Transcript Quantiferon-Gold implementation: Beth Israel Deaconess

IGRAs for Diagnosis of
Tuberculosis:
2010 Update
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 exposure
Interferon-gamma Release Assays
(IGRAs):
basic concepts
• Expose T cells (isolated, or within whole blood) to:
– 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 anergy
– Saline control defines level of background (should be
low)
• Quantify amount of IFN-produced under each condition
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
• NOW FORMALLY REPLACING 2nd GENERATION—
company no longer making prior version!
• Specimen collection: draw whole blood directly into
three proprietary 1 mL blood collection tubes:
– 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 temp).
• Centrifuge 15’ to separate plasma from cells, remove
>150 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
QFT-G IT, continued
• Quantification of IFN-: ELISA, as for QFT-G
– Results readout: positive, negative, or
indeterminate
– 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--12% of population per Cellestis website)
QFT-G IT results interpretation
Note: for QFT-G this value was >50%;
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
indeterminates..
T-Spot.TB (Oxford Immunotec; Elispot
technology)
FDA-approved July 2008
• in vitro diagnostic test based on an enzymelinked 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 interferongamma (IFN-γ) in the vicinity of T cells from
which it was secreted
T-Spot.TB
Each spot=one reactive effector T-cell
TSpot.TB results interpretation
• Positive: (ESAT-6-Nil) and/or (CFP-10-Nil) are > 8 spots.
– (***note: this cutoff used to be 6 spots)
• Negative: both (ESAT-6-Nil) and (CFP-10-Nil) are < 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 >10 spots, OR
– mitogen control count is <20 spots and (TB Ag-nil) counts are <4
spots
Doing T-Spot in MA
• Oxford Immunotec has a testing facility in
Marlborough (since July 2009): CLIA/CAP
certified
• 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 cultureconfirmed 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
<8days treatment prior to testing):
– Japanese study (n=92): QFT-G IT 93.5%, QFT-G 83.7%
– Australian study (n=27): QFT-G IT 88.9%, QFT-G 74.1%
– US study (n=44): 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 in:
– 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 2008)
• Sensitivity in culture-confirmed active disease (n=183):
95.6% using >6 spots, 90.7% using >8 spots.
• Specificity (used individuals with no TB risk factors and
negative TST) (n=306):
97.1% using >6 spots, 99.0% using >8 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 >6 spot cutoff,
whereas current version uses >8 spot cutoff and equivocal
range.
TSpot.TB FDA approval/PI (July 2008):
clinical studies
• 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 (n=1403) (NOTE: used >6 spot
cutoff)
• Included 328 HIV+, 229 recent contacts, 122 druginduced immunosuppression, 97 IVDU, 108 DM, 195
ESRD. Many BCG-vaccinated and foreign-born. 93
children/adolescents.
TSpot.TB FDA approval/PI clinical studies:
aggregate results (not by clinical subgroup)
• After controlling for the other variables, positive
results for both T-SPOT.TB and TST were
significantly associated with history of prior TB
infection.
• 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 T-SPOT.TB
• 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 patients—12 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 <2 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 subjects
• Note: cutoff for TSpot.TB+ was >6 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
factors?)
– 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: 4.4%
– TSpot.TB: 3.8%. In immunosuppressed subgroup: 6.1%
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
immunocompetent)
(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
CD4<100).
• 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
remainder:
• 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”
subgroups:
– 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% TST>10mm, 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 BCGvaccinated.
• 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??)
Children
• 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 >5, IGRAs preferred over TST when specificity is
paramount or when patients might not return for TST reading
– Kids <5: 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 kids?)
Are CFP-10, ESAT-6, +/- TB7.7
sufficient for comprehensive detection of
LTBI?
• Overall: in contact investigations,
sensitivity of IGRA=TST, and IGRAs
correlate better with TB exposure
• For active TB, sensitivity of IGRAs = or
> 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. PPD>15 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 TB—typically, 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+ (>5 mm).
– 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 >5 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%), TST>10 mm (3.1%), TST >15 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+. We’re just
not sure what to do with all the IGRA-…
• We don’t assume (for now) that a negative IGRA
rules out LTBI. Perhaps, in future, we can be
confident that it does—or, 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 (>10 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 data:
– 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 past
– Neither providers nor patients should expect reliable changes
in IGRA results after standard treatment
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,
India)
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 test).
– 6/27 had discordant results of some kind
– variability in the magnitude of IFN-gamma responses between
assays performed for a given individual
– most variability seen in assays that were obtained from an
individual on two different days.
– Conclusion: This intra-individual variability could influence
interpretation of serial measurements
• 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 values—
could be useful for interpreting conversions/reversions
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 read).
– 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
– 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 weak.
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 treatment
n = 2083
TST
QFT*
T-SPOT*
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 < 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 individual’s
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 result….
– Check absolute value to see if they are close to cutoff for
positive
– would repeat, if negative repeat again as tie-breaker..
– Again, consider who should be tested in the first place, and who
shouldn’t
• 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 healthcare 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 assess
• "each QFT-G result and its interpretation should be
considered in conjunction with other epidemiologic,
historic, physical, and diagnostic findings."
New CDC recs for use of IGRAs
in development…coming 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 recommendations
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 kids>5)
• 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)