The presence of IgG to a specific food indicates previous exposure not hypersensitivity. The use of methods other than serum-specific IgE evaluation or skin prick testing in diagnosing allergies is not proven and can result in inappropriate diagnosis and treatment.
Sources:
Bernstein IL, Li JT, Bernstein DI, et al. Allergy diagnostic testing: an updated practice parameter. Ann Allergy Asthma Immunol. 2008 Mar;100:S1–148. PMID: 18431959.
Carr S, Chan E, Lavine E, et al. CSACI Position statement on the testing of food-specific IgG. Allergy Asthma Clin Immunol. 2012 Jul;8(1):12. PMID: 22835332.
Cox L, Williams B, Sicherer S, et al. Pearls and pitfalls of allergy diagnostic testing: report from the American College of Allergy, Asthma and Immunology/ American Academy of Allergy, Asthma & Immunology Specific IgE Test Task Force. Ann All Asthma Immunol. 2008 Dec; 101(6):580–592. PMID: 19119701.
Stapel SO, Asero R, Ballmer-Weber BK, et al. Testing for IgG4 against foods is not recommended as a diagnostic tool: EAACI Task Force Report. Allergy. 2008 Jul;63(7):793-796. PMID: 18489614.