    exposure and progression (n1403) (NOTE used gt6
    spot cutoff)
  • Included 328 HIV, 229 recent contacts, 122
    drug-induced immunosuppression, 97 IVDU, 108 DM,
    195 ESRD. Many BCG-vaccinated and foreign-born.
    93 children/adolescents.

TSpot.TB FDA approval/PI clinical
studiesaggregate results (not by clinical
  • After controlling for the other variables,
    positive results for both T-SPOT.TB and TST were
    significantly associated with history of prior TB
  • A positive result for T-SPOT.TB was significantly
    associated with contact with infectious TB and
    birth in a TB endemic country no such
    association observed for TST.
  • A positive TST was associated with BCG
    vaccination no such association observed for
  • A negative TST was associated with being
    immunocompromised no such association observed
    for T-SPOT.TB
  • TSPOT.TB results were not impacted by age

TSpot.TB FDA approval document/PI (continued)
  • Notes theoretical cross-reaction (false-positive
    test) with M. kansasii, M.szulgai, M. marinum, M.
    xenopi, M gordonae (latter two not mentioned in
    QFT-G IT PI). However, actual data obtained in a
    very small of patients12 with MAC (all
    negative with TSpot), 1 with xenopi (positive), 4
    with gordonae (all positive), 1 with kansasii
    (positive). (note no marinum..)
  • The performance of this test has not been
    adequately evaluated with specimens from
    individuals younger than age 17 years, in
    pregnant women and in patients with hemophilia

Direct comparisons of QFT-G IT, TSpot.TB, and
TST meta-analysis
  • Diel et al, Chest 2010
  • Evaluated comparative sensitivity in studies of
    subjects with active TB confirmed by culture
    and/or PCR and/or histologic evaluation, treated
    for lt2 weeks
  • Evaluated comparative specificity for LTBI in
    studies of subjects who were healthy, native
    residents of low-incidence countries without any
    previously known exposure to TB, irrespective of
    BCG vaccination status.
  • Evaluated indeterminate rates (though no apparent
    distinction between indeterminates due to high
    background, vs low mitogen)
  • Included studies that evaluated immunosuppressed
  • Note cutoff for TSpot.TB was gt6 spots in all
    included studies, which as discussed is different
    than FDA-approved version

Diel et al metaanalysis, cont
  • Pooled sensitivities in active TB
  • TST 69.9
  • QFT-G IT 81.
  • Note that in studies done in developing
    countries, sensitivity was 74.3, vs 84.5 in
    developed countries. (Is this difference due to
    HIV co-infection, malnutrition, or other
  • TSpot.TB 87.5.
  • Majority of studies done in developed countries
    sensitivity in that subgroup was 88.5
  • Pooled specificities in low-risk subjects
  • QFT-G IT (5 studies) 99.2
  • TSpot.TB (3 studies) 86.3
  • Pooled rates of Indeterminates
  • QFT-G IT 2.1. In immunosuppressed subgroup
  • TSpot.TB 3.8. In immunosuppressed subgroup

IGRA performance in specific groups of interest
e.g. contacts of active TB cases
  • Overall consensus, IUATLD NAR meeting, Vancouver,
    2007 overall both IGRAs performing well (and
    comparably) in contact investigations
  • Tspot.TB and QFT-G (including IT version) results
    correlate better than TST results with exposure
    to MTB1, 2
  • Direct comparison TSpot.TB vs QFT-IT vs TST,
    20093 both IGRAs appeared to indicate LTBI more
    accurately than TST, and IGRAs agreed well
  • Suggests that IGRAs may be as or more sensitive
    than TST for recently acquired infection (in

