Biomedical Engineering Reference
In-Depth Information
The above examples expose the severity and scope of allergic diseases and consequently
highlight the importance of IgE as a medical analyte.
20.1.4
Current Diagnosis Methods
The primary aim of allergy testing is to determine whether an individual presenting with
symptoms of an allergic hypersensitivity has demonstrable allergen-specific IgE. The first
step in the diagnostic procedure is an assessment of symptom history and a physical
examination. Once the clinician has concluded that there is a high degree of suspicion of
an allergic disorder, in vivo and in vitro analyses for the presence of IgE can be performed
to strengthen the likelihood of the diagnosis being correct (3). In vivo testing refers to skin
testing, which has become the standard to which other detection methods are compared,
and is the primary method for detection of specific IgE (6). In vitro tests, examined in more
detail below, refer to IgE immunoassays which are used as an alternative to skin tests
when these are not practical or possible, or when equivocal results are obtained (6). This
could be the case, for example, when an allergenic substance is not available as a licensed
extract for skin testing (e.g., latex and industrial chemicals), or when a patient is taking
medication that would preclude a skin test (6). Generally, results of skin testing performed
under optimal conditions agree with those obtained using specific IgE immunoassays, but
experts agree that these two methods are not interchangeable (6). When discrepancies in
results exist, the skin test is usually positive and the immunoassay negative, probably due
to the limitations of the immunoassays (6).In contrast, in the case of a positive immunoas-
say test and a negative skin test, one should question whether the skin test was performed
correctly (6). Most studies however show a sensitivity of ~70-90% when immunoassays
are compared to skin testing (16; 17).
The first commercial in vitro assay designed for the detection of IgE was the Phadebas
Radioallergosorbent test (RAST, Pharmacia, Uppsala, Sweden). In this format, an aller-
gosorbent was prepared by covalently coupling an allergen of a particular specificity onto
cyanogen bromide-activated cellulose paper disks. Allergen-specific antibody of all iso-
types in the serum could then bind after addition of the sample. After washing, radio-iod-
inated antihuman IgE was used to detect bound IgE. After a second wash, radioactivity
relating to the amount of IgE initially present in the sample was measured using a gamma-
counter (3). Although the term “RAST” is still commonly used to refer to an immunoas-
say for allergen-specific IgE, it is in fact a trademark name and refers to an assay which is
now very rarely used (6). So-called “second-generation” allergen-specific IgE antibody
assays have almost all been based on the RAST chemistry, but use a larger number and
higher quality of allergen extract for the preparation of allergosorbents. Furthermore, the
shelf-life, ease of use, and safety of these tests have improved due to the use of nonra-
dioactive, enzyme-labeled antibodies. Automation has also improved precision and repro-
ducibility of results (3). Consequently, these assays have become a lot more competitive
with the in vivo skin tests.
There are, however, a number of problems associated with immunoassays, the greatest
of which is the lack of standardization. This is in most part due to the varying sources of
raw allergenic materials, different methods for binding allergen to the detection matrix,
and different detection systems (6). Although no official test standard exists, the
Pharmacia CAP system is in worldwide use and is a de facto standard to which other tests
are compared (6).
It is important to bear in mind that although these assays are often promoted as allergy
diagnosis tests, they should best be regarded as tests indicating the presence or absence of
detectable specific IgE (6). Furthermore, specific IgE can be found in patients with allergic
diseases as well as in about 15% of normal asymptomatic individuals (6). Immunoassays
