Diagnosis – Allergy tests

Allergy testing is important in allergy and asthma management. It enables the identification of avoidable environmental triggers for asthma and allows appropriate avoidance advice to be given.

Allergy testing:

detects the presence of specific IgE antibodies to selected allergens
identifies allergens to which the patient is sensitive
may be done in vivo by skin prick testing or in vitro using blood (RAST) tests – however, these tests cannot be used to test for food intolerance
results must be considered together with patient history and in some cases results of specific challenge tests to be
clinically relevant.

Skin prick tests

Skin prick testing is indicated in all patients suspected of suffering from moderate to severe episodic asthma or persistent asthma. There are no age limitations, although the very young and the elderly may have diminished wheal sizes compared to other subjects.

Skin prick testing is only useful if there is appropriate allergen selection, based upon patient history, knowledge of local aerobiology and the patient’s environment.

Skin prick testing is:

the gold standard for allergy testing and more sensitive than blood (RAST) tests³⁹
more accurate for identification of allergens than relying on history alone^{40
<LE III-2>}
very safe and usually well tolerated, even by small children as it is quick and causes minimal discomfort
convenient as results are available within 15-20 minutes
a specialised procedure that should only be performed by a person who has been trained in the procedure and interpretation of test results
rebated by Medicare and is relatively inexpensive compared to RAST tests.

Methodology

Skin prick testing is performed by introducing a small amount of allergen into the skin via a sterile instrument. If a person is sensitised, a wheal and flare develops within 15-20 minutes. A wheal diameter of 3 mm or greater than the negative control is considered to represent a positive result.
It is most commonly performed on the forearm, although the back can be used in children.
Medications with antihistamine activity must be withheld for 3-7 days before testing to minimise the likelihood of false negative results. These medications include antihistamines, tricyclic antidepressants and pizotifen (for migraines).
Skin prick testing should be performed on normal skin.
A positive and negative control must be performed.
A medical practitioner with knowledge of the management of acute allergic reactions must be present and access
to onsite resuscitation facilities (oxygen and adrenaline) must be available.

Interpretation of results

Care must be taken to interpret results in the context of a negative control solution, otherwise dermographism may result in false positive results.
Knowledge of any conditions suffered by patients, such as recent anaphylaxis and peripheral neuropathy is required, as these can modify the reaction and may result in false negative results.

Adverse reactions to skin prick tests are rare

Local itch and swelling normally subsides within 1-2 hours. More prolonged or severe swelling may be treated with an oral antihistamine, topical corticosteroid creams and an ice pack. Occasionally patients will experience vasovagal responses. Systemic allergic reactions occur in less than 1 per cent of subjects, and are extremely rare in patients tested for aeroallergen sensitivity alone. ^{41-44 <LE IV>}

Alternative skin test methods

Other methods such as scratch testing have generally been abandoned because of poor reproducibility and greater patient discomfort.

Intradermal skin testing:

has no role in inhalant or food allergy testing
is not as safe or well tolerated as skin prick testing
is more sensitive and more likely to lead to false positive and clinically irrelevant results
is more commonly used for evaluation of patients with sensitivity to antibiotics or insect venom.

Blood (RAST) tests

In vitro allergy tests detect allergen specific IgE circulating in the bloodstream. They are commonly referred to as RAST tests, an acronym for radioallergosorbent test, even though the radioactive technology has now been mostly superseded by enzyme or fluorescence-based systems.^{45,
46}

Compared to skin testing, traditional RAST testing has a sensitivity of approximately 70-75% for the detection of allergen-specific IgE.^{47,
48} Newer technology such as CAP has higher sensitivity than traditional RAST tests.

Results of RAST tests:

• are not immediately available as they are performed in laboratories

• when done for allergen mixes may give false negative or positive results

• when negative, do not exclude that particular allergen as a trigger for asthma.

In vitro testing for allergic triggers in asthma should be considered in situations where diagnosis of suspected allergic triggers by skin testing is impractical or unavailable,⁴⁹ including when the patient:

is likely to be uncooperative with skin testing
has recently taken antihistamines, tricyclic antidepressants or pizotifen (for migraines)
has dermographism or extensive skin rash
has an unusually high risk of anaphylaxis, for example, in cases of occupational asthma due to latex hypersensitivity.

There is a Medicare rebate for a limited number of RAST tests performed by laboratories accredited by the National Association of Testing Authorities (NATA).

NATA is an independent body whose task is to ensure that all laboratories performing diagnostic tests comply with minimum standards in all areas.

Unproven allergy tests

There are several unproven methods that claim to test for allergies. They include cytotoxicity testing, the provocation neutralisation procedure, Vega (electrodermal) testing, applied kinesiology, reaginic pulse testing and chemical analysis of body tissues, such as hair analysis.

Controlled studies of the cytotoxicity test, provocation neutralisation procedure and Vega tests have shown that the results are not reproducible and do not correlate with symptoms of allergy.^{50-52<LE
II>}

The other methods have not been evaluated in controlled trials for accuracy nor correlation with allergic symptoms and have not been scientifically validated.

These methods can be expensive and may lead patients to implement unnecessary, costly and particularly in the case of dietary manipulations, dangerous avoidance strategies. The costs of these tests are not rebated by Medicare and their use is not recommended.