(1 Richeldi, AJRCCM 2006 2 Arend et al, AJRCCM
2007 3 Diel et al, Chest 2009)
e.g. HIV Data mixed can use IGRAs, but watch
for indeterminates, particularly at low CD4
  • QFT-G IT e.g. Brock et al, 2006, Denmark
    indeterminates correlated with low CD4 (24 in
    pts with CD4lt100).
  • ELISPOT assays1 overall perform better than TST.
    E.g. Dheda et al, 2005 T-Spot.TB in
    HIV-positive pts w/o other TB risk factors
    technical performance independent of CD4 count.
    However, another study found more indeterminates
    with Tspot vs QFT-G (Stephan et al, 2008)
  • Tspot may be more sensitive than QFT-G in this
    population (Mandalakas et al, 2008, small study
    in S. African patients)

1Kimura et al 1999 Chapman et al, 2002 Carrara
et al, 2004, Dheda et al 2005
HIV, continued
  • Diagnosis of active TB in HIV
  • QFT-G IT might be a sensitive tool for
    detection/prediction of active TB in HIV
    (Aichelburg et al, CID 2009), or NOT (Aabye et
    al, PLoSONE 2009)
  • Cattamanchi et al, BMC ID 2010 TSpot.TB in 236
    HIV active TB suspects in Uganda mean CD4 of
    49. 126 patients diagnosed with active TB by
    culture. 10 of subjects had insufficient
    mononuclear cell counts for TSpot assay. Of
  • 25 had indeterminate results
  • IGRA sensitivity was 73
  • Proportion of positive test results was similar
    across CD4 count strata
  • IGRA results did not meaningfully alter the
    probability of active TB in patients with
    negative sputum smears
  • If IGRA sensitivity might be lower in HIV
    subjects (vs immunocompetent) with active TB
    (recall also Diel metaanalysis), what does this
    mean re sensitivity for LTBI?

Immunocompromised patients
  • IGRAs (vs TST) do allow optimization of
    experimental conditions in vitro, e.g. incubation
    time or adjustment of cell numbers, allowing
    potential for higher sensitivity. However,
    studies are as usual limited by lack of gold
    standard for LTBI.
  • Overall IGRAs seem to work, but true sensitivity
    for LTBI unknown.
  • In earlier studies, QFT-G had higher rate of
    indeterminates (low mitogen control) than
    TSpot.TB (Ferrara et al, AJRCCM 2005 (Italy)
    Piana et al, AJRCCM 2006 (Italy) Ferrara et al,
    Lancet 2006 (Italy))
  • More recent metaanalysis (Diel et al, 2010)
    rates of indeterminates (note reason for
    indeterminate not defined) in immunosuppressed
  • QFT-G IT 4.4
  • TSpot.TB 6.1
  • Occasional case reports of IGRAs being used to
    help with Dx of active TB in TST-negative
    immunosuppressed patients
  • Disturbing case report of person who was QFT-G
    negative before liver transplant AND in setting
    of post-transplant active (Cx-positive) pulmonary
    TB (Codeluppi et al, 2006)

e.g. health care workers (HCW) depends where you
are and what question you ask. For example
  • Japan (Harada, 2006) QFT-G vs TST
  • 95 s/p BCG. 93 TSTgt10mm, vs 10 QFT-G.
    QFT-G results were a/w LTBI risk factors, while
    TST results were not.
  • Rural India (Pai, 2005) QFT-IT vs TST
  • 50 positive by either test, 31 by both
  • Russia (Drobniewski, 2007) QFT-IT
  • QFT-IT was positive in 8.7 of medical/non-medical
    students, 39.1 of all doctors/nurses, 46.9 of
    TB doctors and nurses
  • Denmark (Soberg, 2007) QFT-G vs TST
  • ID dept employees 34 TST, 1 QFT-G. 89 of
    TST were BCG-vaccinated.
  • Urban US (Pollock, 2008) QFT-G
  • In TST newly hired employees with increased risk
    of having LTBI (large PPD, residence in highly
    endemic area, recent or remote contact,
    conversion, CXR findings c/w old TB, patient
    care) 28 QFT-G, 70 QFT-G-
  • Many more.mostly descriptive (TST results vs
    IGRA results)

