TEST FOR ALLERGIES

 

The diagnostic

The period of limitation, the execution and especially the interpretation of allergy tests require a specific cultural background and experience in not being able to solve everything in an uncritical mechanical operation of pharmacological and dietary requirements depending on the results. In the daily life of a pediatrician allergist is, on the contrary, experience common to see allergy tests recommended for reasons completely irrelevant requests for advice allergy is not justified, allergy testing, especially on the blood which often allow you to by-pass the allergist, evaluated uncritically without taking into account, for example, the seasonal nature of certain diseases or non-significance, especially for certain values ??of the tests for food.
The diagnostic work in allergy must be strict. In the diagnostic of allergic diseases in the first and most important steps are, as always in medicine:

- 'S history , that is, the collection of news about the family and personal history of the child, past diseases in general and in particular those relating to the field allergy, history of present illness, with its characters, the characteristics of its occurrence, its seasonality, if present, the triggers or accentuanti, the effects of treatment, concomitant diseases.

- 'S clinical examination , that a thorough physical examination that in addition to assessing the body or equipment directly affected does not exclude the evaluation of other devices. In fact, it might be possible to understand the underlying diseases (asthma, for example, is often associated with rhinitis, and vice versa) or detect signs that allow to classify correctly a certain disease (eg skin manifestations of atopic dermatitis can be interpreted as allergic a persistent cough, chronic diarrhea, etc..). The tests, skin or blood, represent a further step on the path to diagnosis and should not be interpreted as absolute values ??but should be valued only if inserted in a global context and if they are compatible with the data emerging by history and clinical examination.

What are the test

The test usually used in the first instance are skin tests and blood. Skin tests are the skin prick test . They consist of puncturing the skin with the needles particular, through a drop of liquid containing the substance to be tested placed on the skin of the forearm. Usually more substances are tested for each session. The positive response to the test is the appearance of a swelling (wheal) measurable. The test is considered positive, ie the child is sensitized to that certain substance (mind you sensitized and not necessarily allergic, as will be seen later!) If the wheal has a diameter greater than a certain extent (quantitative determination expressed in mm. in diameter); the evaluation may also be done by comparing the diameter of the wheal on the tested substance with that of a wheal reference induced by a substance called histamine: in this case the positivity is expressed with spreaders according to a predetermined scale (qualitative determination) . Skin tests so described may be performed not only using "extracts", solutions of the substances potentially able to give allergy, made ??available by the pharmaceutical industry but for food can be performed directly using the same foods. In these cases we are talking about prick by prick . This technique is usually used for fruit and vegetables (but you can actually use for any food) and consists of puncturing the skin of the forearm after dipping the needle in the food itself, if liquid, or after point the food to be tested, if solid.
Tests on the blood should be carried out only in those particular conditions, such as when the child is taking antihistamines or presents skin diseases very extended, or have had very serious allergic reactions, where they can not be performed skin prick test. Outside of these particular situations, to ascertain the presence or not of allergic sensitization should be performed, first of all, the skin prick. The tests on the skin that are preferable and not only for economic reasons but also because the prick you can test many substances simultaneously, the answer is quite straightforward, especially because it is test more "specific". Less frequently than is the case with blood tests, in other words, it is possible that the positive test does not correspond to a true allergy but, on the contrary, more often those who test positive on the skin is truly allergic. With the blood test for the presence in the blood of the same patient of particular types of antibodies, the so-called specific IgE , that are involved in allergic reactions of a certain type (so-called reactions of type I ° or IgE-mediated). The determination of these IgE occurs with different methods (RAST, ImmunoCAP, ELISA). The presence in the blood of specific IgE directed to a specific substance is expressed both in terms of number according to a classification into classes. For simplicity we use to call the test of research on the blood serum of these IgE specific for the various substances with the name of the method used; generally is used to speak of RAST.
RAST I therefore consist essentially in the demonstration of the presence in the blood of the patient IgE specifically directed to certain potentially allergenic substances (allergens) and their measurement. Same target also the methods skin. Both the prick that the prick by prick, in fact, are able to determine the formation of the wheal (ie the measurable swelling) only if the skin of children tested, attached to specific cells called mast cells, are present the same specific IgE direct against the tested substances. In other words, the RAST are positive if there are specific IgE in the blood, skin prick and prick by prick if there are specific IgE in the skin. Are different methods to highlight in different tissues of the same substance: IgE specific to certain allergens. Therefore, as we shall see later, valid tests and indicated to investigate one type of allergy, allergy fact that IgE-mediated or type 1, that, for instance responsible for some allergies such as anaphylaxis or immediate 's urticaria. There may be, however, other mechanisms underlying allergic reactions, independent of IgE codeste, for which so this type of investigation is not useful. For example, much of the pathology "allergic" gastrointestinal, from chronic diarrhea to growth failure, by gastroesophageal reflux disease, constipation is not normally supported by a mechanism type allergic IgE-mediated diseases for which in these tests are usually negative , although it could be responsible foods of clinical symptoms.
To forms not IgE mediated and, in particular, for those allergic forms in which probably is in play a mechanism involving special cells called T lymphocytes, the so-called cell-mediated allergies or type IV or delayed, you can use the patch test. It 'also a skin test and is currently used for the diagnosis of contact dermatitis. It consists in applying on the skin, in particular on the back, with patches of cells containing the various substances to be tested. The patches are left attached to the skin for 48 hours and the reading of the test is made ??at 48 and 72 hours. The cell-mediated allergy to some substance tested causes a local reaction with redness, swelling, blistering that can be quantified with a score from 0 to 3. In this way you can see if a subject with a particular type of dermatitis is allergic to nickel, to rubber, to dyes tissue etc.
A similar method is used for some time ( atopy patch tests ) for atopic dermatitis a common skin disorder in which the prevalent mechanism is often that cell-mediated. In the specific case are tested foods (milk, egg, wheat etc.) And inhalants (mites) to assess whether these allergens, with a mechanism different from that of the immediate IgE mediated allergies can be, in some way responsible or co-responsible for the disease. It is a non-routine tests because it still has not been standardized, encoded.
Something like that can, in some cases, be used also in other diseases where cell-mediated component may be prominent as in many gastrointestinal diseases "allergic".

