Page 1324 - Clinical Immunology_ Principles and Practice ( PDFDrive )

CHaPtEr 95 Assessment of Human Allergic Diseases 1285

of asthma, whether or not IgE-mediated allergic processes are of allergen extract that is injected only slightly influences the
present. In a bronchitic sputum specimen, neutrophils predomi- size of the wheal-and-flare reaction, whereas concentration is
nate. A neutrophilic nasal discharge is characteristic of sinusitis. the most important determinant of the final ID skin test result.
Other laboratory tests that may be ordered, as indicated, include ID testing allows an investigator to perform a skin test titration
pulmonary function tests and a chest X-ray or sinus computer to quantitatively determine the patient’s skin sensitivity. For serial
tomography (CT) scan. titration, the same volume (e.g., 0.02 mL) of 3–10-fold serial
dilutions of allergen extract are injected into different skin sites
IN VIVO PROVOCATION TESTING and the concentration of allergen required to produce a wheal
or erythema of a defined mean diameter (e.g., 8-mm wheal) is
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Both the skin test and nasal/bronchial/GI provocation tests are interpolated. The higher the concentration of allergen required
useful in vivo diagnostic tools for the confirmation of immediate- to induce the defined size of wheal or erythema, the less sensitive
type hypersensitivity reactions associated with allergic disease. is the patient to that allergen preparation and/or the lower is
They can also allow the identification of offending allergens in the allergenic potency of the extract.
an allergy patient’s workup for avoidance, or management with
pharmacotherapy, immunotherapy or anti-IgE therapy. Variables That Influence Skin Test Responses
The quality (composition, potency, heterogeneity) of the allergen
Skin Tests extract is the single most important variable that affects skin
19
Historically, Guerin and Watson described a three-phase skin test performance. Most skin test extracts are nonstandardized,
response during an immediate-type skin test reaction following and their potency is reported in biological or weight per volume
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the administration of a stimulus (allergen or histamine-positive units. Allergen extracts used in puncture skin testing contain
control). First, a bluish-white area appears that involves the 50% glycerin, which is present to enhance stability. However,
constriction of capillaries and that typically disappears within the glycerin causes skin irritation and false-positive skin test
minutes. Second, an erythematous peripheral halo or flare appears results if used intradermally without dilution. Other factors that
as a result of arteriole dilatation. Finally, a circular urticarial influence the skin test response include the area of the body that
papule or wheal is observed, as a result of extravasation of plasma is tested (back vs forearm), age of patient (skin wheals increase
into the skin. The response is generally maximal by 15–20 minutes. in size from infancy to adulthood), race (clarity of reading on
The immediate “wheal and flare” reaction can be followed by a dark vs light skin pigmentation) and preadministered drugs (e.g.,
late-phase reaction 5–6 hours later that appears as a poorly defined antihistamines, tranquillizers, corticosteroids).
edema-like reaction and that usually disappears by 24 hours. An In terms of controls, the saline (negative control) can identify
allergen extract can be administered either by a prick or puncture dermatographic patients and trauma-induced reactivity produced
or by ID injection. 20 by the puncturing device. The histamine positive control (for
Puncture skin testing was first described by Lewis and Grant prick or puncture at 5.43 mmol/L or 1 mg/mL of histamine
22
21
in 1924 and modified by Pepys. It involves placing a drop of base) is useful in detecting medication- or disease-induced
each test allergen extract or control solutions (histamine and suppression of the skin test response. It is also used as a quality
saline) on the skin of the forearm or back and the introduction control reagent to document the reproducibility of technician
of allergen into the epidermis by a needle puncture. Importantly, performance. 27
drops are spaced at least 2 cm apart to prevent cross-over bleeding
that can produce false-positive reactions or difficulty reading Relationship Between Puncture and Intradermal Skin
each discrete test site because of overlapping erythema. A variety Test Responses
of single-point (23–26 gauge), multipoint, and bifurcated needles Fig. 95.1 shows the relationship between the ng/mL level of
have been used. 20,23 After the prick or puncture, the excess allergen Dermatophagoides pteronyssinus (Dpt, dust mite) specific IgE
is removed by blotting with tissue paper or gauze approximately antibody in sera from 30 subjects with dust-mite allergy, as
1 minute later. With single devices, separate needles are used to measured in serum by an in vitro assay, as well as the ID skin
insure no cross-contamination. An immediate reaction (wheal test midpoint Dpt allergen extract titer required to produce an
and erythema) is read at 15–20 minutes as it reaches its maximum 8 mm wheal in the same individual. Using the same Dpt extract
size. Because of the direct skin irritation with some crude allergen in both tests, a higher degree of skin sensitivity (i.e., lower titer
extracts, bleeding can produce false-positive results. of antigen required to induce an 8-mm wheal) was strongly
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2
The ID skin test, first reported by Mantoux in 1908, is correlated (r = 0.77; p < 0.001) with higher serum IgE antibody
1000–30,000 times more analytically sensitive by concentration levels in those with the higher levels of skin sensitivity (<10 ng/
than the puncture skin test. A 0.02- to 0.05-mL volume of diluted mL midpoint). Fig. 95.2 shows the strong correlation between the
allergen extract or controls (histamine or saline) in a 0.5- to wheal size that is observed in the same patients with Dpt allergy
1.0-mL tuberculin syringe is injected intracutaneously through receiving the same dust-mite extract by a single puncture skin test
a 26- to 27-gauge needle. Importantly, the bevel of the needle and a midpoint ID skin test titration. Both the puncture and the
needs to face up and injection should be no deeper than the ID skin test procedures produce a maximal wheal and flare size
superficial layers of the skin. A 0.02-mL injection will initially by 15–20 minutes, which is measured with a millimeter ruler or
produce a superficial 2- to 4-mm-diameter bleb. Like the puncture caliper. The maximal diameter and the midpoint perpendicular
test, the ID skin test is read at 15–20 minutes, when the reaction diameter are averaged to generate an index. A permanent record
is maximal. Dilutions of extract >1 : 1000 weight/volume (w/v) of the skin reaction can be made by applying adhesive cellulose
are commonly used to minimize false-positive reactions because tape over the wheal-and-flare skin area, which has previously
of irritation and the potential for systemic reactions, which range been outlined with a felt-tip or ballpoint pen. Using a single
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from 0.02% of 1.4% of patients tested. Subcutaneous administra- concentration of allergen, the ID skin test can be graded according
tion of the allergen may lead to a false-negative result. The volume to one of several reported systems (Table 95.1). 20,23 Alternatively,