Tests for Allergy :
Allergy testing can let you know for certain which allergens are affecting you.
Testing may reveal allergens that you didn't even realize were causing
you problems.
Scratch/skin test:
The skin prick or scratch test is the most common and reliable test for most
allergies. The procedure is fairly painless. A small needle or plastic device is
used to lightly prick or scratch your back or forearm with a tiny
amount of allergen.
After 15-20 minutes, your allergist will be able to interpret the
results by examining
each spot where allergens were scratched or pricked into your skin.
The spots where
you are allergic will become red and swollen, and the others will remain normal.
Blood Test : RAST test or Radioallergosorbent Test
Allergic or immediate hypersensitivity reactions are known as Type I
immunopathologic
reactions that may occur within minutes of an allergic challenge.
A radioallergosorbent immunological test system is a device that consists of the
reagents used to measure by immunochemical techniques the antibodies that cause an
allergic reaction specific for a given allergen.
If you are experiencing symptoms of allergies - such as sneezing,
itchy eyes, runny
or congested nose, swollen sinuses, coughing, and wheezing - RAST test can be
done to identify which specific substances (allergens) are triggering
your allergic
reaction.
Common allergens include the waste products from dust mites (tiny bugs
you cannot see),
proteins found in pet dander, saliva and urine, molds inside and
outside your home,
tree, grass and weed pollen, and cockroach droppings. Other possible
substances that
can cause allergic reactions include, venom from insect stings,
certain foods, food
additives, natural rubber latex, or drugs.
1. How is this test performed ?
A technician, doctor, or nurse will use an antiseptic to clean the
skin above a vein
on your arm. A blood sample will then be drawn. You may feel a brief pricking
sensation when the needle is initially inserted. The blood sample will
be collected
into a tube and then sent to a lab for analysis.
The lab will expose a sample of your blood to different test allergens.
Allergen-specific IgE antibodies will attach to a substance if you are
allergic to it.
A radioactive chemical is used in testing to help measure the results.
2. How long does the procedure take?
A RAST takes about three minutes. Results are usually available within
7 to 14 days.
3. What happens if an abnormality is found?
Results are interpreted along with information on your symptom history
for a positive
diagnosis of allergies. If the results are incomplete, your doctor may
order a skin
test or provocation test.
If allergen-specific antibodies are found, allergies are suspected.
Your healthcare
provider may suggest ways to reduce exposure to the allergen and treatment may be
recommended to ease symptoms.
Allergy shots or allergy desensitization involves injections of small
amounts of the
triggering allergen. This works in a way similar to vaccinations or immunizations.
That is, your body will react to the injected allergen by building
immunity to it. As
immunity builds, subsequent environmental exposures to this allergen
will not be as
likely to trigger an allergic response.
4. Risks associated with this test?
No risks are associated with this test.
5. Interpretation:
Test results show the amount of antibodies in a milliliter of serum.
If you are not
allergic to the tested substance, the amount may be very low or
nonexistent. If you
are allergic, it may be elevated.
A RAST result of IgE sensitivity below 09 IU/ml indicates that the
substances tested
are not allergy-triggering agents for the person tested.
6. Advantages:
It can be used in people with extensive eczema or dermatitis who
cannot undergo skin
testing.
7. Disadvantages:
a. Results must be interpreted in the light of your symptoms and
history: A positive
response to an allergen indicates only a potential allergic
reaction that may not be
the cause of your symptoms.
b. It's at least twice as expensive as skin testing.
c. Results are not immediately available.
d. It may produce negative results for drugs or insect venom when in
fact an allergy
exists.
e. Only a limited number of allergen extracts is available for this test.
8. Factors affecting results:
i. Presence of radioactive substances in your blood (from other procedures in the
previous few days).
ii. Presence of diseases such as atopic dermatitis and parasitic
infections, which cause
increased Ig E antibody levels even if there is no allergic
reaction to the substance
being tested.
9. Variations:
Variations on the RAST test differ in the material to which the
allergen is attached
and the method by which antibodies are counted. A recent variation is
called the CAP
RAST. It is more sensitive and specific than the RAST because more
allergen molecules
are attached to the supporting material.
