SOS Allergo · I understand & manage
Laboratory tests in allergy
Specific IgE, molecular IgE, tryptase, multiplex tests, total IgE, BAT and LTT: what are they used for?
A blood test can help answer a specific question in allergy, but no set of blood tests can summarise an allergy on its own. Results must be compared with symptoms, reaction timing and exposures; they sometimes complement skin tests or a challenge test.
In this sheet
3 essential points
1. A positive test is not enough to make a diagnosis
Detectable specific IgE shows sensitisation. On its own, it does not prove that the tested allergen causes your symptoms.
2. Molecular IgE refines interpretation
It looks for specific components of an allergen and may help distinguish primary sensitisation from cross-reactivity. The clinical history remains essential.
3. Large panels and total IgE are not routine
More tests do not necessarily provide a better diagnosis. A test is useful when it answers a genuine clinical question and its result can be interpreted correctly.
Main types of tests
- Specific IgE
- Look for sensitisation directed against a specific allergen.
- Molecular IgE
- Identify recognised allergen components and clarify certain cross-reactivities.
- Tryptase
- Document acute mast-cell activation and/or investigate elevated baseline tryptase.
- ISAC / ALEX2
- Explore many allergens and components simultaneously in selected situations.
- Total IgE
- Measure the overall amount of IgE; this is non-specific information useful only in certain contexts.
- BAT / LTT
- Specialist tests considered in certain complex situations, particularly drug reactions.
| Examen | What is it used for? |
|---|---|
| Specific IgE | Look for sensitisation directed against a specific allergen. |
| Molecular IgE | Identify recognised allergen components and clarify certain cross-reactivities. |
| Tryptase | Document acute mast-cell activation and/or investigate elevated baseline tryptase. |
| ISAC / ALEX2 | Explore many allergens and components simultaneously in selected situations. |
| Total IgE | Measure the overall amount of IgE; this is non-specific information useful only in certain contexts. |
| BAT / LTT | Specialist tests considered in certain complex situations, particularly drug reactions. |
Blood tests and specific IgE
Blood test or skin tests?
The choice depends on the situation. A blood test may be useful when skin testing cannot be performed or needs to be supplemented, and it is not affected by antihistamines in the way skin tests may be. In many cases, the two approaches complement each other.
What is specific IgE?
Specific IgE testing looks in the blood for antibodies directed against a given allergen, such as house dust mites, cat, pollen, peanut or Hymenoptera venom. The result helps identify sensitisation, but an allergy diagnosis always depends on whether it matches the symptoms and exposure.
How should the value be read?
Laboratories often report specific IgE in kUA/L. In some contexts, a higher value may increase the probability that sensitisation is clinically relevant, but there is no universal threshold that predicts the future severity of a reaction on its own.
What is molecular IgE used for?
Molecular IgE testing, or component-resolved diagnosis, looks for specific allergen proteins rather than a whole extract. It may help determine whether sensitisation is more consistent with a primary allergen source or with a protein family responsible for cross-reactivity.
- Peanut — Ara h 2
- May strengthen or qualify the interpretation of peanut sensitisation according to the clinical history.
- Birch — Bet v 1
- May help identify a pollen-food allergy syndrome profile related to PR-10 proteins.
- House dust mites — Der p 1 / Der p 2
- May clarify sensitisation to major house dust mite components.
- Venoms — molecular components
- May help distinguish some double-positive or complex profiles.
| Example | What this may help to understand |
|---|---|
| Peanut — Ara h 2 | May strengthen or qualify the interpretation of peanut sensitisation according to the clinical history. |
| Birch — Bet v 1 | May help identify a pollen-food allergy syndrome profile related to PR-10 proteins. |
| House dust mites — Der p 1 / Der p 2 | May clarify sensitisation to major house dust mite components. |
| Venoms — molecular components | May help distinguish some double-positive or complex profiles. |
What a molecular result cannot tell you on its own
It does not predict the severity of a future reaction with certainty and cannot, by itself, decide on avoidance, treatment or the need for a challenge test.
Total IgE: do not confuse it with specific IgE
Total IgE measures the overall amount of circulating IgE. It does not identify the allergen to which you are sensitised. A normal value does not rule out allergy, and a high value proves neither a particular allergy nor its severity.
- Not routine
- Testing may be useful in some specialist assessments or for certain treatment decisions, but it should not automatically be performed in every allergy assessment.
| Key information | Key points |
|---|---|
| Not routine | Testing may be useful in some specialist assessments or for certain treatment decisions, but it should not automatically be performed in every allergy assessment. |
Tryptase
Tryptase is not a general allergy test and is not an IgE. It is a marker associated with mast cells. It may be useful during certain severe immediate reactions and when investigating elevated baseline tryptase or a mast-cell disorder.
- Reaction compatible with anaphylaxis
- An acute sample, ideally within the first few hours, followed by a later baseline value may help document mast-cell activation.
- Elevated baseline tryptase / suggestive context
- It contributes to specialist assessment but cannot diagnose mastocytosis or another mast-cell disorder on its own.
| Situation | Why tryptase may be useful |
|---|---|
| Reaction compatible with anaphylaxis | An acute sample, ideally within the first few hours, followed by a later baseline value may help document mast-cell activation. |
| Elevated baseline tryptase / suggestive context | It contributes to specialist assessment but cannot diagnose mastocytosis or another mast-cell disorder on its own. |
Emergency care comes first
During a severe reaction, treatment must never be delayed in order to take a blood sample.
