What does a positive intradermal skin test result really mean?

Optimal cutoff values of allergen-specific immunoglobulin E to house dust mites and animal dander based on skin-prick test results: Analysis in 16,209 patients with allergic rhinitis

BACKGROUND

The most common tests for allergen sensitization in patients with allergic rhinitis are the skin-prick test (SPT) and an in vitro test to detect serum specific immunoglobulin E (sIgE). However, in vitro allergen test results were interpreted dichotomically as positive or negative at a threshold of 0.35 kU/L of sIgE, regardless of the patient characteristics or antigen types.

OBJECTIVE

The purpose of this study was to determine the cutoff value for sIgE in house-dust mites and animal dander, and to analyze differences in cutoff value according to age and gender.

METHODS

A total of 16,209 patients with more than one allergic rhinitis symptom who underwent both SPT and serum sIgE testing were retrospectively evaluated between March 2008 and May 2012. There were 9374 male (57.8%) and 6835 female (42.2%) patients. The mean age was 31.8 years (range, 2-89 years). The criterion standard for allergen sensitization was defined as a wheal of >3 mm or an allergen-to-histamine ratio of ≥1 in SPT results. The Youden index was used to calculate the cutoff value of sIgE.

RESULTS

Cutoff values of sIgE for Dermatophagoides pteronyssinus, Dermatophagoides farinae, cat, and dog were 0.69, 1.16, 0.13, and 0.45 kU/L, respectively. The cutoff value of sIgE changed according to age for D. pteronyssinus and D. farinae but not for cat and dog allergens. When categorizing according to age group, the cutoff values of sIgE for D. pteronyssinus and D. farinae had a tendency to decrease with age. There was no significant difference in cutoff value according to gender.

CONCLUSION

The cutoff value for sIgE differed for each antigen and changed with age. Physicians should select the proper cutoff value for sIgE for appropriate criteria according to antigen and patient age rather than using a uniform cutoff value.

Intradermal skin test results correlate with atopy

Intradermal skin testing (IDST) is performed by most allergists, but its value remains controversial. In most previous studies assessing the value of IDST, a positive result was based on a 5- to 6-mm wheal and erythema. While a subject's immediate reaction upon allergen exposure is a common endpoint to determine "allergy," additional known allergic phenomena such as the delayed or late-phase response to allergen exposure suggest that correlation only with current history or acute challenge may result in an incorrect labeling of false-positive results when, in fact, there is a physiologic response to exposure, albeit a reaction not immediately evident. We conducted a retrospective study to determine if positive IDST reactions represent nonspecific irritation or if they correlate with atopy. We retrospectively reviewed the records of 100 patients who had undergone skin prick testing (SPT) and IDST and compiled data on their age, sex, diagnosis, and number of skin and intradermal reactions. Results were analyzed according to a general linear model to see if the development of a positive IDST result correlated with atopy (defined as SPT positivity). We found statistically significant differences between SPT-positive and -negative patients with regard to diagnoses of asthma, rhinitis, or both (p=0.008). Controlling for asthma and rhinitis, we also found a significant association between atopy and a positive IDST result; among atopic (SPT+) patients, a mean of 25.9% of IDST results were positive, compared with a rate of only 6.7% IDST positivity among the nonatopic (SPT-) patients (p<0.0001). We conclude that IDST is more commonly positive in atopic (SPT+) than nonatopic (SPT-) patients, suggesting that a positive IDST represents genuine atopy and bona fide sensitization rather than nonspecific irritation.

Analysis of allergen specific IgE cut points to cat and dog in the Childhood Allergy Study

BACKGROUND

The application of a single allergen specific IgE (sIgE) cut point, such as 0.35 kU/L, to determine sensitization for all allergens may be suboptimal.

OBJECTIVES

To analyze self-reported symptoms suggestive of dog and cat allergy in relation to the test performance characteristics of low level, but reliably detectable, sIgE and to compare these cut points to the traditional 0.35-kU/L cut point.

METHODS

Interviews and blood samples were collected among 564 young adult participants of a general risk birth cohort. Data collected from the participants' parents were analyzed as validation populations. A history of symptoms consistent with allergy on exposure to pets was obtained by standardized questionnaire. Allergen sIgE levels for dog and cat were evaluated with Pharmacia CAP. Receiver operating characteristic curves were constructed and the performance characteristics of the traditional sIgE cut point of 0.35 kU/L were compared with cut points as low as 0.1 kU/L.

RESULTS

Using the Youden J criteria, based on the highest sum of sensitivity and specificity for a diagnostic test, cut points of 0.12 kU/L for cat and 0.2 kU/L for dog were identified as performing optimally among the participant population. In 2 validation populations, consisting of the participants' mothers and fathers, the performance of these alternative cut points were superior or similar to the traditional 0.35-kU/L sIgE level.

CONCLUSIONS

Accurately measured sIgE at levels approaching the lower limit of detection of current assays may be useful in confirming sensitization. Optimal clinical application of these tests will continue to require careful integration of the result and the strength of the patient's history.