The positive test does not always mean allergy

Awareness, ie the presence of specific IgE against inhalant allergens and especially food, which is manifested by a positive skin tests and RAST, not necessarily accompanied by clinical manifestations of allergic disease, as represents an essential step towards the onset of the disease. Even more distant in the scale that leads to the allergic disease is the condition of atopy, ie the tendency, hereditary, genetic, to the production of IgE, instead of other types of antibodies, directed against food or inhalant substances present in the environment (environmental antigens ).
On the other side, as often occurs in allergies to foods, the acquisition of the state of tolerance, ie the disappearance of the food intake of reactions previously harmful, may precede the disappearance of sensitization, ie the positive tests, which may never occur. For this reason, the proof test of tolerance to foods, after a period of exclusion diet, should be performed, regardless of the persistence or less of positivity of the test, as soon as the assessment of the clinical history of the child, the age, dietary transgressions without random reactions allow to assume that the child has become tolerant. Ie before the child loses the allergy and then loses awareness.
then a positive test does not necessarily mean allergy. Obviously, however, is difficult for a strongly positive test for a certain allergen inhalation or food is present in a child who has no reaction when exposed to them, more positive is the test easier it is accompanied by a true allergy and not a simple awareness.
Failure absolute correspondence between positive test and the presence of a true condition of allergy is particularly true for food and for some of them in particular. For this reason, except in special cases, a positive skin tests or examination of the serum, perhaps done for completely arbitrary reasons, can not and should not determine, quite simply, the suspension for long periods of food. A positive test should be confirmed that on a practical level: the suspension of the food in respect of which the child tests positive for the test must be accompanied by a significant improvement in conditions for which it was suspected allergic origin and, later, the reintroduction of it must determine the reappearance of disorders significant, though not necessarily identical to the previous ("test of elimination" and "challenge test").
In this way it will be appreciated that for both methods (Prick and RAST) in relation to food
- the probability that the positive test correlates an authentic presence of allergic disease, that is, the positive predictive value (PPV), is very low,
- while, on the contrary, there is a high probability that the negativity of the test correlates an authentic absence of IgE-mediated allergic disease type, that is, the negative predictive value (NPV).

