False negative SPT after anaphylaxis.

Abstract

. A 58-YEAR-OLD woman with a history of childhood eczema and food allergies underwent intravaginal ultrasound examination because of vaginal bleeding. The gynecologist used plastic gloves and Hibitane gel. The ultrasound transducer had a natural rubber latex (NRL) cover (Ultracover International Medical). Within 5 min, the patient developed edema and erythema on her face, and a sense of laryngeal swelling; her blood pressure was 70/50 mmHg. She was treated with intravenous epinephrine and hydrocortisone, and the symptoms soon disappeared. She also received peroral cetirizine for 5 days. The first evaluation was made 12 days after the anaphylactic reaction. She had a positive skin prick test (SPT) to grass pollen, but not to standardized NRL solution (Stallergenes, SA, Fresnes, France) or chlorhexidine chloride (1% aqueous solution) (Table 1). There was only a suggestive 2-mm reaction to one NRL glove extract (Lic) (3). Two other glove extracts (Triflex, Baxter and Armi) remained negative. The NRL use test with a NRL glove (Triflex) (1) was negative. The commercial NRL CAP-RAST (Pharmacia, Uppsala, Sweden) was negative. In immunospot (2), no binding of specific IgE antibodies to the NRL cover or Hibitane gel could be demonstrated in the patient’s serum. Three months after the anaphylaxis, our patient had positive SPT reactions to standardized NRL solution, to several NRL glove extracts, and to an extract made from the ultrasound NRL cover (Table 1). The commercial NRL RAST was negative. In immunospot, no binding of specific IgE antibodies could be demonstrated to the ultrasound cover or several other NRL extracts despite a very high NRL allergen content in the shield (393 RLU/ml) as compared to low-allergenic NRL gloves (,10 RLU/ml) in latex RAST inhibition. Experimental studies indicate that stimulation of mast cells results in two opposing reactions, activation events that cause degranulation and desensitization events that inhibit mediator release. Mast cells display both specific and nonspecific desensitization (3). Our patient had not taken cetirizine for 7 days before the first SPT. Reactions to histamine were equal in the two tests. The patient reacted to grass pollen at the same time that the NRL tests were negative. This could be explained by specific desensitization of skin mast cells. Specific mast-cell desensitization is also thought to be the principal mechanism of acute penicillin desensitization. In penicillin-desensitized patients, SPT reactions to penicillin determinants diminish or become negative, while reactions provoked by histamine and environmental allergens are not affected by the desensitization procedure (4). In one report of 38 patients with insectsting anaphylaxis, the accurate diagnosis could not be confirmed in eight patients 1 week after anaphylaxis with SPT or RAST (5). Our study also supports the recommendation to postpone the SPT after anaphylaxis or repeat the negative tests after some months. However, in vitro tests may remain negative even after this period, because the SPT is more sensitive than in vitro tests (1).