Patients approaching TNF-alpha blocker therapy
  • The problem many have underlying diseases or
    are on immunosuppressive medications which can
    compromise TST sensitivity. But how sensitive
    are the IGRAs in this group? Again, limited by
    lack of a gold standard.
  • E.g. Laffitte et al, Br J Dermatol 2009
    retrospective study of TST vs T-Spot.TB in 50
    patients with psoriasis considering TNF-alpha
    blocker (in Switzerland)
  • Positive TSpot was strongly a/w presumptive Dx of
    LTBI (by risk factors), while TST was not
  • 20 of subjects had positive TST and negative
    TSpot and were NOT treated for LTBI no
    reactivation detected with median f/u of 64 weeks
    (but note, small numbers overall)
  • E.g. Diel et al, Pneumologie 2009 (German
    recommendations) due to expectation of false
    negative AND false positive TST in these
    patients, they recommend highly specific IGRA
    instead (but what about IGRA sensitivity??)

  • Lewinsohn, Lobato, and Jereb, Curr Opin
    Pediatrics 2010
  • Overall, performance of IGRAs equivalent or
    superior to that of the TST, but evidence
    supports usage of IGRAs in children aged 5 years
    or older only (insufficient evidence re
    performance in younger kids, and sensitivity
    poorly defined in that group)
  • In kids gt5, IGRAs preferred over TST when
    specificity is paramount or when patients might
    not return for TST reading
  • Kids lt5 TST preferred
  • E.g. Bianchi et al, Pediatr Infect Dis J 2009
  • QFT-G IT was positive in 15 of 16 (93.8)
    children with active pulmonary TB
  • Among IGRA children (excluding active TB), TST-
    were significantly younger than TST children (so
    could IGRA be more sensitive than TST in younger

Are CFP-10, ESAT-6, /- TB7.7 sufficient for
comprehensive detection of LTBI?
  • Overall in contact investigations, sensitivity
    of IGRATST, and IGRAs correlate better with TB
  • For active TB, sensitivity of IGRAs or gt to TST
  • Could IGRAs be sensitive to recent/active
    infection, but not remote infection? (Pollock et
    al, ICHE 2008)

Relying on IGRAs for making clinical decisions
how much caution should we use at this point?
  • if we base Tx decisions on IGRA results alone,
    many individuals with clinical risk factors
    historically considered suggestive of true LTBI
    will suddenly be exempt from treatment. Is this
    good or bad?
  • AND, some of these risk factors have historically
    been associated with increased reactivation risk
    (e.g. PPDgt15 mm, recent immigration from high
    risk country, various CXR findings)
  • But can IGRAs actually distinguish those at
    higher reactivation risk? Should we only care
    about the IGRA?

Studies of predictive value of IGRAs
  • Hard to do studies of predictive value of
    positive IGRA for development of active
    TBtypically, ethically would need to consider
    treatment of LTBI if IGRA.
  • E.g. Diel et al, AJRCCM 2008, Germany evaluated
    rates of progression to active TB in close
    contacts (immunocompetent) within 2 years of
    contact screening.
  • 11 of contacts were QFT-G IT, vs 40 TST (gt5
  • 41 QFT-G IT subjects refused LTBI treatment 6
    (14.6) developed active TB. 219 TST subjects
    refused treatment 5 (2.3) progressed to active
    TB. Concluded that QFT-G IT is a more accurate
    indicator of LTBI than the TST and provides at
    least the same sensitivity for detecting those
    who will progress to active TB.
  • Vs. e.g. Kik et al, Eur Respir J 2009 looked at
    immigrants who were close contacts of smear TB
    cases, all found to have TST gt5 mm during contact
    investigation followed for 2 years.
  • PPV for progression to TB disease was comparable
    and LOW for QFT-G IT (2.8), T-Spot TB (3.3),
    TSTgt10 mm (3.1), TST gt15 mm (3.8)