VPN and VPP skin tests and serum specific IgE to milk, egg, fish, soy, peanuts, cereals
VPN    VPP
Prick    approximately 100%    <20%
Specific IgE    > 97%    <30%
In other words, for these allergens, which are the main food allergens in pediatric age, a negative test virtually excludes the diagnosis of IgE-mediated food allergy and allows the administration of food with ease without fear of important reactions. The negative test, however, does not allow to exclude, as will be seen below, the presence of a non-IgE mediated allergy, but in any case it would, however, not sudden and violent reactions but slow and delayed as typical of cell-mediated allergies.
The situation is different for any positive, as can be inferred from the data, only a smaller percentage of cases expressing a true allergic disease and in most of them expresses only an awareness without corresponding symptoms.

The negative test result does not always exclude allergy

According to the most recent classification, allergies can be divided into IgE-mediated and non-IgE mediated. Previously, in the course of these notes concerning the interpretation of allergy testing we mentioned in this distinction trying to give some brief information on the two forms. IgE mediated reactions are the so-called type 1 ("classification of Gell and Coombs"), those characterized by the suddenness and immediacy of symptoms on exposure to allergenic substances, are in practice also potentially dangerous allergic reaction. Both the prick test that the RAST have as target the specific IgE, that is, their qualitative assessment at the level of the skin, in particular at the level of cells called mast cells, in the case of skin prick, and their quantitative evaluation in the blood serum in the case of RAST. Both these tests, of course, are not able to explore the non-IgE mediated allergies. Therefore the negativity of these tests if from a verse allows to exclude a condition of IgE mediated allergy, on the other hand can not allow to exclude a disease that is mediated by different mechanism.
Therefore, for example, in an infant with persistent diarrhea or with a stunted growth for which assume an allergy to cow's milk protein (CMA), the negativity of the serological and skin test does not rule out the cow's milk-dependence of the two diseases being the immunological mechanism responsible for this disease more frequently not IgE mediated.
The real test will be, as always, the practical test, ie, the elimination in diagnosis of cow's milk, followed, if necessary, but not necessarily, from the observation of possible resurgence of diarrhea and / or growth slowed to rechallenge, after a short period, the food.
patch testing, as assessed from the table, only vice versa explores the cell-mediated allergic reaction and is therefore the test of choice in allergic contact dermatitis (ACD). Recently, it is also enhancing its use in atopic dermatitis and in various gastrointestinal disorders in which the cell-mediated mechanism plays an important role.

Provocation test

We have seen that a negative test does not always mean the absence of allergy and that a positive test does not always expressed with certainty the presence. Obviously, however, if the overall picture, that is the history of the child and his illness and the data emerging from the medical examination, it is perfectly compatible with the results of the tests do not need any further diagnostic assessment to deny or affirm the presence of a disease allergic. These cases represent the majority of situations in practice reale.Viceversa there may be doubtful cases, discrepancies between medical history (anamnesis) and test at other times although not present that your child has a positive discrepancy not to one but to many allergens and can therefore be necessary to establish among the substances tested positive which may actually be the most important for those certain symptoms in that particular child. For these reasons you can proceed to the execution of the so-called provocation tests which consist in exposure of direct child allergic to the allergen or deemed or stimuli of different nature. These are the findings of the third level competing centers allergy. In this seat is sufficient to recall that the provocation for allergens inhalers is performed by instilling usually at the level of the nose or the conjunctivae progressively increasing amount of the substance to be tested and evaluated both with a score based on the symptoms induced with instrumentation systems that, in the case of the nose , the effects of these doses.
For food, as said earlier, the provocation test (challenge) is sometimes almost a requirement since, generally, is fairly modest the probability that a positive test is accompanied by authentic disorders recruitment of the food. Obviously not in all cases it is so, and it is the pediatrician allergist to assess the situation in each case. However, for the less strong positivity, positivity for hardly correlated to the child's symptoms, in particular for certain foods for which the statistical probability that a positive test is related to a true allergy is low, you should proceed to the practical test.
Similarly for diseases which the mechanism responsible is most often not addictive specific IgE we can not of course limit the negativity of the test, cutaneous or serological that is, to carry the food. Even in this case the importance of the food suspected to be tested practically with its suspension for a certain period of time and its subsequent re-entry after the improvement of diarrhea or constipation or of growth failure or the symptoms of gastroesophageal reflux esophagitis or of atopic dermatitis etc.. etc..

