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Understanding Allergy Test Results: A Parent's Guide
If you're exploring whether your child might have allergies, testing at our London clinics can help uncover what’s behind their symptoms. We offer a range of tests that can point to potential triggers, but making sense of the results isn’t always straightforward. That’s where our specialists come in — they don’t just look at the numbers, but take your child’s full medical picture into account. This guide walks you through the different types of allergy tests, what the results can tell you, and why expert interpretation really matters.
The three pillars of allergy testing
Allergy diagnosis typically relies on three key elements
- Detailed clinical history – your description of symptoms, their timing, and potential triggers
- Allergy testing
- Challenge testing (in some cases) – supervised exposure to suspected allergens
Each component provides valuable information, but none alone can definitively diagnose an allergy. Together, they create a comprehensive picture that guides diagnosis and management.
Types of allergy tests
There are two main methods for detecting IgE antibodies – the proteins your immune system produces in response to allergens that can trigger allergic reactions.
Skin prick testing (SPT)
Skin prick testing works by introducing a tiny amount of allergen extract into the skin's surface layer:
- A drop of allergen extract is placed on the skin
- A small lancet gently pricks the skin through the drop
- If allergic antibodies (IgE) are present, a raised, itchy bump (wheal) develops
- The diameter of this wheal is measured in millimetres after 15–20 minutes
- A positive reaction is typically a wheal 3mm or larger than the negative control
Skin testing provides immediate results during your appointment and allows for testing multiple allergens simultaneously.
Blood testing for specific IgE
Blood tests (sometimes referred to as RAST or ImmunoCAP tests) measure allergen-specific IgE antibodies directly in the blood:
- A blood sample is drawn and sent to a laboratory
- The amount of IgE antibodies specific to each allergen is measured
- Results are reported in kilounits per litre (kU/L) or international units per millilitre (IU/mL)
- Higher numbers indicate greater amounts of allergen-specific IgE
- Results typically take several days to become available
Interpreting test results
Understanding test results requires nuance and expertise. Here's what parents should know:
What positive tests indicate
A positive test result (whether skin or blood) indicates sensitisation to an allergen – meaning your child has developed IgE antibodies to that substance. However, sensitisation alone doesn't necessarily mean your child will experience allergic symptoms when exposed to the allergen.
Some key points about positive results
- The size of a skin test reaction or the level of specific IgE in blood testing correlates with the likelihood of a clinical allergy, but not perfectly
- Very large skin test reactions (e.g., ≥8mm) or high blood test levels generally indicate a high probability of clinical allergy
- Smaller positives (e.g., 3–4mm skin reactions) may represent sensitisation without clinical allergy
What negative tests indicate
Negative test results generally suggest a low likelihood of IgE-mediated allergy, but they don't completely rule it out. A 0mm skin test combined with undetectable specific IgE in blood makes IgE-mediated allergy unlikely.
However, false negatives can occur, particularly if testing is done during very early stages of allergy development, when the allergen extract used in testing is of poor quality, or when recent antihistamine use suppresses skin test reactivity. This is why we emphasise the importance of proper preparation before testing and may recommend repeat testing in certain circumstances.
The relationship between test size and reaction severity
A common misconception is that larger test reactions predict more severe allergic reactions. This is not true.
Test size does not reliably predict the severity of future reactions, nor does it indicate the amount of allergen required to trigger symptoms.
Additionally, test size does not confirm that a reaction will happen with every exposure. Rather, test size primarily reflects the likelihood of allergy, not the intensity of response.
This distinction is crucial for understanding what your test results actually mean for managing your condition.
Complementary nature of skin and blood tests
Each testing method has specific advantages, and they often work well together:
When skin testing has advantages
Skin testing may be preferred when:
- Immediate results are needed
- Testing for environmental allergens like pollens, dust mites, and animal dander
- Investigating fresh fruit and vegetable allergies
- Examining potential cross-reactivity between related allergens
When blood testing has advantages
Blood testing may be more appropriate when:
- The patient has widespread eczema or other skin conditions
- The patient cannot stop taking antihistamines before testing
- Testing very young infants where skin testing is more challenging
- The patient has a history of severe allergic reactions
The role of challenge testing
When skin and blood test results are inconclusive or conflict with the clinical history, controlled challenge testing may be recommended:
- Food challenges involve consuming gradually increasing amounts of the suspected food under medical supervision
- Drug challenges may be used to confirm or rule out medication allergies
- Challenge tests are considered the gold standard for diagnosing or ruling out allergies
Challenge testing is particularly valuable when:
- Tests show mild positives but there's no history of reaction
- Tests are negative but there's a convincing history of reaction
- Assessing whether a previously diagnosed allergy has been outgrown
Case examples: Interpreting different test scenarios
These examples illustrate how different test results might be interpreted in clinical practice:
Strong positive results
Example: A child has a 10mm skin test to peanut and a blood test level of 30 IU/mL.