Predictive value of IGRAs, cont
  • E.g. Hill et al, PLoS One 2008, The Gambia risk
    of progression to active TB after positive
    ELISPOT (similar to TSpot) or TST in case
    contacts, over 2 year period. Noone got
    preventive therapy, per local guidelines.
  • Rates of progression in ELISPOT was similar to
    rates in TST.
  • Because initial ELISPOT and TST were each
    positive in just over half of secondary cases,
    while 71 were initially positive by one or the
    other test, they concluded that positivity by
    either might be the best indication for
    preventive treatment.
  • Note there were clearly some NEW infections over
    study time period (discordant genotyping between
    index and secondary case isolates) so this really
    confuses this study.
  • San Francisco IGRA experience--?? Not seeing
    spike in TB cases after switching to IGRA only
    for TB screening programs..

Our clinical response to all this data We feel
great about the IGRA. Were just not sure what
to do with all the IGRA-
  • We dont assume (for now) that a negative IGRA
    rules out LTBI. Perhaps, in future, we can be
    confident that it doesor, at least, that it
    rules out high baseline reactivation risk.
  • Consider offering treatment to certain high-risk
    populations even with a negative IGRA result
  • 1. Patients with medical risk factors placing
    them at higher risk of TB reactivation if they do
    have LTBI, i.e. HIV, chronic oral steroid
    treatment, TNF-alpha blocker treatment, renal
    insufficiency, diabetes, some malignancies.
  • 2. recent TB contact (debatable, given good IGRA
    performance in contact studies)
  • 3. PPD conversion (gt10 mm increase) in past 2
    years (also debatable, given performance in
    contact studies)
  • 3. Abnormal CXR potentially consistent with old
    TB in significant burden (e.g. large scar,
    nodule, after r/o with smear/culture)

FAQ Do positive IGRA results turn negative with
TB or LTBI treatment?
  • Multiple studies on this topic data mixed, but
    general consensus is NO, not reliably.
  • E.g. local study Pollock et al, ICHE 2009 HCW
    treated for LTBI with 9 months INH still had
    positive QFT-G after treatment.
  • Suggested approach to this issue based on current
  • IGRA results should not be used to assess the
    effectiveness of recent or remote treatment
    courses for TB/LTBI many (if not most)
    individuals will continue to test positive after
    standard therapy
  • Do not assume that an individual who reports
    prior TB/LTBI therapy but still tests positive by
    IGRA has not been appropriately treated in the
  • Neither providers nor patients should expect
    reliable changes in IGRA results after standard

Serial testing with IGRAs
  • Primarily relevant to HCW or other individuals
    requiring annual screening
  • Multiple issues to think about
  • reproducibility of test results in a given
    individual tested repeatedly over time, without
    intervening exposures to TB
  • appropriate definition of reversion/conversion
  • optimal test cutoffs
  • (e.g. initially raised by Pai et al, 2006,2009,

From QFT-G IT package insert
  • The magnitude of the measured IFN-g level cannot
    be correlated to stage or degree of infection,
    level of immune responsiveness, or likelihood for
    progression to active disease.

Reproducibility of IGRA results in serial testing
  • E.g. Detjen et al, Clin Vaccine Immunol 2009 27
    S. African HCW, tested with QFT-G IT on day 1 (2
    tests, by different operators) and day 3 (1
  • 6/27 had discordant results of some kind
  • variability in the magnitude of IFN-gamma
    responses between assays performed for a given
  • most variability seen in assays that were
    obtained from an individual on two different
  • Conclusion This intra-individual variability
    could influence interpretation of serial
  • E.g. Van Zyl-Smit, AJRCCM 2009 26 S. African
    subjects repeated IGRAs (T.SpotTB, QFT-G IT) 4x
    over 21D prior to TST (to assess within-patient
    variability), and then again on days 3,7,28, 84
    post-TST (to assess for boosting of IGRA by TST).
  • Pre-TST tests 7/26 had spontaneous
    conversions/reversions (6 for TSpot, 1 for QFT-G
    IT). 95 of variability was 3-spot or 80
    IFN-gamma response variation on either side of
    baseline valuescould be useful for interpreting

