Researchers in Southampton have called on healthcare professionals not to rule out food allergies in children based on blood or skin prick tests alone.
Dr Kate Grimshaw, a specialist paediatric dietitian at Southampton Children's Hospital, warned that not all allergies could be detected purely by measuring levels of a protein in the blood - immunoglobulin E (IgE) - which is linked to allergic reactions.
She spoke out following new research into cow's milk allergy, published online by the medical journal Allergy, which found that for every UK child who had a cow's milk reaction due to IgE in their blood, there was a child who suffered a reaction who did not.
"We know that sometimes if a child is seen for a possible food allergic reaction - to any food, not just milk - but tests show there is no measurable IgE, then a possible food reaction may be ruled out when in fact the child may be reacting to the food, just not via IgE," explained Dr Grimshaw, who was involved in the research alongside colleagues from eight other European countries via a European Union-funded project known as EuroPrevall.
"This research will hopefully highlight to GPs and non-allergy specialists that just because an IgE test is negative, the child may in fact be reacting to a food and further investigations should be carried out."
Food allergy reactions are split into two groups, IgE-mediated, which cause a range of immediate symptoms such as skin rashes, vomiting, respiratory issues and, in some cases, potentially life-threatening reactions, and non-IgE mediated.
Non-IgE mediated reactions occur around four to 28 hours after an incident and may cause stomach complaints such as diarrhoea or constipation among other problems.
The study, which saw more than 9,000 babies from nine European countries followed up until the age of two, found that 1.3% of UK children reacted to milk within two hours - the highest rate of all the countries involved - but only just under half (45%) had IgE at a level likely to cause symptoms.
Dr Grimshaw, who is also a senior research fellow at the University of Southampton, added: "At present, when an IgE test is negative, non-allergy experts may just think the condition doesn't exist or is not serious enough to worry about and that is something we need to address.
"Reactions that don't involve IgE are often referred to as intolerances, which implies the condition may not be serious but children may actually be suffering a different type of allergic reaction that can still be extremely serious."
Professor Graham Roberts, a consultant in paediatric allergy and respiratory medicine at Southampton Children's Hospital and principal investigator for the UK study, said: "Our study shows that a significant number of children can be reacting to a food despite having a negative IgE test.
"The take-away message for any non-allergy specialist is, if the patient's history is convincing, then further investigations should be made if IgE tests are negative to ensure the child isn't actually reacting to a food."
The research, funded in the UK by the Food Standards Agency, was part of a project locally called Prevalence of Infant Food Allergy (PIFA), which assessed the development of food allergies in children in Hampshire in the first two years of life.
Dr Grimshaw and Professor Roberts now plan to reassess all 1,140 children who participated in that research at school-age to discover what happens to allergies that started in early childhood and find out which older children develop allergies and asthma.