Allergy diagnosis: pros and cons of different tests, indications and limitations

Key points

• Allergy and sensitisation are not the same.

• Sensitisation to both food and airborne allergens may change over time.

• Double-blind placebocontrolled food challenges are by far the best way to diagnose food allergy.

• Size matters: the degree, not the presence, of sensitisation determines risk of allergy.

Educational aims

• To describe different methods of allergy diagnosis, with pros and cons.

• To explain the difference between sensitisation and allergy.

• To demonstrate that allergic sensitisation is not an all-or-nothing phenomenon, but that the degree of sensitisation is also important.

Summary Although related, allergy and sensitisation are not identical. It is quite common for sensitisation to occur asymptomatically, while “allergic” symptoms may also appear independent of the presence of sensitisation. In addition, the risk of allergic disease (symptoms) depends on the degree of sensitisation, rather than whether sensitisation is present.

The results of tests for sensitisation, such as skin prick tests (SPTs) and specific immunoglobulin (Ig)E tests, must be interpreted with this in mind. Because of their low cost and rapid results, SPTs are the preferred method to screen for allergic sensitisation. Specific IgE tests are more useful to confirm or reject the suspicion of specific sensitisation to a certain allergen. Caution is necessary when interpreting published results from tertiary-care reference centres; because the population studied is usually distinctly different from the patients in your centre, the results of such studies may not be applicable in your own clinical practice.