Blood Test for Dairy Allergy
The recommended blood test for dairy allergy is cow's milk-specific IgE (sIgE) measured using modern fluorescence-labeled antibody assays such as ImmunoCAP, which has replaced the outdated RAST test. 1, 2
Primary Laboratory Test
Cow's milk-specific IgE testing is the standard blood test for evaluating suspected dairy allergy, though it identifies sensitization rather than confirming clinical allergy and must be interpreted alongside clinical history. 2
Modern fluorescence-labeled antibody assays (ImmunoCAP) are the preferred testing platform over the obsolete radioallergosorbent test (RAST), which should no longer be used. 1, 2
This blood test is particularly valuable when skin prick testing cannot be performed due to extensive eczema, dermatographism (skin that welts easily when scratched), or when antihistamines cannot be safely discontinued. 1, 2
How to Interpret Results
A negative result (sIgE <0.35 kU/L) effectively rules out IgE-mediated dairy allergy with greater than 95% negative predictive value. 1, 2
A positive result (sIgE ≥0.35 kU/L) indicates only sensitization—not necessarily clinical allergy—and requires correlation with the patient's clinical history of reactions to dairy. 1, 2
Higher sIgE levels correlate with increased probability of clinical reactivity on oral challenge, though the exact predictive values vary between studies and patient populations. 1, 2
The positive predictive value of sIgE testing is only 40-60%, meaning many patients with positive tests can actually tolerate dairy without symptoms. 1
Diagnostic Algorithm
Step 1: Obtain detailed clinical history looking for symptoms occurring within minutes to 2 hours after dairy ingestion, particularly if reactions are reproducible on multiple occasions. 2
Step 2: Order cow's milk-specific IgE using a validated fluorescence-labeled assay system (ImmunoCAP). 2
Step 3: Interpret results in clinical context:
- If sIgE <0.35 kU/L: IgE-mediated dairy allergy is effectively ruled out. 2
- If sIgE ≥0.35 kU/L: Sensitization is present, but clinical correlation is mandatory—positive tests alone do not diagnose allergy. 1, 2
Step 4: Consider oral food challenge if diagnosis remains uncertain after testing, as this remains the gold standard for definitive diagnosis. 1
Component Testing Considerations
Specific IgE can be measured to individual milk proteins including casein, alpha-lactalbumin, beta-lactoglobulin, and bovine serum albumin, though no single dominant allergenic protein has been identified. 3, 4
Children with persistent dairy allergy beyond age 9 years have significantly higher levels of milk-specific and casein-specific IgE compared to younger children who typically outgrow the allergy. 3
Critical Pitfalls to Avoid
Never diagnose dairy allergy based solely on positive sIgE testing—sensitization does not equal clinical allergy, and many sensitized individuals tolerate dairy without symptoms. 1, 2, 5
Do not assume undetectable sIgE excludes allergy if clinical history is highly suggestive—proceed with supervised oral challenge in these cases. 1
Results from different laboratory platforms cannot be compared—predictive values established for ImmunoCAP do not apply to other testing systems like Turbo-MP or Immulite, as they use different antigen preparations and yield significantly different measurements. 1, 2
Do not measure total serum IgE levels—this provides no useful information for diagnosing food allergy. 1
Non-IgE-Mediated Dairy Allergy
Approximately 23.6% of children with dairy allergy have non-IgE-mediated disease, defined as positive oral challenge despite sIgE <0.35 kU/L and negative skin testing. 2
Diagnosis of non-IgE-mediated dairy allergy relies on clinical history, elimination diet, and supervised reintroduction rather than blood testing, as sIgE will be negative by definition. 2