In this case, the reported symptoms are not convincing or consistent with an IgE-mediated reaction. This point is important as it helps to inform next steps. There are no clinical cut points for allergen-specific IgE to fish. I recommend that you ask the parent to provide tilapia so that you may perform prick skin test with the fish. If the skin test is negative, in the context of the reported history, the likelihood of true allergy is low, so you could proceed to do an oral food challenge. If there is no reaction, then I would not recommend any further testing. You could advise the family to introduce other fish as desired, especially since the child is already tolerating fish sticks.
If the child does react to the tilapia, then I recommend a discussion with the family about whether fish consumption is a high priority. There is a 30-50% rate of cross-reactivity among finned fish, so if it is not a high priority the child could avoid other fish. If the family frequently consumes fish and wants their child to eat it, then you could perform skin tests to the desired fish. If the tests are negative, then the caregiver could introduce these into the child's diet as desired.
Of note, many people who are allergic to tuna are able to tolerate canned tuna. It is thought that processing alters the allergenicity of the allergen.
I hope this information helps you with your patient.
Jacqueline A. Pongracic, MD, FAAAAI