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A RAST test (short for radioallergosorbent test) is a blood test used to determine to what substances a person is allergic. This is different from a skin allergy test, which determines allergy by the reaction of a person's skin to different substances.

Because there are other tests that help with confirmation, results are best interpreted by a doctor.

Contents

Indication

The RAST test is an alternative to skin tests to elucidate the causal allergen to an allergy.

Advantages of the RAST test range from: improved sensitivity without loss of specificity, to excellent reproducibility across the full measuring range of the calibration curve. In general, this method of blood testing (in-vitro, out of body) vs skin-prick testing (in-vivo, in body) has a major advantage: it is not always necessary to remove the patient from an anthihistamine medication regimen, and if the skin conditions (such as eczema) are so widespread that allergy skin testing can not be done.

Still, when possible, allergy skin testing is the preferred method in comparison with various in vitro tests for assessing the presence of specific IgE antibodies because it is more sensitive and specific, simpler to use, and less expensive.[1] [2]

Method

The RAST test is a radioimmunoassay test to detect specific IgE antibodies to suspected or known allergens. IgE is the antibody associated with Type I allergic response: for example, if a person exhibits a high level of IgE directed against pollen, the test may indicate the person is allergic to pollen (or pollen-like) proteins. A person who has outgrown an allergy may still have a positive IgE years after exposure.

The suspected allergen is bound to an insoluble material and the patient's serum is added. If the serum contains antibodies to the allergen, those antibodies will bind to the allrgen. Radiolabeled anti-human IgE antibody is added where it binds to those IgE antibodies already bound to the insoluble material. The unbound anti-human IgE antibodies are washed away. The amount of radioactivity is proportional to the serum IgE for the allergen.[3]

History

The market-leading RAST methodology was invented and marketed in 1974 by Pharmacia Diagnostics AB, Uppsala, Sweden, and the acronymn RAST is actually a brand name. In 1989, Pharmacia Diagnostics AB replaced it with a superior test named the ImmunoCAP Specific IgE blood test, which literature may also describe as: CAP RAST, CAP FEIA (fluorenzymeimmunoassay), and Pharmacia CAP. A review of applicable quality assessment programs shows that this new test has replaced the original RAST in approximately 80% of the world's commercial clinical laboratories, where specific IgE testing is performed. The newest version, the ImmunoCAP Specific IgE 0-100, is the only specific IgE assay to receive FDA approval to quantitatively report to its detection limit of 0.1kU/l. This clearance is based on the CLSI/NCCLS-17A Limits of Detection and Limits of Quantitation, October 2004 guideline.

RAST often are used to test for allergies when:
• a physician advises against the discontinuation of medications that can interfere with test results or cause medical complications;
• a patient suffers from severe skin conditions such as widespread eczema or psoriasis; or
• a patient has such a high sensitivity level to suspected allergens that any administration of those allergens might result in potentially serious side effects.

Scale

The RAST test is scored on a scale from 0 to 6:

RAST rating IgE level (KU/L) comment
0 < 0.35 ABSENT OR UNDETECTABLE ALLERGEN SPECIFIC IgE
1 0.35 - 0.69 LOW OF ALLERGEN SPECIFIC IgE
2 0.70 - 3.49 MODERATE LEVEL OF ALLERGEN SPECIFIC IgE
3 3.50 - 17.49 HIGH LEVEL OF ALLERGEN SPECIFIC IgE
4 17.50 - 49.99 VERY HIGH LEVEL OF ALLERGEN SPECIFIC IgE
5 50.0 - 100.00 VERY HIGH LEVEL OF ALLERGEN SPECIFIC IgE
6 > 100.00 EXTREMELY HIGH LEVEL OF ALLERGEN SPECIFIC IgE

References

  1. ^ http://www.mayoclinicproceedings.com/inside.asp?AID=3978&UID=
  2. ^ Ten, R (1995). "Allergy Skin Testing". Mayo Clin Proc 5 (70): 783–4. PMID 7630219.  
  3. ^ WebMD > Medical Dictionary > radioallergosorbent test (RAST) Citing: Stedman’s Medical Dictionary 28th Edition. Copyright 2006

External links

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