Method:
(1) All tubes are properly labeled with the patient’s name and other identifiers.
(2) Venous blood is collected into a sterile glass tube without additives.
(3) The blood is allowed to clot for 30 minutes at room temperature.
(4) The tube is centrifuged for 10 minutes at 450 to 500 g.
(5) The site for testing is selected, usually on the volar forearm. The area should be free of spontaneous wheal formation during the past 24 hours.
(6) Gently clean the testing area with an antiseptic to which the patient is not allergic.
(7) Prepare 2 sterile syringes with a 27 gauge needle with:
(7a) 0.05 mL of the patient’s serum
(7b) 0.05 mL of normal saline.
(8) Prepare a histamine control. This may be a skin prick test device with a histamine solution (10 mg/mL) OR a syringe 0.05 mL with 0.5 to 1.0 micrograms of histamine.
(9) Perform intradermal injections with each syringe with sites 3 to 5 cm away from each other.
(10) Wait 30 minutes.
(11) Cross-light each injection site and mark the edge of edema with a marking pen.
(12) The test is valid if the reaction to histamine is positive.
(13) Measure the maximum perpendicular diameters of the serum and normal saline injection sites in millimeters.
(14) Take the average of the diameters for each site.
Interpretation:
• The test is positive if (1) the histamine reaction is positive, (2) the site of the serum injection was red, and (3) the mean of the diameters diameters of the serum injection site is >= 1.5 mm than the mean of the diameters for the normal saline injection site.
Critical factors:
(1) There must be controls in place to ensure that the injected serum sample is from the patient.
(2) The injection sites must be at areas that have not had a spontaneous wheal in the previous 24 hours.
(3) The method of measuring the size of skin lesions must be precise and accurate. The pen marks to delimit lesions must be thin.
Performance:
• Sabroe et al reported a sensitivity of about 70% and specificity of about 80%.