ISAC and ALEX2: multiplex tests
ISAC and ALEX2 are platforms that use a single blood sample to test for IgE against many allergens or components. They may be useful for some polysensitised patients or when the allergist needs a broad molecular overview. Their panels and methods differ, so results are not directly interchangeable.
- Potential value
- A broad overview that is useful for some complex profiles and for investigating cross-reactivity.
- Limitation
- A very broad panel may reveal sensitisation unrelated to symptoms. It is not a routine screening test.
| Key information | Key points |
|---|---|
| Potential value | A broad overview that is useful for some complex profiles and for investigating cross-reactivity. |
| Limitation | A very broad panel may reveal sensitisation unrelated to symptoms. It is not a routine screening test. |
BAT and LTT: specialist tests
BAT — basophil activation test
The BAT measures basophil activation in the laboratory after contact with an allergen or medicine. It may complement assessment in selected situations, especially when usual investigations do not answer the question clearly. Performance depends on the indication and laboratory.
LTT — lymphocyte transformation test
The LTT investigates a T-lymphocyte response, mainly in some delayed drug hypersensitivity reactions. It remains a specialist test: sensitivity varies with the medicine and type of reaction, and a negative result alone does not rule out hypersensitivity.
Why not request ‘the whole panel’?
A very broad test increases the chance of finding sensitisation unrelated to your symptoms. If results are interpreted outside their context, this may cause anxiety or unnecessary avoidance.
How is the right test selected?
- Suspected allergy to a specific allergen
- Clinical history + targeted skin tests and/or specific IgE.
- Cross-reactivity or a profile that is difficult to interpret
- Targeted molecular IgE; sometimes multiplex testing if the profile is complex.
- Severe immediate reaction / anaphylaxis
- Acute and baseline tryptase according to the context, followed by a targeted allergy assessment.
- Selected suspected immediate drug allergy
- BAT in some centres and for certain medicines, as an adjunct to the assessment.
- Selected suspected delayed drug reaction
- LTT in certain specialist situations, with limitations in sensitivity.
| Question clinique | Examens parfois utiles |
|---|---|
| Suspected allergy to a specific allergen | Clinical history + targeted skin tests and/or specific IgE. |
| Cross-reactivity or a profile that is difficult to interpret | Targeted molecular IgE; sometimes multiplex testing if the profile is complex. |
| Severe immediate reaction / anaphylaxis | Acute and baseline tryptase according to the context, followed by a targeted allergy assessment. |
| Selected suspected immediate drug allergy | BAT in some centres and for certain medicines, as an adjunct to the assessment. |
| Selected suspected delayed drug reaction | LTT in certain specialist situations, with limitations in sensitivity. |
Common misconceptions
- ‘A blood test always replaces skin tests.’
- False: the tools complement each other.
- ‘A positive result always proves allergy.’
- No: it may show sensitisation without symptoms.
- ‘The higher my IgE, the more severe my reaction will be.’
- Not necessarily; severity cannot be inferred from an isolated value.
- ‘Normal total IgE rules out allergy.’
- No.
- ‘ISAC or ALEX2 is useful for everyone.’
- No: their value depends on the clinical question.
- ‘BAT or LTT always gives a definitive answer.’
- No: these are specialist investigations with limitations.
| Misconception | SOS Allergo guidance |
|---|---|
| ‘A blood test always replaces skin tests.’ | False: the tools complement each other. |
| ‘A positive result always proves allergy.’ | No: it may show sensitisation without symptoms. |
| ‘The higher my IgE, the more severe my reaction will be.’ | Not necessarily; severity cannot be inferred from an isolated value. |
| ‘Normal total IgE rules out allergy.’ | No. |
| ‘ISAC or ALEX2 is useful for everyone.’ | No: their value depends on the clinical question. |
| ‘BAT or LTT always gives a definitive answer.’ | No: these are specialist investigations with limitations. |
Before my appointment
If possible, bring previous blood and skin-test results · a report of the reaction · the time between exposure and symptoms · any photographs · medicines taken · the precise name of the suspected food, medicine, venom or allergen.
You may also want to read
More to read on SOS Allergo
Skin prick tests · Preparing for allergy tests · Food allergy · Anaphylaxis · Hymenoptera venom · Mastocytosis / tryptase · Medicines · Allergen immunotherapy
References and resources
- Dramburg S, Hilger C, Santos AF, et al. EAACI Molecular Allergology User’s Guide 2.0. Pediatr Allergy Immunol. 2023;34 Suppl 28:e13854. doi:10.1111/pai.13854.
- Ansotegui IJ, Melioli G, Canonica GW, et al. IgE allergy diagnostics and other relevant tests in allergy: a World Allergy Organization position paper. World Allergy Organ J. 2020;13:100080.
- Golden DBK, Wang J, Waserman S, et al. Anaphylaxis: A 2023 practice parameter update. Ann Allergy Asthma Immunol. 2024.
- Mayorga C, Çelik GE, Pascal M, et al. Flow-based basophil activation test in immediate drug hypersensitivity: an EAACI task force position paper. Allergy. 2024;79:580-600.
- Mayorga C, Celik G, Rouzaire P, et al. In vitro tests for drug hypersensitivity reactions: ENDA/EAACI position paper. Allergy. 2016;71:1103-1134.
- Platteel ACM, van der Pol P, Murk JL, et al. Comparison between ISAC and ALEX2 multiplex test systems. Clin Chem Lab Med. 2022;60:1046-1052.
This information sheet does not replace a medical consultation. The indication for and interpretation of investigations depend on your clinical history and context.
Scientific review: September 2026 · sosallergo.fr
Back to the library