How are allergies diagnosed? Allergy tests explained

There are only 2 scientifically validated tests to help with the investigation and diagnosis of food allergies – skin prick tests (SPT’s) and specific IgE antibody blood tests (sIgE).

SPT results are available 15-20 minutes after being performed, whereas sIgE blood test results take a few days to be available, as the samples need to be processed in a lab.

Other tests such as IgG testing, cytotoxic testing, vega testing, hair analysis, iridology and kinesiology are not recommended by NICE as there is no scientific evidence for their use.

Allergy tests alone cannot diagnose a food allergy – an allergy focused diet and clinical history is essential to guide the clinician as to which foods should be tested. A skilled clinician experienced in diagnosing and treating food allergies should interpret the results of allergy tests, as it is not simply a matter of a ‘positive’ or ‘negative’ result.

If a food is being eaten and tolerated in the diet, then it should not be tested.

SPT’s are measured as the size of the wheal (red swelling) on the arm in millimetres – if the wheal is 0mm, then the likelihood of an immediate type allergy occurring is very low.

If the SPT size is 3mm or more above the negative control, then this increases the likelihood of an immediate type allergy occurring, but does not rule out the possibility of a delayed reaction. The larger the size of the wheal, the more likely the chance of a clinical reaction, but the size does not predict the severity of the reaction.

BUT

Allergy tests CANNOT:

  • Help with the diagnosis of non-IgE mediated allergies as different antibodies of the immune system are involved (not immunoglobulin E)
  • Predict the severity of an allergic reaction…they can only predict the likelihood of a future (immediate-type) allergic reaction.
  • Give a definite ‘yes / no’ answer to the question “does my child have a food allergy?”

Allergy tests can also be useful in guiding doctors and dietitians about when a child may be growing out of their allergy…

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