Even though the risk of adverse reactions with prick testing is low61,86 an emergency tray was made available. It contained all the emergency drugs and basic equipment to be used in the event of an anaphylactic reaction. The contents of the tray included:
Vials of Adrenaline injection B.P in 1000
Vials of Adrenaline injection B.P in10,000
Chlorpheniramine injection B.P
Chlorpheniramine tablets
Prednisolone tablets
Vials of Hydrocortisone injection
Vials of water for injection
Nebules of Salbutamol
Nebules of pulmicort (budesonide inhalation)
Vials of Aminophylline injection
Bags of 0.9% Normal saline infusion
Infusion giving sets
Plaster
Ambu bag
Endotracheal tubes
Tourniquet
Needles and syringes
Nebuliser
A cylinder of oxygen was also available with a face mask. However, none of the patients had any significant adverse reaction to the tests. The only reaction noted was mild itching
at the site of prick test in few patients. One asthmatic patient reported sneezing for a few days.
3.9.1 Prick testing method
Each subject was seated comfortably, and the procedure was explained prior to
commencing the tests. The pulse and blood pressure were recorded. The flexor aspect of the forearm was the site used for the tests. The forearm was rested comfortably on the table. The skin was then cleaned with soap and water and allowed to dry. Strong antiseptics and organic solvents were not used to clean the site because these may react with test solutions and give unreliable results.
A ball pen was used to make 9 horizontal lines on the forearm, at about 5cm intervals.
Since each vial was used more than once, aseptic precautions were taken to ensure that the applicator did not touch the skin of the subjects. This was to avoid the risk of microbial contamination. With the applicator that was attached to the vial cap, one drop of each of the control and test solutions were placed at the intervals made on the flexor aspect of the forearm.
The prick lancet supplied was held at an angle of 300 (almost parallel) to the skin with its tip in the drop solution. This was pushed gently just to allow the tip to enter the
superficial skin layer taking care not to draw blood. The tip was slightly raised and then withdrawn. The procedure was repeated with each test and control solution, rinsing the lancet in normal saline and then wiped cotton wool between each test. The excess test solution was carefully dotted away with cotton wool. A lancet was used per patient.
The subject was instructed not to scratch the test area. The skin prick test technique was strictly adhered to as the test solutions were not recommended for intradermal testing.
Intradermal skin testing is usually done by injecting into the superficial layer of the dermis through a fine bore (26 or 27g) needle with its bevel pointing upwards. The quantity which may conveniently be injected varies from 0.01 to 0.1ml. For clinical purposes, 0.05 ml or the amount which just causes a visible weal (0.01-0.02ml) is sufficient.
3.9.2 Interpretation of Skin Reactions
Assessing the strength of each reaction is important. Major or minor allergies can be differentiated based on the measurement of skin reactions. The grading of each reaction can be done by assessing the degree of erythema and the area of the weal formed.
However in this study the assessment was done by grading only the weal reactions.
Erythema measurement was not done because this was difficult to assess in dark coloured individuals. The reactions were examined fifteen minutes after testing and graded according to the manufacturer’s written directive. Reactions to histamine were examined and assessed ten minutes after the test. The measurement of the weal was done with a transparent ruler. The weal size was measured using the transparent ruler and the chart provided by the manufacturer.
Grade Reaction
- No wheal. Erythema absent or less than 1mm diameter + Wheal absent or very slight. Erythema present, not more than 3mm diameter
++ Wheal not more than 3mm diameter, with associated Erythema
+++ Wheal between 3mm and 5mm diameter, with erythema.
++++ Any larger reaction, possibly with pseudopodia.
.
The size of the reaction to the tested allergens was assessed by subtraction of the size of reaction to the negative control. Wheal that were 1-2mm greater than the size of the prick control solution were considered minor allergens and those greater by 3mm or more above the size of the prick control solution were the major allergens. A positive reaction was defined as a reaction size of 3mm or greater above the size of the prick control solution.
When all tests were completed, moist cotton wool and soap were used to clean the pen markings and to wipe off the residual extracts from the patients forearm. The total duration of each test from the time the solutions were administered to the end of the procedure was approximately 30 minutes. The pulse and the blood pressure of the patients were measured again at the end of the procedure. The patients were observed for another 30 minutes for adverse reaction before they were allowed to leave.
3.9.3 Measures taken to eliminate variability
Measures that were put into consideration to eliminate variability of the skin test results were:
- The allergen extracts were purchased from the same vendor.(Allergy Therapeutics England)
- The same technique and criteria were used for all the skin tests - The tests were all performed by the same person (the author)