Effect of TST on IGRA results
  • QFT-G IT package insert in U.S. specificity
    study (individuals with no reported TB risk
    factors), a subset of subjects were retested 4-5
    weeks after initial QFT-G IT/TST. Agreement
    between 2 QFT-G IT tests was 98.5 (out of 530
    subjects, 5 went pos?neg, and 3 went neg?pos.)
  • Van Zyl-Smit, AJRCCM 2009 26 S. African
    subjects after baseline IGRAs (T.SpotTB, QFT-G
    IT), repeated IGRAs on days 3,7,28, 84 post-TST
    (to assess for boosting of IGRA by TST).
  • Post-TST tests 8 subjects boosted above
    defined baseline variability by day 7, but not
    day 3. 2 initially IGRA-negative subjects
    converted to IGRA-positive.
  • Conclusion safe to do QFT-G IT or TSpot within
    3 days of performing TST (i.e. on day of TST
  • Cohort as a whole showed some persistently
    elevated IFN-gamma responses up to day 84 after
    TST, though some individuals had returned to pre-
    TST levels by day 28.(So what are implications
    for long-term boosting effects, e.g. in those
    receiving annual testing?)

Effect of TST on IGRA, continued
  • Review by van Zyl-Smit et al, PLoSOne 2009 13
  • Studies used different TU for TST, different time
    points for IGRAs after TST, and varied re
    initial TST/IGRA status of individuals
  • 5 studies concluded boosting of IGRA by TST did
    NOT occur in 4/5, earliest timepoint of repeat
    IGRA was 28 days-9 months after TST. In 5th,
    IGRA was repeated only on day 3 after TST.
  • 7 studies demonstrated TST-induced boosting
    of IGRA responses in 5/7, repeat IGRA was done
    within 21 days after TST.
  • Conclusions
  • Boosting more pronounced in IGRA-positive (i.e.
    sensitized) individuals, but also occurred in a
    smaller but not insignificant proportion of
    IGRA-negative subjects
  • Time frame of repeat IGRA is key. TST appeared
    to affect IGRA responses only after 3 days, and
    may be issue particularly between days 7-28
    boosting effect may apparently persist for up to
    3 months and then wane, but evidence for this is

Preliminary (unpublished) data from a 4-site
(U.S.) collaborative study of serial IGRAs in HCW
  • Longitudinal study of HCW undergoing routine
    testing for LTBI overall low risk for TB
    acquisition at work
  • 15 born in high-burden country
  • 10 s/p BCG
  • 0.4 HIV, 3 DM, 2 other immunocompromise
  • Baseline 2-step TST, QFT-G IT, TSpot.TB
  • IGRAs done BEFORE placement of 1st TST
  • Repeat all 3 tests at 6, 12, and 18 months

Slides obtained from Dr. John Bernardo, BMC
Baseline Results in subjects with no prior ()
TST or LTBI treatmentn 2083
Positive 43 (2.1) 76 (3.7) 108 (5.2)
Negative 2040 (97.9) 2007 (96.3) 1907 (91.6)
Borderline 68 (3.3)
p lt 0.0001 compared to the TST (borderline
T-Spots categorized as negative)
6 month Follow-up
Conversion Reversion
TST 6 / 1503 (0.4) 11 / 21 (52.4)
QFT-GIT 56 / 1516 (3.7) 28 / 56 (50)
T-SPOT 52 / 1473 (3.3) 47 / 85 (55.3)
Conversion (-) baseline () 6 month Reversion
() baseline (-) 6month
Total Baseline Positive 43 TST, 76 QFT-GIT,
108 T-SPOT
12 month Follow-up
Conversion Reversion
TST 1 / 362 (0.3) n/a
QFT-GIT 9 / 384 (2.3) 7 / 11 (63.6)
T-SPOT 3 / 356 (0.8) 10 / 16 (62.5)
Conversion (-) baseline (-) 6 month () 12
month Reversion (-) baseline () 6month (-)
12 month
Some take-home points
  • IGRAs should not be used alone to exclude the Dx
    of active TB
  • In particular, sensitivity in question for
    extra-pulmonary TB1
  • IGRAs cannot distinguish between active and
    latent TB
  • IGRAs may remain positive even after appropriate
    treatment of active or latent TB.
  • Sensitivity for diagnosis of LTBI is impossible
    to calculate, given absence of a gold standard
    for this Dx. Exercise caution when interpreting
    negative IGRA results in individuals with major
    risks for TB reactivation.
  • A negative result must be considered with the
    individuals medical and historical data relevant
    to probability of M. tuberculosis infection and
    potential risk of progression to tuberculosis
    disease, particularly for individuals with
    impaired immune function. (QFT-G IT package
    insert, 2009)

