Diagnosis

DIAGNOSIS OF ALLERGY : The diagnosis of allergic diseases is made primarily by performing a detailed physical examination and recording the patients history. It is useful to confirm the presence of specific anti-bodies against the allergens identified on the basis of history. There are various in-vivo and in-vitro methods for assessing the presence of specific IgE antibodies.

Skin testing is comparitively simple and inexpensive method of diagnosis, which is easy to perform and the results are available within half an hour. In this process the patient is also able to see the inflammatory response to the positive skin test and appreciate the differential in severity of the allergens.

For sucessful treatment of patient with respiratory allergic disorder, it is essential that the allergens causing the symptoms are accuratly identified. There are different ways to arrive at a diagnosis. The clinical history is exteremy important in all alllergy investigations. Based on the patient's symptoms, the doctor can decide to carry out various laboratory skin tests, provocations tests or more likely a combination of these.

CASE HISTORY : The case history should be form the basis of all alllergy investigations, which gives the doctor an idea of causative allergens. The doctor questions the patient or ask him to fill the questionnaire. It is important to know, when and how the symptoms develops. Also important is the relationship with seasons., damp weather, physical activity and certain foods, etc. A knowledge of personal habits such as smoking, occupation, hobbies, the pets or fitted carpets etc. Will be helpful.

The basis of allergic diagnosis is a thorough and detailed case history. The case history enables the clinican firstly to assess the important of allergy as an etiological factor, and secondly, to recognized those allergens which are the probable cause of the patient's condition.

Some allegic conditions can be identified relatively easily from the case history. Classical hay fever and grass pollen asthma, for example, have a clear-cut patterns of symptoms which can be related to the incidence of grass pollens in the atmosphere.

Similarly, many cases of asthma and perennial rhinitis may be caused by, or complicated by allergy to the house dust mite. A correlation of the frequency and severity of attack with the incidence of the house dust mite in the patient's envoirment will help considerably in the correct diagnosis of the condition.

THE PATIENT : Age may help indicate whether allergy involved in the patient's condition. Occupation may be significant, since materials contracted or inhaled at work may cause allergic reactions.

PRESENT COMPLAINT : Allergic asthma attacks are characteristically episodic, severe, of sudden onset and short duration.

ALLERGIC HISTORY : Allergic patients are likely to have personal and/or family history of allergy. A variety of allergic symptoms may appear at diffenet ages, e.g. a patient may have eczema as an baby, hay fever as an child and asthma as adult.

EVENTS PRECEDINGS PRESENT COMPLIANT : Answer to this section may indicate whether the patient has come into contact with new inhalants or contactants as a result of an environmental change.

LIKELY CAUSATIVE ALLERGENS : Question in this section are designed to indicate more important allergens implicated in the patients condition.

PREPARATION OF THE PATIENT : The patient should avoid anti-histamines and tranquilizers for 48-72 hours and other sympathomimetic drugs such as ephedrine, adrenaline, isoprenaline, salbutamol, etc. for about 8 hours prior to the time of allergen skin tests. Although experiments have shown that the influence of the sympathomimetics on skin tests start wearing off after an hour or two, it is better to avoid them for a longer period. Long acting antihistamines such as astemizole should be stopped at least for four weeks.

Prick test is preferred for intital testing because it is more rapid, less painful and the glycerinated extracts, used are more stable. However, prick test can give false results, if not performed properly.

SITE FOR TESTING : The common site for skin test ate the volar aspect of the fore-arms starting about 5 cm. proximal to the crease of the wrist and the lateral aspect of the upper arms. If there is abundant growth of hairs on the arms, this may be shaved off or the injections may be given on the back of the patient. The reactivity of skin in different areas of the body to an allergen extract, is not the same due to variation in numbers of mast cells.

