Background Although cow's milk allergy (CMA) is one of the most common food allergies, mislabelling non-allergic infants as being allergic to cow's milk is more common. Despite this, characteristics of families and infants with mislabelled CMA are lacking.

Methods Using a prospective population-based study, we identified infants with any possible adverse reaction to cow's milk (n=381) from a cohort of 13 019 infants followed from birth. They had a detailed history taken, skin prick tests and an oral food challenge when indicated. Infants with symptoms for which the causative relationship to cow's milk protein was excluded were compared with infants with IgE-mediated CMA and with a control group, and followed for 2–5 years.

Results Overall, 243 infants (1.87%) with mislabelled CMA were identified. Compared with 66 infants with IgE-mediated CMA, those with mislabelled CMA presented earlier and with symptoms usually involving a single organ system. Doctor-diagnosed atopic dermatitis (AD) was associated with mislabelled CMA (p<0.001), manifested primarily as skin rashes, compared with control infants. Higher maternal and paternal education were also associated with mislabelled CMA (p=0.007 and p=0.035, respectively) and manifested primarily as non-specific symptoms.

Conclusions Mislabelled CMA typically presents within the first 3 months of life involving a single organ. Infant AD and higher parental academic education are associated with mislabelled reactions. Better parental and physician awareness of the importance of objectively diagnosing milk allergy is required to avoid mislabelling of infants as being allergic to cow's milk and to prevent potential nutritional deficiencies.