1. Dewan et al, CID 2007
Some take-home points, cont.
  • Specificity of IGRAs is very high, but
    occasionally you will see a patient with NO
    apparent TB risk factors and a positive IGRA
  • Check absolute value to see if they are close to
    cutoff for positive
  • would repeat, if negative repeat again as
  • Again, consider who should be tested in the first
    place, and who shouldnt
  • It is still not clear how well IGRAs will perform
    in serial testing situations (e.g. HCW) or what
    the true impact of TSTs on subsequent IGRAs
    actually is. Can we trust conversions if IGRAs
    are used for annual testing in relatively low
    risk settings? Would those conversions be
    stable if we waited 6 months and retested?

December, 2005 CDC guidelines for use of QFT-G
  • CDC recommends that QFT-G may be used in all
    circumstances in which the TST is currently used,
    including contact investigations, evaluation of
    recent immigrants, and sequential-testing
    surveillance programs for infection control
    (e.g., those for health-care workers).
  • left open the possibility that "QFT-G sensitivity
    for LTBI might be less than that of the TST,"
    while acknowledging that the lack of a
    confirmatory test would make this difficult to
  • "each QFT-G result and its interpretation should
    be considered in conjunction with other
    epidemiologic, historic, physical, and diagnostic

New CDC recs for use of IGRAs in
developmentcoming in 2010!!??
  • Likely to advocate broad use (including in annual
    testing), and use in place of TST, rather than as
    confirmatory test.
  • My opinion if we are going to make clinical
    decisions based on IGRA results, then we need to
    focus on estimating IGRA sensitivity/NPV for LTBI
    and also potentially revisit the clinical
    guidelines regarding increased reactivation risk.
    What will we do with TST/IGRA- individuals who
  • Have various forms of relative immunocompromise,
    or are going to become immunocompromised (e.g. by
    transplant, TNF-alpha blockers)?
  • Are recent immigrants from endemic areas?
  • Have CXR findings consistent with past TB (and
    which CXR findings, specifically, matter?)

MACET recommendations on use of IGRAs 6-13-08
  • Recent contacts IGRAs seem to perform well
    (good sensitivity and correlation with TB
    exposure) can use IGRA or TST
  • Immunocompromised two groups
  • Pre-immunocompromisation (awaiting transplant,
    going on TNF-alpha blocker or steroids, etc)
    use both tests, Tx if either positive
  • Already immunocompromised (including HIV) same

MACET recs 2008, cont.
  • Recent immigrants panel unable to reach
    consensus, as negative test does not appear to
    rule out LTBI, and some of this population could
    be recently infected. Clinical f/u after testing
    is optimal.
  • HCW same caveats as above. Agreed that either
    IGRA or TST could be used. Those with key
    reactivation risk factors who are IGRA negative
    should have clinical f/u if possible.

MACET recs 2008, continued
  • Children limited data, no recommendations
    (could update, given recent analyses suggesting
    good performance in kidsgt5)
  • Low-risk individuals given low pre-test
    probability, test results difficult to interpret.
    Best to NOT test with either IGRA or TST.
  • Adults with recent BCG IGRAs can be helpful
    given high specificity
  • Active TB can use IGRA to rule IN infection
    (either latent or active), but NOT to rule OUT
    active disease (given limits to sensitivity)
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