The AlaSTAT IgE assay (Diagnostic Products Corp., Los Angeles, CA) employs biotin-labeled allergen extracts that bind in the fluid phase to specific IgE (5). The biotin-IgE complexes are added to microtiter wells or plastic tubes where biotin-albumin is coated to the surface. Avidin is added to the solution and the IgE-biotin complexes are linked to the biotin solid phase. An enzyme-linked anti IgE is used to calibrate the specific IgE.
Quantitative IgE calibrators are used to assess specific IgE in the complexes and the results are calculated and classified as kU/L bound or with specific classes. The results of the AlaSTAT-specific IgE are similar, though not necessarily identical, to those of the ImmunoCAP IgE. Like the ImmunoCAP system, the AlaSTAT system has been automated.
The specific IgE bound in the AlaSTAT system depends on the individual extracts (such as cat dander, perennial ryegrass, and so on) that are coupled with biotin. The specific extract and or the coupling procedure may, by altering the binding of the allergen or the extract content of major allergens, give different values than the ImmunoCAP procedure.
The Hycor EIA and the Hycor Turbo-MP RIA (Hycor Stratogene, Garden Grove, CA) methods for specific IgE detection are both FDA cleared (6). In these tests, allergens are linked to cellulose discs and serum IgE is tested after incubation with the allergen-coated discs. The Hycor-EIA uses a modified RAST scheme in which low-level specific IgE is recorded as positive. These low-level tests may be clinically unimportant. The Hycor test is similar to the original RAST technology, but may be less well standardized owing to the purity of the extracts and the comparative validity of the specific IgE.
In a chemiluminescent system from Hitachi Chemical Diagnostics (formerly called MAST), the allergens are linked to a thread (7). After incubation with sera containing IgE antibodies, the threads are washed, incubated with anti IgE, washed again, and then developed with a chemiluminescent indicator. The test results are reported as specific classes, 0-6, and the assay is considered a qualitative rather than a quantitative test.
5. Cell- and Microarray-Based Technologies to Detect Allergen-Specific IgE 5.1. Basophile Histamine Release
Histamine release using leukocytes from atopic individuals has been studied for the last half century (8,9). Leukocytes are separated from heparinized blood to remove red blood cells, and the volume of the leukocyte preparation is restored to the initial blood volume. The specific allergen under study is added to the leukocyte preparation (containing 0.5-1.0% basophiles) as well as controls. After an appropriate incubation, the cells are separated from the supernatant. Released histamine is then determined by comparing lysed basophiles (100% histamine) to buffer and other controls. A good correlation (~90%) between histamine release and skin testing as well as specific IgE is usually found with pollen allergens. With venoms and B-lactam drugs, the sensitivity vs skin testing is approx 50% (10). This discordance may be related to false-positive skin test reactivity or secondary to relative nonsensitivity of separated basophiles. In any event, basophile histamine release is more time-consuming and difficult to perform than specific IgE immunoassays, but is a relevant confirmatory test for unclear allergen biologic activity.
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