Although an individual could be allergic to any food, such as fruits,
vegetables, and
meats, they are not as common as the following eight foods which
account for 90% of
all food-allergic reactions:
Cows Milk Protein, Hens Egg White, Soya Milk,
Codfish, Wheat and Peanut
Potentially allergenic foods in adults
Tree Nuts (Brazilnut, Hazelnut, Almond, Walnut, Cashew, Pistachia)
Fruit (Peach, Apple, Strawberry, Citrus fruit)
Vegetables (Tomato, Celery, Onion, Potato, Parsley)
Meats (Pork, Beef)
Shellfish (Mussels, Crab, Shrimp, Squid)
Seeds (Sesame and Poppy seeds)
Foods considered to have a very low potential for causing allergies:
Meat: Lamb, chicken
Vegetables: Rice, sweet-potato, carrots, rhubarb, asparagus.
Fruit: Pears, banana, apricots, apple, pineapple (All peeled)
Fat: Non-dairy margarine, sunflower and olive oil
Other: Herb or Rooibos tea, water, honey, sugar, sago
Supplement: Calcium 400 - 800mg/day and added vitamin B
List of Potential Allergenic Food Ingredients to be labelled in EU
countries by 2005
Cereal containing Gluten and products thereof.
Crustaceans and products thereof.
Eggs and products thereof.
Fish and products thereof.
Peanuts and products thereof.
Soybeans and products thereof.
Milk and dairy products (including lactose)
Nuts and nut products
Celery and products thereof.
Mustard and products thereof.
Sesame and products thereof.
Sulphur dioxide and sulphites at concentrations of more than 10mg/kg
or 10mg/litre.
Mechanism of Allergy:
Allergies can cause from minor discomfort, like a little tingling in the mouth
sneezing, itchy eyes, runny or congested nose, swollen sinuses, coughing to
sevre symptoms like wheezing causing difficulty breathing and anaphylaxis.
Allergic reactions are Type I Hypersensitivity rection Immediate or
Type 1 patterns
of allergy can largely be attributed to IgE(a type of antibody) and a
sub-population
of immune cells (mast cells and basophils). These cells degranulate if sufficient
antigen reacts with IgE antibodies which act as receptors on the body
cell's surface.
An inherited tendency to make excessive amounts of IgE antibody is one
characteristic
of allergy senstive individuals.
The most explosive, immediate, and obvious allergic reactions begin with allergens
attaching to basophil and mast cells triggering their degranulation.
Allergens meet
reactive cells just under the surface of the skin, respiratory,
gastrointestinal and
genitourinary tracts. Mast cells release three sets of mediators with dramatic,
immediate, local and systemic effects.
Hay fever is the most clear-cut case of allergy. Ads for
antihistamines proclaim the
simplest mechanism of allergy:
An inhaled allergen (antigen), grass pollen, meets antibody-coated
mast cells waiting
in the mucosal surface of the nose.
A typical hay fever attack with sneezing, itching and nose congestion results.
A similar reaction in the throat produces soreness, mucus flow, swelling, and
difficulty in swallowing and breathing (pharyngitis, laryngitis).
A similar reaction in the lungs produces cough, mucus obstruction to airflow, and
asthmatic wheezing (bronchitis, asthmatic bronchitis).
Food antigens can trigger the mast cell mechanism at any point in their journey
through the body. Lip, tongue, throat burning and swelling are most obvious. Acute
abdominal pain, vomiting, and diarrhea follow. Antigen may not react in GIT but
activate Type 1 responses in blood or target organs, especially lungs
and connective
tissue. The onset of "immediate reactions" may be delayed hours after
eating offending
foods. Major symptoms include: flushing, swelling, shortness of breath, fast heart
rate, panic, pain, and confusion.
The drug, cromolyn, blocks mast cell degranulation and can prevent the
whole allergic
cascade from developing. Antihistamines (e.g, cetrizine), prostaglandin and
leukotriene inhibitors knock out a fraction of the subsequent responses. Once
underway, corticosteroids are the only drugs that block the late and delayed phase
responses.
These two are World class Diagnostics Laboratories having centres in
most of the European
countries.
Quest Diagnostics
Web site : http://www.questdiagnostics.com/
Abbott Laboratories
Web site : http://abbott.com/
You can get List of Diagnostic Laboratories on following URL
http://dir.yahoo.com/Business_and_Economy/Business_to_Business/Health_Care/Diagnostic/Laboratories/
Thanx |