1 NEGATIVE CONTROL Phosphate buffered saline (PBS)
2 POSTIVE CONTROL Histamine acid phosphate in PBS (100ug/ml)
3 DOSE 0.01-0.02 ml with tuberculine syringe
4 ALLERGEN EXTRA 1:500 W/V concentration

TECHNIQUE OF INTRADERMAL TEST

 

The skin is cleaned with 70% alcohol swab and allowed to dry. Sterile tuberculine syringes with 26 G (1/2") short bevelled sterile hypodermic needle are filled with 0.05 ml. of test solution taking aseptic precautions. The syringe is then placed at the angle of 45 degree to the arm and the needle is inserted in the superficial layer of skin (intradermal), keeping the bevel of the needle down and facing the skin. Then suitable amount 0.01 to 0.02 of the allergen/ buffered saline/ Histamine solution is injected, raising a bleb of about 2 to 3 mm. A seperate syringe should be used for each antigen. The minimum distance between the two injection site should be 5 cm to avoid overlapping of reaction.

Seperate syringes should be used for each patient.

A negative control of buffered saline and positive control of Histamine solutions is also used in exactly similar way and suitably marked with a ball pen.

GRADING : The tests are read after 15 to 20 min and graded according to the criteria given below. The means of the two diameters of the wheal is recorded for grading the reactions.

OBSERVATION : The immediate (TYPE-I) skin reaction usually starts appearing within five minutes after the injection. Therefore, all the intradermal tests (I.D) should be graded after 15-20 minutes.

(I.D is used most commonly for drug senstivity has got limited use for diagnostic purpose).

Criteria of Grading Intradermal Test
Grading of Reaction Intradermal Test
- Nearly same as buffer saline control
+/- Less than 1+
+

W : Over twice the size of control but more than 6 mm.

E : About 15 to 20 mm.

2+

W : About 3 to 3.5  times the size of control

E : Over 20 mm.

3+

W : About 3.5 to 4 times the size of control with one or two pseudipodia

E : Over 30 mm.

4+

W : About 4 to 6 times the size of control with several pueudopodia.

E : Over 35 mm.

Note : The intensity of reaction depends not only the presence and the level of the LgE antibodies but also reactivity of skin. The abtibodies level differs from person to person. The criteria in table are only a guide and not a mathematical equation.

SKIN PRICK TEST

In Prick test the following reagents are used :

1 Allergen Extracts 1:10 w/v in glycerinated buffered saline (50%)
2 Negative Control Glycerinated Buffered saline (50%)
3 Positive Control Histamine acid phosphate in glycerinated buffer saline (50%) (5.0mg/ml)

As in intradermal tests, the skin is cleaned with cotton ball soaked in 70% alcohol and is allowed to dry. Small drop of glycernited buffered saline (Negative control), Histamine solution (Positive control) and the allegen extracts, are placed on the skin in linear rows and each one is numbered. Not more 8-10 drops are placed in rows, each 2-3 cm apart. The distance is maintained so that large reactions may not overlap, making the interpretation difficult.

A sharp disposable prick test needle/blood lancet is used for prick test. It is inserted at a shallow angle into the skin, through the drop of test solution and then raised slightly. So that a small amount of test solution penetrates the skin, Excess of the test solution is wiped off with the tissue paper, two minutes after the performance of tests.

The skin prick tests are read after 10-15 minutes and are graded according to the criteria given in the table

Criteria used for grading skin responses
Grading of Reaction Intradermal Test Prick Test
Negative >3mm less >2 mm. less
1+ Upto 3 mm less Upto 3 mm less
2+ Equal to histamine response Equal to histamine response
3+ Upto 3 mm more;pseudopodia may or may not be present Upto 2mm more
4+ > 3 mm; pseudopodia usually present >2mm more
Merits and limitations of intradermal and prick testing techniques
Intradermal Test Prick Test
Less Specific More Specific
More sensitive Less sensitive
Low Potency extracts best evaluated High Potency as well as high concentration of extract required
Risk of anaphylaxis Safe
More painful Less painful

False positive skin tests are more frequently met due to :

  1. Faulty technique.
  2. Injection of too much material.
  3. Stronger concentration of antigen.
  4. Presence of chemical irritants or histamine liberating substances in the allergen extracts.
  5. Trauma with blunt or bent point of needles.
  6. Contaminated test solutions.
  7. Injection of a little air.

False negative skin test may be seen due to following reasons  :

  1. Fever.
  2. Dehydrated
  3. Diseased skin (Hyporeactive)
  4. Wrinkled skin due to old age.
  5. Injection of allergen extract of low potency.