Doctors are warning that allergies could be missed in children because many health-care professionals are relying solely on blood or skin prick tests.
Dr Kate Grimshaw, a specialist pediatric 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.
Her advice follows new research into cow's milk allergy, published online by the medical journal Allergy, that 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," Dr Grimshaw said.
"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 about four to 28 hours after an incident and may cause stomach complaints such as diarrhoea or constipation, among other problems.
The study, which monitored more than 9000 babies until the age of two in various European countries, found 1.3 per cent of UK children reacted to milk within two hours.
But just under half had IgE at a level likely to cause symptoms.
Professor Graham Roberts, the study's lead investigator, 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."
