title: Positive serum specific IgE has a short half-life in patients with penicillin allergy and reversal does not always indicate tolerance.
abstract: BACKGROUND
  The positive and negative predictive values of specific IgE to penicillins are not well established for penicillin hypersensitivity. One reason may be that serum IgE levels to penicillin diminish over time. The objective in this study was to investigate variations in serum half-life (T½) for specific IgE to penicillins (s-IgE) and to evaluate the outcome of penicillin challenges in patients with previous but not present specific IgE to penicillins.
  
  
  METHODS
  Two subgroups were investigated. All included patients had a history of penicillin allergy with reported symptoms such as urticaria/angioedema or unclassified cutaneous rash. T½ of specific IgE to penicillins was calculated based on sera from 29 patients with repeated measurements of s-IgE. Twenty-two patients with a previous positive s-IgE was followed and challenged with penicillin when IgE had become negative.
  
  
  RESULTS
  The T½ for s-IgE varied between the 26 patients with decreasing s-IgE from 1.6 months to 76.4 months and 52% had a T½ of less than a year. The three patients with stable and increasing IgE-values showed T½ approaching infinity A total of 29 challenges with β-lactams were performed. Four different patterns were seen when evaluating the clinical reaction to challenge (positive/negative) and post-challenge boost of s-IgE (yes/no). Eight (36.4%) had negative challenge and negative post-challenge s-IgE, eight (36.4%) negative challenge, but positive post-challenge s-IgE levels. 3 (13.6%) had positive challenge and positive post-challenge s-IgE whereas 3 (13.6%) were challenge positive, but had negative post-challenge s-IgE.
  
  
  CONCLUSIONS
  Specific IgE to penicillins declines over time stressing the importance of a close time relation between diagnostic work-up and clinical reaction. Reversal of previously positive s-IgE may still be associated with positive penicillin challenges and/or re-boostering of s-IgE to positivity.
authors.affiliation: Department of Dermatology and Allergy Centre, Odense University Hospital, Sdr. Boulevard 29, DK-5000 Odense C, Denmark.
authors.forename: Janni
authors.initials: J
authors.lastname: Hjortlund
authors.affiliation: Department of Dermatology and Allergy Centre, Odense University Hospital, Sdr. Boulevard 29, DK-5000 Odense C, Denmark.
authors.forename: Charlotte Gotthard
authors.initials: CG
authors.lastname: Mortz
authors.affiliation: Clinical Pharmacology, Institute of University of Southern Denmark, Odense C, Denmark.
authors.forename: Tore Bjerregaard
authors.initials: TB
authors.lastname: Stage
authors.affiliation: Department of Dermatology and Allergy Centre, Odense University Hospital, Sdr. Boulevard 29, DK-5000 Odense C, Denmark.
authors.forename: Per Stahl
authors.initials: PS
authors.lastname: Skov
authors.affiliation: Department of Dermatology and Allergy Centre, Odense University Hospital, Sdr. Boulevard 29, DK-5000 Odense C, Denmark.
authors.forename: Ronald
authors.initials: R
authors.lastname: Dahl
authors.affiliation: Department of Dermatology and Allergy Centre, Odense University Hospital, Sdr. Boulevard 29, DK-5000 Odense C, Denmark.
authors.forename: Carsten
authors.initials: C
authors.lastname: Bindslev-Jensen
keywords: 7-day oral challenge; Half-life; Penicillin allergy; Prolonged challenge; Specific IgE to penicillin