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IGRA / Tuberculin Skin TestDr.Haifa Naser


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Interferon-gamma release assays (IGRAs) are diagnostic tools for

latent tuberculosis infection (LTI).

T!ey are surrogate mar"ers of #ycobacterium tuberculosis

infection and indicate a cellular immune res$onse to #.

tuberculosis.

IGRAs


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IGRAs cannot distinguis! bet%een latent infection and acti&e

tuberculosis (T) disease' and s!ould not be used as a sole met!odfor diagnosis of acti&e T' %!ic! is a microbiological diagnosis.

A $ositi&e IGRA result may not necessarily indicate acti&e T

!o%e&er' a negati&e IGRA result rules out t!e $ossibility of bot!acti&e and latent tuberculosis.


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ecause IGRAs are not affected by acille almette-Gu*rin

(G) &accination status' IGRAs are useful for e&aluation of

LTI in G-&accinated indi&iduals' $articularly in settings

%!ere G &accination is administered after infancy or multi$le(booster) G &accinations are gi&en.

In contrast' t!e s$ecificity of tuberculin s"in test (T+T) &aries

de$ending on timing of G and %!et!er re$eated (booster)&accinations are gi&en.


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QuantiFERON

also "no%n as ,T' is t!eregistered trademar" of t!e test

for tuberculosis infection or

latent tuberculosis.

,T is an interferon- release

assay (IGRA) used in

tuberculosis diagnosis.


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T!e ,T-GIT assay is an /LI+A-based' %!ole-blood test t!at uses

$e$tides from t!ree T antigens (/+AT-0' 1-23' and T4.4) in an

in-tube format.

T!e result is re$orted as 5uantification of IN-gamma in international

units (I6) $er mL. An indi&idual is considered $ositi&e for #.

tuberculosis infection if t!e IN-gamma res$onse to T antigens is

abo&e t!e test cut-off (after subtracting t!e bac"ground IN-gammares$onse in t!e negati&e control).


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QuantiFERON-TB Gold Test .

T-+17T-TTest .

Commercially Available IGRAsTests


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om$arison

of commercially a&ailable IGRAs

,uanti/R7N 8T Gold

ollect 2ml of blood into

eac! of t!ree tube9 nil

control /+AT-0 :1-

23:T4.4 .

Incubate for 20-;< ! at =4

c co; not re5uired .

T +17T-T ollect ml blood in lym$!cyte

se$aration tube . ertrifuge for =3 min.

ollect 1# layer and add to 23 ml

sterile culture media .

?as! 1# t%ice by resus$ension and

centrifugation 4 min .

+tain and count &iable cells in culture

media .

Add cells and anti gens /+AT-0:1-23

to coated microtiter $late.

Incubate for 20-;3 ! at =4 c co;

incubator re5uired .


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om$arison of commercially a&ailable IGRAs

QuantiFERON-TB Gold

Har&est $lasma @ centrifuge tube 2

minutes o$tional .can store sam$les for u$

to > %ee"s at < c.

Add $lasma and conBugate to /LI+A $late

incubate for 2;3 min at room tem$erature. ?as! microtiter $late and add substratefor

=3 minutes.

Add sto$ reagent and reed o$tical densities.

+oft%are calculates and $rint results.

T !OT-TB

"as! off cells and add conBugate to/LI+17T $late . Incubte for 03 min at

room tem$erature .

?as! microtiter $late and add

$reci$tating substrate for 4 minutes .

?as! $late dry for = ! and count s$ots

&isually or using an /LI+17T $late read.

+oft%are %it! reader calculates results .


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,uanti/R7N-T (,T).

,uanti/R7N-T Gold In-Tube (,T-GIT)' t!e t!ird

generation test' !as re$laced ,uanti/R7N-T (,T)

,uanti/R7N-Gold' %!ic! are no longer mar"eted.According

to t!e 6.+. enters for Disease ontrol'@2C in ;332' t!e

,uanti/R7N-T test (,T) %as a$$ro&ed by t!e ood and

Drug Administration (DA) as an aid for detecting latent#ycobacterium tuberculosis infection.


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T!is test is an in &itro diagnostic aid t!at measures a

com$onent of cell-mediated immune reacti&ity to #.

tuberculosis.

T!e test is based on t!e 5uantification of interferon-gamma

(IN-) released from sensitied lym$!ocytes in %!ole blood

incubated o&ernig!t %it! $urified $rotein deri&ati&e (11D)

from # tuberculosis and control antigens.


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Advanta#es o$ QuantiFERON %TBGold

Re&uires a sin#le 'atient visit to dra( a blood

sam'le.

Results can be available (it)in *+ )ours.

,oes not boost res'onses measured bysubse&uent tests ()ic) can )a''en (it)tuberculin sin tests /TT0.


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Is not subBect to reader bias t!at can occur %it! T+T.

Is not affected by $rior G (bacille almette-Gu*rin)&accination.


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,isadvanta#es and limitation

o$ QuontiFERON-TB Gold

lood sam$les must be $rocessed %it!in 2; !ours after collection

%!ile %!ite blood cells are still &iable.

