For the study, diagnosis and interpretation of the results
of treatment, various tests are used. These are described throughout the text,
but those used specially in allergic skin diseases are given here.
Patch Test.
The patch test is the
simplest. It is a method of testing the skin for the identification of
sensitization to various substances, and is particularly useful in eruptions of
contact dermatitis (dermatitis venenata), in which the suspected causes are
plants, industrial chemicals, medicines, cosmetics, foods, household articles
and kindred products. The excitant may contact the skin internally through the
blood stream or directly from the outside.
The patch test consists of the
application to uninjured skin, contiguous to the involved area, of substances
suspected to be causes of the dermatitis or eczema. This is done by saturating
a small piece of gauze with one of these substances in a concentration which
will not cause irritation in the average person, and then applying this patch
to the patient's skin. It is covered by adhesive plaster. By using the
patient's lower back or thighs, forty or more patch tests can be made at once
in perpendicular rows. Whenever many tests are made the site of each should be
marked with a number to identify it and assure accuracy of future readings. The
patches are usually allowed to remain in place forty-eight hours (unless there
is pronounced irritation). Readings
should not be made until the patches have been off for twenty minutes as
positive reactions may not show immediately. It may be two or three days later
before a positive test shows, so it is important to watch for delayed
reactions.
Tomato juice, egg, asparagus, orange,
or other foodstuff may be applied similarly. The raw food is better than cooked
food for these tests. In housewives, the cause of chronic eczema of the hands
often may be determined by this means. Carrots, oranges, tomatoes, turnips,
onions, garlic and asparagus commonly cause dermatitis and give positive
reactions.
The clinical response to the patch test shows wide variations. The important
reactions are diffuse erythema, often intensified at the follicles or with
small vesicles at the follicles, a bullous reaction, or a distinct follicular
vesicular reaction. All may later be followed by crusting and exfoliation.
Although refinements in the method of making this test have improved its
usefulness, it is still impossible exactly to duplicate the actual contacts
encountered by the exposed person. For this and for other reasons the results
of the test are far from conclusive, particularly when negative. Due to the fact
that sensitivity may be localized in one area and not be present elsewhere, the
test should be applied near to or upon the affected site. During an acute
outbreak this might lead to extension and increased severity of the eruption,
for which reason it is often wiser to defer such a test until the dermatitis
has subsided, although there may be some change in the local tolerance at that
time.
A localized miliariform dermatitis may be produced by patch
testing due to the high humidity caused locally by the occlusive dressing. Such
dermatitis is apt to occur under adhesive tape or dressings that become
saturated with sweat and other materials in which fungi and bacteria flourish.
It is
There are marked differences in the sensitivity of the skin
proper, the mucocutaneous junctions and the mucous membranes. Frequently local
hypersensitivity will exist in one of these without being present in the
others. For instance, the skin and lips may be sensitive to poison ivy although
the mucous membranes usually are not. The buccal mucosa, on the other hand, may
become irritated from menthol in cigarettes, whereas patch tests on the skin
with menthol show no particular skin sensitivity. In a like manner sensitivity
to lipstick, which is usually due to the dye, is apt to be confined to the lips
so that patch tests on other areas with lipstick give negative results. In
addition it may be pointed out that hypersensitivity of the vaginal mucous
membranes to quinine and other ingredients in douches and contraceptives is
usually limited and not accompanied by positive skin tests to these substances.
Percutaneous
and intracutaneous tests with various foods, pollens, epidermal and bacterial
proteins are made in persistent or recurrent cases of chronic pruritus, and
eczema. Whereas the results of such tests are not in themselves sufficiently
reliable to warrant a conclusion as to the etiology of the disease, they may
arouse a suspicion of the cause and thus be of some assistance. Epinephrine
should always be at hand.
