What happens during a skin-prick test?
A skin-prick test, usually abbreviated to SPT, is one of the standard ways of looking for IgE sensitisation.
A small amount of an allergen preparation is placed on the skin, usually on the forearm or back.
The skin is then gently pricked through the drop so that a tiny amount reaches the superficial layers.
This isn't an injection deep into the arm.
If allergen-specific IgE is sitting on mast cells in the skin and recognises the substance being tested, those cells can release mediators locally.
Within minutes, a raised itchy swelling known as a wheal, often surrounded by redness, may develop.
It resembles a small nettle sting.
Controls are used so the result can be interpreted properly. A positive control demonstrates that the skin is capable of producing the expected response; a negative control helps show whether the skin is reacting nonspecifically.
The resulting wheal is measured.
And this is where another misconception appears.
A bigger wheal does not simply translate into:
bigger reaction next time you eat the food.
Skin-prick testing can help estimate the likelihood that somebody is clinically allergic in the right context, but it cannot reliably tell an individual how severe their next reaction will be.
The test is measuring a biological response in the skin.
It is not staging a miniature version of the person's next meal.
Sometimes the food itself can become part of the test
Commercial allergen extracts are useful because they provide standardised testing material.
But foods — particularly less commonly tested foods and some fruits, vegetables or spices — can be diagnostically awkward.
In certain circumstances specialists may use a prick-to-prick technique, where a lancet is introduced into the food itself and then used for skin testing.
That does not mean someone who suspects mustard allergy should start experimenting on their arm with the contents of the condiment cupboard.
Skin testing belongs in an appropriate clinical setting because the result needs proper interpretation and, although uncommon, systemic reactions to skin testing can occur.
The principle is simply worth knowing:
Sometimes diagnosing a food allergy requires getting closer to the actual food rather than relying entirely on a manufactured test extract.
The blood test is looking for antibodies, not mustard
The other familiar investigation is a blood test for allergen-specific IgE, usually written as sIgE.
A blood sample is taken and the laboratory measures IgE antibodies capable of binding to the allergen being investigated.
The result may be expressed as a concentration.
Again, it is tempting to turn the number into a severity gauge.
Low number = mild allergy.
High number = severe allergy.
That is not how it works.
Higher levels can, for particular allergens and populations, be associated with a greater probability of clinical reactivity. But the test does not provide a reliable personal forecast of how severe the next reaction will be.
Someone with a relatively low measured value can still have clinically important allergy.
Someone with a higher value may have a different clinical history.
The result has to return to the thing we started with:
What happens when this person actually encounters the food?
Mustard makes this particularly interesting
Mustard is not investigated as extensively as peanut, milk or egg, and the evidence base for some diagnostic questions is consequently smaller.
But clinical studies of mustard allergy have demonstrated exactly why testing and history need to be combined.
Research involving children with positive mustard skin tests used food challenges to establish which children were genuinely clinically allergic. Other mustard research has likewise combined history, skin testing, specific IgE and controlled food challenge rather than treating one laboratory result as the diagnosis.
That distinction becomes particularly important because mustard participates in the cross-reactivity patterns we discussed earlier.
Someone sensitised to pollens or other plant proteins may have IgE that recognises related structures.
A whole-mustard test tells us that IgE recognises something in the mustard preparation.
But what if we could ask exactly what it recognises?
We increasingly can.
From "mustard" to Sin a 1: component-resolved diagnostics
Traditional allergy testing often treats a food rather like a box.
The question is:
Does this person's IgE react to something inside the mustard box?
Component-resolved diagnostics attempt to open the box.
Instead of measuring IgE only against an extract containing many mustard proteins, component testing can investigate IgE directed against particular allergenic proteins.
We met several of these earlier:
Sin a 1 — a 2S albumin seed-storage protein.
Sin a 2 — an 11S globulin seed-storage protein.
Sin a 3 — a non-specific lipid transfer protein.
Sin a 4 — a profilin.
Why does that matter?
Because those proteins have different biological properties and different relatives elsewhere in the plant kingdom.
If somebody's IgE recognition is largely associated with a cross-reactive protein family found in pollens and many plants, that can potentially tell a different clinical story from recognition of a stable seed-storage protein.
This is one of the most exciting changes in allergy diagnosis.
We are moving from:
"Which food are you allergic to?"
towards:
"Which molecules within that food does your immune system recognise, and what might that pattern mean?"
Modern reviews of food-allergy diagnostics describe component testing as a way of refining diagnosis and helping distinguish primary food sensitisation from some cross-reactive patterns.
But there is an important brake to apply here.
Component testing is not a molecular crystal ball.
Its usefulness differs considerably between allergens. For some foods, particular components are well validated and clinically very useful. For others, including less extensively studied allergies, the evidence and availability are more limited.
Mustard science is developing, but we should not pretend that a laboratory can currently examine someone's Sin a 1, Sin a 2, Sin a 3 and Sin a 4 results and print out a precise prediction of their future life with mustard.
Medicine isn't there yet.
Can a test tell you whether your next reaction will be anaphylaxis?
This is one of the questions people understandably want answered most.
If I am allergic, how allergic am I?
Will I get an itchy mouth?
Will I develop hives?
Could I have anaphylaxis?
Unfortunately, conventional allergy testing cannot reliably answer that question for an individual.
Skin-prick wheal size and specific-IgE concentration can provide information about the probability of clinical allergy in appropriate circumstances.
They do not function as a dependable severity meter.
That is why a person should not conclude:
"My number is low, therefore I don't need to take the allergy seriously."
Nor should someone with a high result assume:
"This proves the next exposure will kill me."
Both interpretations ask the laboratory test to predict something it cannot reliably predict.
This is an important area of current research. Scientists are investigating combinations of molecular diagnostics, basophil activation, epitope recognition and other biomarkers in the hope of describing individual allergy more precisely.
The direction of travel is towards better prediction.
But better prediction is not yet perfect prediction.
Asking the immune cells themselves
One particularly interesting developing tool is the basophil activation test, or BAT.
Basophils are immune cells involved in allergic reactions.
Instead of merely measuring whether IgE antibodies capable of recognising an allergen are present, the BAT exposes a person's basophils to allergen in the laboratory and examines whether those cells become activated.
Conceptually, this is a step closer to asking:
"Does this person's allergic machinery actually switch on when it encounters this allergen?"
rather than simply:
"Can we detect antibodies that bind it?"
The BAT has shown considerable promise for some food allergies and is incorporated into modern discussions of advanced allergy diagnosis, although availability and validation differ by allergen and healthcare setting.
For the ordinary patient today, skin testing, specific-IgE testing, clinical history and — where necessary — food challenge remain much more familiar.
But BAT gives us a glimpse of where allergy diagnostics are heading.