T!ere is limited data on t!e use of ,T-G in c!ildren younger t!an

24 years of age' among $ersons recently eE$osed to #. tuberculosis'

and in immunocom$romised $ersons (e.g.' im$aired immunefunction caused by HIF infection or ac5uired immunodeficiency

syndrome @AID+C' current treatment %it! immunosu$$ressi&e

drugs' selected !ematological disorders' s$ecific malignancies'

diabetes' silicosis' and c!ronic renal failure).


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/rrors in collecting or trans$orting blood s$ecimens or in

running and inter$reting t!e assay can decrease t!e accuracy of

,T-G.

Limited data on t!e use of ,T-G to determine %!o is at ris" for

de&elo$ing T disease.

alse $ositi&e results can occur %it! #ycobacterium sulgai'

#ycobacterium "ansasii' and #ycobacterium marinum.


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Tuberculin s"in tests (T+T) are administered to detect t!e

$resence of #ycobacterium tuberculosis' t!e bacterium t!at

causes tuberculosis (T).

T!e terms 1antou2 TB sin test tuberculin sin testand !!,s are o$ten used interc)an#eably. 1antou2re$ers to t)e tec)ni&ue $or administerin# t)e test.

Tuberculin in Test


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Tuberculin (also called $urified $rotein deri&ati&e or 11D) is t!e

solution used to administer t!e test.

T!e $referred term for t!e test is tuberculin s"in test' or T+T.

Tuberculin in Test


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#antouE Tuberculin +"in Test


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Administration of Tuberculin s"in test

T!e T+T is $erformed by inBecting 3.2

ml of tuberculin $urified $rotein

deri&ati&e (11D) into t!e inner surfaceof t!e forearm.

T!e inBection s!ould be made %it! a

tuberculin syringe' %it! t!e needle

be&el facing u$%ard. T!e T+T is an intradermal inBection.

?!en $laced correctly' t!e inBection

s!ould $roduce a $ale ele&ation of t!e

s"in (a %!eal) 0 to 23 mm in diameter.


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Reading of Tuberculin s"in test

T!e s"in test reaction s!ould be read

bet%een and 4; !ours after

administration.

A $atient %!o does not return %it!in 4;

!ours %ill need to be resc!eduled for

anot!er s"in test.

T!e reaction s!ould be measured in

millimeters of t!e induration ($al$able'

raised' !ardened area or s%elling).


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T!e reader s!ould not measure eryt!ema(redness).

T!e diameter of t!e indurated area

s!ould be measured across t!e forearm

($er$endicular to t!e long aEis).


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+"in test inter$retation de$ends on t%o factors9

#easurement in millimeters of t!e induration.

1ersons ris" of being infected %it! T and of

$rogression to disease if infected.

3o( Are TT ReactionsInter'reted4


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+ome $ersons may react to t!e T+T e&en t!oug! t!ey

are not infected %it! #. tuberculosis. T!e causes of

t!ese false-$ositi&e reactions may include' but are not

limited to' t!e follo%ing9

Infection %it! nontuberculosis mycobacteria

1re&ious G &accination

Incorrect met!od of T+T administration

Incorrect inter$retation of reaction

Incorrect bottle of antigen used

")at Are False-!ositiveReactions4


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+ome $ersons may not react to t!e T+T e&en t!oug! t!ey are

infected %it! #. tuberculosis. T!e reasons for t!ese false-negati&e reactions may include'

but are not limited to' t!e follo%ing9

utaneous anergy (anergy is t!e inability to react to s"intests because of a %ea"ened immune system)

Recent T infection (%it!in >-23 %ee"s of eE$osure)

Fery old T infection (many years)

")at Are False-Ne#ativeReactions4


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Fery young age (less t!an 0 mont!s old). Recent li&e-&irus &accination (e.g.' measles and small$oE). 7&er%!elming T disease. +ome &iral illnesses (e.g.' measles and c!ic"en $oE). Incorrect met!od of T+T administration. Incorrect inter$retation of reaction.


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Performance and

operationl

characteristics

T.S.T IGRAS

Estimatedsensitivity in 't(it) active TB

567-897 lo(erimmunocom'romised 'o'ulations

567-867:inade&uate datainimmunocom'romised 'o'ulations;

Estimateds'eci


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Cross- reactivitywith BCG

YES Less i!ey

Cross-reactivity(it) NT1

>E ?ess liely

Associationbet(een test-@veand subse&uentris o$ active TB

durin# $ollo(-u'

1oderate to stron#'ositive association

Insucientevidence

Correlation (it)mycobacteriumtuberculosise2'osure

>E >E

BeneE NO


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Performance and

operationl

characteristics

T.S.T IGRAS

1otential forcon&ersions andre&ersions

/+ Insufficient e&idence

Ad&erse reactions Rare Rare

#aterial costs L7? #oderate to !ig!1atient &isits T?7 7N/

Laboratoryinfrastructurere5uired

N7 /+

Time to obtain a ; to = days 2 to ; days


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