Skin, blood testing not very useful for identifying ‘co-allergy,’ study suggests

by Salynn Boyles
Contributing Writer March 27, 2017

More than 50% of people with documented allergies to peanuts or a single tree nut passed an oral food challenge to other tree nuts, despite showing skin or blood test sensitivity to those other foods, researchers said.
Findings from the retrospective analysis involving more than 100 patients with tree nut allergies or sensitization suggest that skin and blood testing is not particularly accurate for identifying tree nut co-allergy, according to Christopher Couch, MD, of the University of Michigan in Ann Arbor, and colleagues.
Nearly all patients with peanut allergy and tree nut co-sensitization passed the tree nut challenge, questioning the clinical relevance of ‘co-allergy’,” the researchers wrote online in Annals of Allergy, Asthma and Immunology. Although conducted at a single center, the study is one of the largest ever reported to address the accuracy of standard tests for “co-allergies.”
Couch told MedPage Today that controlled oral food challenge in an allergist’s office is the best test available for identifying additional tree nut allergies.
“What this study shows is that patients can have positive [skin or blood] tests, without being truly allergic,” he said. “This is where oral food challenge really gives a much clearer, objective result.”
The study included results from all tree nut oral food challenges conducted at the University of Michigan Division of Allergy and Clinical Immunology clinics from 2007 through 2015. All patients had tree nut skin prick testing and/or corresponding tree nut sIgE testing before undergoing the food challenges. Delayed oral food challenge was defined as longer than 12 months from the time of an sIgE level lower than 2kUA/L.
Among the main study findings:
• The overall passage rate was 86% for 156 tree nut oral food challenges (OFCs) in 109 patients (54 almond, 28 cashew, 27 walnut, 18 hazelnut, 14 pecan, 13 pistachio, and two Brazil nut)
• Passage rates were 76% (n=67) in patients with a history of tree nut allergy who were challenged to another tree nut to which they were sensitized and 91% (n=65) in those with tree nut sensitization only (mean sIgE 1.53 kUA/L; range 0.35-9.14)
• Passage rates were 89% (n=110 of 124) for a tree nut sIgE level lower than 2 kUA/L and 69% (11 of 16) for a tree nut sIgE level of at least 2 kUA/L
• Among 44 challenges in patients with peanut allergy and tree nut co-sensitization, the tree nut OFC passage rate was 96%
• In 41 tree nut OFCs with a tree nut skin prick test wheal size of at least 3 mm, 61% passed, with a mean wheal size of 4.8 mm (range 3-11) in those passing versus 9 mm (range 3-20) in those failing
“The discovery of tree nut sensitization in individuals who have never ingested any tree nut or might be reactive to another tree nut but have never ingested the particular tree nut in question has become problematic in clinical practice,” the researchers wrote. “Sensitization in this context is difficult to interpret, is poorly specific and can lead to potentially unnecessary food avoidance.”
In an analysis of these patients, 91% of 42 patients with sensitivity to a tree nut undergoing 65 tree nut OFCs were successful, leading the researchers to conclude that controlled food challenge in this scenario “might have high utility.”
The retrospective design of the study was cited as a study limitation, as was the fact that the data were obtained from electronic medical record review. The potential for selection bias was also cited, since most challenges were performed in patients with s IgE levels lower than 2 kUA/L and/or skin prick test wheal smaller than 3 mm.
“We propose that positive tree nut skin prick results (wheal size ≥3 mm) might be a better predictor of oral food challenge outcome than sIgE levels in individuals with tree nut allergy when the two tests are available,” the researchers wrote.
“We found that almond might be introduced into the diet of patients with peanut allergy without the need to perform skin prick testing, sIgE, and/or oral food challenge because 100% passed the almond challenge in our sample. Although this study contributes valuable data on tree nut allergy and tree nut sensitivity, additional studies are needed to help guide clinical decision making in this area.”

Funding for this research was provided by the National Center for Advancing Transplational Science, and others.
• Reviewed by F. Perry Wilson, MD, MSCE Assistant Professor, Section of Nephrology, Yale School of Medicine and Dorothy Caputo, MA, BSN, RN, Nurse Planner
• Primary Source
Annals of Allergy, Asthma and Immunology
Source Reference: Couch C, et al “Characteristics of tree nut challenges in tree nut allergic and tree nut sensitized individuals” Ann Allergy Asthma Immun 2017; DOI: 10.1016/j.anai.2017.02.010.