Antunes J et al. wrote a nice review about skin prick test (SPT).1 We congratulate them and we also report a case of adverse reaction to SPT elicited by an iatrogenic procedure.
Skin prick test is an essential diagnostic tool in allergy practice. The simplicity, rapidity of performance, improvement of patient adherence, high sensitivity and low cost make it preferable to in vitro testing for determining the presence of specific IgE antibodies.
Systemic reactions with SPT for inhalant extracts are rare and have decreased dramatically to an overall risk below 0.02% for anaphylactic reactions.1,2 Studies have identified some risk factors for systemic reactions: SPT with fresh food, latex, and non-standardised extract; SPT in duplicate; SPT in young children, pregnant women, and patients with extreme eczema.1,2
Another main cause of adverse reactions to SPT is iatrogenic procedures. SPT should always follow the pre-established recommendations, and safety procedures should be adopted as a precaution in the event of adverse reactions during or after the test. The correct technique, the extract quality and its concentrations are crucial to obtain a reliable and safe SPT.
We report a 27-year-old patient with mild persistent allergic rhinoconjunctivitis and asthma; the diseases were well controlled with low doses of nasal and inhaled corticosteroids. The patient was being evaluated for immunotherapy, during a clinical trial, when he underwent a SPT with a standardised Dermatophagoides pteronyssinus (Der p) extract. After 10min, our patient started to complain of ocular and nasal itching and developed a rhinoconjunctivitis crisis (Figure 1). Mean wheal diameter, calculated as the sum of the largest diameter (25mm) and its largest orthogonal diameter (15mm) divided by 2, was 20mm. He was promptly treated with intravenous anti-histamine and presented clinical improvement.
Reviewing the extract preparation, we realized that our extract was twenty times more concentrated than a SPT extract should be. By mistake, its dilution had not been made appropriately, and it was not double checked, a procedure that could have prevented the systemic adverse reaction observed.
A month later, the patient was resubmitted to the SPT and performed a specific bronchial challenge, both tests using an appropriate Der p extract. At this time, the SPT elicited a mean wheal diameter of 6mm. The specific bronchial challenge was positive, showing an early and a late reaction.
This case illustrates that an allergen can be absorbed through the skin, depending on its concentration, and elicits distant reactions in primed tissues. We have also shown that an allergen can be absorbed by the airway mucosa during a specific bronchial challenge and cause delayed urticaria.3 Skin prick test, a hallmark of allergy practice, is a safe procedure, but pre-established recommendations have to be followed.
