In a food allergy, especially in a cow milk allergy, only the immediate reactions that develop after a few minutes having injested the allergenic food, i.e. the cow milk, are likely to give a positive blood or skin test, as these detect that Immunoglobulin E (IgE) is involved in the immediate type allergic reaction. If the patient history points to an IgE-mediated food allergy reaction, tests are available that can detect food-specific IgE antibodies in allergic patients.

Skin Prick Tests (SPTs or PSTs)
Skin Prick Tests are an in vivo test of a specific IgE antibody. The Skin Prick Test can provide a limited indication of the likelihood that a patient will react to a particular food protein, depending on the size of the skin reaction.
Skin Prick Tests can be used and are especially accurate in the young child with cow milk allergy. Small drops of the suspected cow milk or other foods are placed on the forearm. A small prick is made through the drop into the skin. A wheal and flare reaction after 15 minutes will indicate that the patient is allergic to the cow milk.
Skin Prick Tests will often initially be performed to assess to which allergenic foods a patient actually produces IgE antibodies. Once the food allergy is known, specific Radioallergosorbent Tests can be performed to assess how much antibody is present in the blood and thus how likely they are to react.

Radioallergosorbent Tests (RAST)
Radioallergosorbent Tests (RAST) are in vitro tests that can measure the concentration of food specific Immunoglobulin E (IgE) in a patient’s blood: the higher the concentration of IgE the higher the likelihood of reacting to a food protein.
ImmunoCAP is the most commonly used type of Radioallergosorbent Tests.

Atopy Patch Test (APT)
Atopy Patch Test (APT) involves prolonged contact of the allergenic food extract with intact skin under occlusion for 48 hours. Positive reactions to patch tests consist of erythema and induration.
In several food allergy related disorders, such as atopic dermatitis and allergic eosinophilic esophagitis, there is a large cell-mediated component to allergic reactions: the Atopy Patch Test is thought to detect this cellular component as opposed to just the IgE-mediated allergic response.

Other diagnostic tests
A number of other diagnostic tests claim to have utility in the diagnosis of cow milk allergy or another food allergy. None of these are fully clinically validated in the same way as Skin Prick Tests, Radioallergosorbent Tests and exclusion and challenge approaches.
One of the most common is testing IgG antibody to foods. This is flawed in respect that everyone raises IgG antibody to foods and no studies have demonstrated a direct correlation between specific levels of IgG antibody and clinical reactivity.