Scratch Tests
The percutaneous tests are usually made by the scratch
method. A separate scratch is made for each substance tested and in addition a
control scratch is made to prove the harmlessness of the decinormal sodium
hydroxide solution which is used for dissolving the test substances. As a rule,
the lower back or the anterior surface of the thigh is chosen for the
scratches. These are made 1/2 inch long with a pointed scalpel through the
epidermis, without causing any bleeding. As each scratch is finished a few drops of decinormal sodium
hydroxide solution (in infants and on sensitive skins physiologic saline
solution is preferable) are placed upon it and into this a bit of the test
substance is dropped, the two then being mixed with a clean toothpick. As many
as one hundred tests may be made at a sitting. The test substances are left on
for fifteen minutes, after which they are rinsed off with sterile water and the
skin is dried.
Readings are then made, first of the control
scratch, which had only decinormal sodium hydroxide solution applied to it, and
then of the others. Positive reactions consist of wheals with pseudopods,
dime-sized or larger. Rarely an edematous plaque, several centimeters across,
will appear and even develop into a generalized but transient erythema. In this
event there may supervene systemic symptoms such as nausea, a chilly feeling, a
flush, and dyspnea. Negative results are those in which the scratch mark, after
removal of the test substance, shows little or no change. The patient is
instructed to observe the test region throughout the next day for possible
tardy reactions. The positive tests may suggest a cause. Proof of the cause is
given by actual trial. If possible, the suspected substance is eliminated from
the patient's environment and then he is exposed to it freely, so that his
response to its absence and presence may be studied.
Intracutaneous Tests
It is also routine to do intracutaneous tests by using a
hypodermic needle on a syringe. In this method no scratches are made. An
aqueous solution of the test substance is introduced intradermally. This is
accomplished by putting the skin under tension with the
The results are read at the end of a half hour and also each
following day for a week. With this method of testing, positive reactions are
red, edematous nodules larger than reactions obtained by the scratch technique.
Scratch tests are particularly valuable for
pollens and intracutaneous tests for foodstuffs; but the patch test is of
greatest value, being used for contactants because it reproduces the epidermal
reaction.
Prausnitz-Kustner Reaction
In a great number of individual cases, the existence of
specific antibodies has been demonstrated by the method of Prausnitz and Kustner—by
the local transfer of hypersensitiveness. Much of our knowledge of the behavior
of antibodies is derived from this test. It is performed by making two
intracutaneous injections on the back of a normal test subject, one consisting
of a dilution of serum from the idiosyncratic patient and the other of a
corresponding dilution from a non-allergic control. The allergen is introduced
intracutaneously at both sites of injection after an interval of twenty-four
hours. The area sensitized by the passive transfer of serum from the allergic
patient gives an immediate positive reaction and the other, none.
A modification of this test is the
"distant reaction" in which the allergen is introduced into a part of
the skin other than that used for the serum injections. The effect is as
before; a wheal is produced only at the site of the idiosyncratic serum
injection. It proves that the antibodies in the idiosyncratic serum have
remained fixed to the cells where the serum was injected, and that only a very
small quantity of allergen is needed. The, distant reaction was carried still
further by the experiments of M. and A. Walzer, who showed that, in cases of
food allergy, the idiosyncratic injection sites will react as usual even if the
allergen is given to the test subject by mouth. Sulzberger carried the work
still further by showing that a positive P.-K. test may result from inhalation
of the allergen in cases of silk sensitivity.
The
Prausnitz-Kustner test has been shown to be positive in hay fever, asthma of
some types, and some kinds of urticaria. Where it is positive the patient's own
skin almost always gives a strong urticarial reaction to the offending
substance, either injected intracutaneously or rubbed on a scarified area. But
the P.-K. test is not by any means positive in all instances of allergy, nor
for all allergens.
Elimination Diets
Tests by elimination diets are described in the chapter on
eczema. They are particularly valuable, often being more dependable than the
tests. For detailed elimination diets, reference should be made to special
works on food allergy. In all cases of sensitization, exposure to the allergen
must be avoided if possible.