Explanation

Fish and seafood mix (allergy): what is measured?

The test measures, in one go, IgE antibodies against a fixed mix of 5 kinds of fish and seafood: cod, tuna and salmon (fish), shrimp (a crustacean) and mussel (a shellfish that belongs to the mollusks). IgE is the type of antibody that, in an allergic reaction, triggers mast cells to release histamine; the body only makes these antibodies after it has previously been in contact with an allergen, and this is called sensitization. Because the 5 foods are tested together, the result gives one number and a verdict of positive or negative, but not which food the antibodies are directed against. Fish on the one hand and crustaceans and mollusks on the other contain different allergens: an allergy to fish says nothing about shrimp or mussel, and the other way round. Shrimp and mussel do share a protein, tropomyosin; according to Stichting Voedselallergie (Dutch food allergy foundation), 50 to 75 in 100 people with a shrimp allergy also react to mollusks.

What can it detect?

Sensitization to at least one of the 5 foods in the mix: the body has made IgE antibodies against one or more of them
A first indication of whether an acute reaction shortly after eating fish, crustaceans or shellfish may be related to one of these foods

Result

What does the result mean?

Positive result

A positive result means that you have IgE antibodies against at least one of the foods in the mix: sensitization. That is not the same as an allergy, and the test does not say whether the antibodies are directed against fish, shrimp or mussel. A positive result only counts if there has also been a clear reaction shortly after eating: itching in the mouth and throat, hives, swelling, vomiting or shortness of breath, according to Stichting Voedselallergie usually within an hour. Antibodies against house dust mite can, through cross-reaction, give a positive result for shrimp or mussel without you having symptoms from eating them. Which food it is becomes clear from separate IgE tests together with the history of the symptoms.

Negative result

A negative result means that no IgE antibodies against the 5 foods were found. An IgE-mediated allergy to these kinds of fish, shrimp or mussel is then less likely, but not completely ruled out. Other kinds of fish, crustaceans such as crab and lobster, and mollusks such as oyster and squid are not in the mix, although antibodies against related species can cross-react. Symptoms after eating fish or seafood can also have another cause, such as food poisoning or an intolerance.

After an abnormal result

The GP mainly looks at the history: which food, which symptoms, how soon after eating and how often. With a positive result, separate IgE tests follow if needed to see whether it is fish, shrimp or mussel. With a suspected real allergy or after a severe reaction, referral to an allergist follows, where a food challenge is done if needed.

Reliability

How reliable is the test?

A positive IgE result shows sensitization, not allergy: it only says that there are antibodies, not that you get symptoms from that food. The Dutch medical specialists' guideline (FMS) on food challenges advises against IgE tests for many foods at once and food mixes if there is no suspicion based on a good history of the symptoms, and calls a supervised food challenge the gold standard. With shrimp and mussel, a positive result without symptoms also occurs through cross-reaction with house dust mite. The mix is therefore mainly a first step after a clear reaction: a negative result makes an allergy to these 5 foods less likely, a positive result calls for separate tests.

What can affect the result?

Sensitization without symptoms occurs, with shrimp and mussel also through cross-reaction with house dust mite; the result can only be interpreted together with the history of the symptoms.
The test does not say which food is the cause, nor whether it concerns fish or crustaceans and mollusks; that requires separate IgE tests.
Kinds of fish, crustaceans and mollusks outside the mix are not tested separately.
Laboratories use different mixes, measurement methods and cut-off values; results from different labs cannot be compared one to one.

Who it is for

Who is the test useful for?

Useful for

Useful after a clear reaction shortly after eating fish, shrimp or mussels, such as itching in the mouth, hives, swelling, vomiting or shortness of breath, when it is unclear which food is the cause. If it clearly concerns one food, a separate IgE test says more. With vague symptoms without a clear link to a meal, and in people who eat fish and seafood without problems, the test adds little; the FMS guideline then advises against screening.

How common is it?

According to Stichting Voedselallergie, about 1 in 100 people have an allergy to fish or crustaceans; fish allergy alone is rarer, an estimated 0.1 to 0.2 in 100 people in Europe. Unlike allergy to milk or egg, an allergy to fish or crustaceans is more common in adults than in children, and it usually does not go away. After peanut and nuts, crustaceans are, according to the same source, the most common cause of a severe allergic reaction (anaphylaxis).

Stichting Voedselallergie

Drawbacks and risks

A positive result without a clear reaction can lead to unnecessarily cutting out fish or seafood, with a risk of a one-sided diet.
A negative result can give false reassurance with an allergy to a kind of fish, crustacean or mollusk that is not in the mix.

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