Interpretation: This combination of strongly positive results indicates a very high probability (>95%) of clinical peanut allergy. If this aligns with the clinical history, a diagnosis can usually be made without challenge testing.
Negative results
Example: A child has a 0mm skin test to milk and an undetectable specific IgE level in blood testing.
Interpretation: These results make IgE-mediated milk allergy highly unlikely. However, if the child has experienced clear reactions after consuming milk, non-IgE mechanisms might be involved.
Borderline results
Example: A child has a 3mm skin test to egg and a blood test level of 0.7 IU/mL.
Interpretation: These borderline results indicate sensitisation but don’t clearly confirm or rule out allergy. Clinical history becomes particularly important, and a supervised food challenge may be recommended.
Common questions about allergy test results
"My child has a positive test but has never reacted to that food. Does this mean they're allergic?"
Not necessarily. This often represents sensitisation without clinical allergy. Your allergist might recommend cautious continued consumption if the child has been eating the food without issues, or suggest a supervised food challenge if exposure has been limited or unclear. In cases where the positive result is very strong, they may advise temporary avoidance and retesting to monitor how the sensitivity evolves over time.
"Can my child outgrow a positive allergy test?"
Yes. Many children, especially with egg or milk allergies, develop tolerance over time. Periodic retesting helps track changes, and declining test values may indicate growing tolerance.
"Should we avoid foods that show any positive result, no matter how small?"
No. Avoiding foods based on small positives can lead to unnecessary restrictions. Especially without symptoms, low-level positives may not represent true allergy. Your allergist will help decide what’s safe to continue.
"Can we use home test kits to check for allergies?"
Home test kits are not reliable for allergy diagnosis. They often test for IgG (which is not relevant to allergy), can be misleading, and lack clinical context for accurate interpretation. Always consult a qualified specialist.
The importance of specialist interpretation
The most crucial message about allergy testing is that results require expert interpretation.
Your allergy specialist integrates the specific test results with your child's detailed clinical history, knowledge of the reliability of tests for specific allergens, understanding of cross-reactivity patterns, and awareness of how conditions like eczema impact test interpretation.
Self-interpretation of results can lead to unnecessary anxiety, inappropriate dietary restrictions, or failure to recognise true allergies. This is why we strongly recommend that you discuss your test results with your specialist rather than attempting to interpret them yourself. Your specialist has the expertise to provide accurate, personalised guidance based on your complete clinical picture.
What to expect from your specialist consultation
When discussing test results with your allergist, they will typically explain what the tests have shown in the context of your child's symptoms and discuss the likelihood of true allergy versus sensitisation without clinical significance.
They will recommend which allergens should be avoided and which can likely be safely consumed, provide emergency management plans if significant allergies are confirmed, and suggest when retesting might be appropriate.
Throughout the consultation, they will address any questions or concerns you have about the results, ensuring you leave with a clear understanding of your child's allergic status and next steps.
Beyond the tests: The complete clinical picture
While testing provides valuable information, allergy diagnosis and management consider many other factors. These include the pattern and timing of symptoms, the consistency of reactions, and response to previous treatments. We also consider family history of allergic conditions, impact on quality of life, and nutritional considerations with food allergies, alongside the practical aspects of allergen avoidance.
This holistic approach ensures that recommendations are both medically sound and practically manageable in daily life.
Moving forward with test results
Once your allergy test results have been properly interpreted, they serve as a foundation for creating clear management plans, developing emergency action plans if needed, and making informed decisions about dietary modifications. They also help us understand which environmental measures will be most helpful and enable us to track changes in allergic status over time.
With expert guidance, allergy test results become a powerful tool for managing allergic conditions effectively, avoiding unnecessary restrictions, and improving quality of life. Remember that allergy testing is not a one-time event but part of an ongoing process. As children grow and their immune systems develop, their allergic profile may change, making periodic reassessment valuable in many cases.
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