8 skin prick test

7/24/2019 8 Skin Prick Test

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SKIN PRICK TEST

FOR THE DIAGNOSIS OF

ALLERGIC DISEASE


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Three types of skin testing use in !""ergy i!gnosis

Skin pri#k testing $SPT%

The pri&!ry &oe of skin testing for i&&ei!te

IgE'&ei!te !""ergy $type (%

Intr!er&!" testing $IDT%

Re"e)!nt to *oth i&&ei!te IgE'&ei!te !""ergy

!n e"!ye'type hypersensiti)ity

P!t#h testing

App"i#!*"e to the i!gnosis of #ont!#t

hypersensiti)ity !n other for&s of e"!ye'type

hypersensiti)ity $type +%


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Skin prick testing provides information about the presence

of specific IgE to protein and peptide antigens(allergens).

Small amounts of allergen are introduced into theepidermis and non-vascular superficial dermis and

interact with specific IgE bound to cutaneous mast cells.

Histamine and other mediators are released, leading to avisible wheal-and-flare! reaction peaking after about "#

minutes.


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Indicationof

S$%

&llergic rhinocon'unctivitis

&sthma

ood reactions such as those

manifested b anaphla*is,

immediate acute urticaria, or acute

flare of ec+ema

Suspected late* allerg

onditions in which specific IgE is

considered likel to pla a pathogenic

role (eg. selected cases of chronicurticaria if the histor suggests an

e*ogenous allergic cause)

arer disorders such as allergic

bronchopulmonar aspergillosis,

eosinophilic oesophagitis or eosinophilicgastroenteritis

&topic dermatitis


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&llerg testing has been shown to increase the

accurac of diagnosis when added to histor and

clinical e*amination

It ma lead to allergen avoidance strategies,

improved use of medications, and for some

patients, desensitisation treatment

(immunotherap).

%he strongest indications for skin prick testingare where there is good evidence for the

effectiveness of allergen avoidance or

immunotherap


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P!tient se"e#tion in skin pri#k testing

$atient age

ontraindications

rugs that interfere with the skin prick test response


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PATIENT AGEPATIENT AGE

/o strict age limits but skin reactions are often diminished in

the ver oung and the elderl interpretation more

difficult in both cases

Infants often show larger flares and smaller

wheals

Sstemic allergic reactions ma rarel occur in

response to skin testing in infants (as in patients of

an age)


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ontraindications

iffuse dermatological conditions

Severe dermatographism

$oor sub'ect cooperation

Sub'ect unable to cease

antihistamines0other interfering

drugs


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Positi)e !n neg!ti)e #ontro"s

%he negative control is the same solution as the allergens are made

up in, eg. saline buffer0#12 glcerol, without an allergen. It is also

available commerciall

%he positive control can be a solution of histamine (usuall histamine

phosphate "1mg0ml) (directl induces cutaneous wheal and flareresponse) or codeine (usuall 32 solution) (degranulates cutaneous

mast cells, indirectl causing wheal and flare). &vailabilit of positive

control solutions is problematic


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Positi)e !n neg!ti)e #ontro"s

It is recommended that a wheal of 4mm to the positivecontrol is acceptable (or 4mm greater than the negative

control) and if it is 54mm the test should be considered

uninterpretable

6heals of 78mm to the negative control indicate severe

dermatographism and would re9uire re'ection of the test


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De)i#es use for skin testing

Sharp lancets are used to prick through the drop into the

epidermis and superficial dermis.

Some devices consist of a point on a flat stopper, so that the

device can be 'abbed! onto the patients skin entering the

epidermis and upper dermis, without penetrating too deepl.

& sharp pointed device such as a prick lancet can be used with

an obli9ue prick and lift! techni9ue, without inserting the needletoo deepl.

%he prick should not be deep enough to draw blood, although in

the elderl with thin skin this ma be unavoidable.


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,etho


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Ti&e of re!ing resu"ts

%he reaction to the histamine positive control is at its ma*imum si+eat appro*imatel "1 minutes whereas the allergen reaction reachesits ma*imum at around "# minutes.

In practice the histamine wheal is usuall still showing at "# minutesand this is recommended as the optimal time for reading skin testresults.


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INTERPRETATIONOF SKIN PRICK TEST RES-LTS


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,e!ning of .positi)e/ !n .neg!ti)e/ tests

Skin prick test results need to be interpreted in the conte*t of the

patient>s histor, clinical signs, and allergen e*posures.

In the presence of a histor of an allergic condition with a positiveskin prick test and known e*posure to the allergen, particularl

when the pattern of smptom e*acerbation relates to variations in

allergen e*posure, it is reasonable to conclude that the allergen is

relevant to the smptoms, and the positive test is significant.

& wheal of 8mm or greater is taken to indicate the presence of

specific IgE to the allergen tested


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,e!ning of .positi)e/ !n .neg!ti)e/ tests

It is evident that in general, larger skin test reactions predict a

higher likelihood of a positive response to a challenge, but do not

predict severit of smptoms.

%hese studies have indicated that for man allergens, a wheal si+e

(lower cutoff) set at a larger si+e than 8mm would correlate better

with clinical allergen reactivit. or e*ample, a wheal si+e of 7?mm ma provide more specificity

for the diagnosis of clinical dust mite allergy than the 3mm wheal.


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8 skin prick test

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