Common Mistakes

1. Do not be a prick or RAST in children <3 years. Can still be heard to say, as some physicians, that allergy tests could not be performed in young children because, at that age, "the allergy is not seen." Far from it: the tests can be performed even in a newborn. Obviously their assessment, but this applies to any age, must be made ??by a physician experienced in the field 2. Give more importance to RAST compared with skin tests. Allergy testing is always to be preferred, except in cases where it can not be done for technical reasons, the Prick. With it you can test multiple allergens at the same time, the response is immediate, it is much cheaper. It is also more specific, or more rarely you will have false positives are more common than vice versa with RAST;
3. Require the determination of total IgE (the so-called PRIST). Total IgE express only atopy, ie the constitutional tendency to allergies, no sensitization to specific substances nor, even less, the real allergy. The PRIST expresses nothing more than to express the family and personal history of the child and has no utility from the operational point of view;
4. Overestimate the importance of the low positivity of skin prick and RAST for food. In most cases the low positivity for food allergy are not accompanied by real and these children have no trouble taking concrete offending food;
5. Exclude allergy, especially in the gastro-intestinal forms, because the prick and RAST were negative. Allergies so-called non-IgE mediated, cell-mediated, can not, by definition, accompanied prick and RAST positive. In most forms of gastrointestinal which are, precisely cell-mediated, regardless of the tests and their results, it must, therefore, perform the test of elimination of the food indicted.

The alternative tests

Food intolerance has become almost a fashion, but not only this. It is in fact a real, concrete fact. Some people think that in the end there is no person who does not have some intolerance to some food, and perhaps rightly so. About 20% of people believe that you have some food intolerance with symptoms that are very different. For these people, the allergy tests "official" are of little use or not at all because it is not demonstrable allergy prick and RAST, ie so-called IgE-mediated allergies. Tests are scientifically established then only for lactose intolerance (milk sugar) and gluten intolerance (celiac disease). In all other cases the only way to ascertain an intolerance would be the usual: identification of the suspect, its elimination for some time, its reintroduction after a period of improvement. But the suspicion is often very vague, the temporal relationship between the intake of food and the symptoms very labile.
Among the feeling of being intolerant to foods that have a lot of people, and poverty aid that can be given by the medical establishment, there is a large empty space in which it has come to include a number of so-called alternative tests, often highly publicized, often very expensive, but surely everyone completely devoid of media that have shown any scientific validity.
many are:

A. Cytotoxic leukocyte test or test of cytotoxicity or cytotoxic test. This is a test born in distant 1956, even before the discovery of IgE. It is based on the concept that the addition to the blood of the patient to determine the allergen food, in case of intolerance / allergy, the rupture of white blood cells. In fact, the changes which may be viewed, could be due to several other factors (eg changes in pH, etc..) That can alter the white blood cells.
E 'test very much in use and for which many patients are labeled as allergic often in many foods. These supposed positivity do not match, the clinical point of view, a real food allergy.
E 'a free test of reliability: all the studies that have been done show that the test is not "reproducible", ie the same blood sample may correspond results entirely different depending on the laboratories.

B. Vega test. It is based on electrical variations, measured with an ammeter, determined by hold a vial containing the suspected food allergen. For this test, many studies, even very recent, they have ruled out any reliability.

C. DRIA tests. It is based on evaluation of changes in muscle strength (measured in various ways) of a limb after it has been exposed to the food. In fact, the food allergy does not cause changes in muscle strength and voluntary muscle activity so that even this test is not valid. Several studies conclude that the test is not scientific validity and that the results are completely random.

D. Kinesologia applied. In this case, even, the reduction of muscular strength that would be determined by hold the tube, is evaluated directly, as if it were a "tug of war", by the examiner. In the case of young children, the test is performed with a parent holding the baby. The reduction in muscle strength of the parent who is holding the tube with the suspected food, measured in the same way by the examiner, would be indicative of intolerance in children. Comment is superfluous!.

E. Hair analysis. Rate the content of heavy metals hair to guide the diet. It is based on the assumption that in some types of food intolerance is present a lack of trace elements. This type of test requires absolutely unbalanced diets.
Again, this test did not pass the hurdle of scientific evidence.

All of these tests therefore have no scientific reliability . The seemingly good that some people seem to get should not surprise. All medical practices "alternative", even completely far-fetched, have supporters and ardent supporters. In the specific case diets always greatly reduced, with the exclusion of many foods including, but not randomly hardly likely to contain the food really responsible, suggestion, the beneficial effect of being able to think that they have finally identified the cause of their problems , are likely to play an important role.