The use of intradermal skin testing in allergy diagnosis

High volume of polysorbate-containing (Tween® 80) solutions induces false-positive results in intradermal test

BACKGROUND

Intradermal test is used to detect causative allergens in IgE-mediated hypersensitivity. The surfactant polysorbate 80 can be added to intradermal test solutions to more reliably dissolve the allergen and ensure a constant bioavailability of the injected allergen. Polysorbate 80 has, however, some histamine-releasing properties which could blur the difference to the histamine-induced wheal. Routinely serving as a control.

OBJECTIVE

Allergen-free polysorbate 80 containing (0.005%) test solutions were therefore systematically tested at different injection volumes to see whether polysorbate can falsify skin reactions and if yes whether conditions can be identified to avoid this.

METHODS

In a partly blinded study, 30 patients were tested intracutaneously at the back: each received at three separate sites 0.05 mL of polysorbate-containing solvent, 0.02 mL polysorbate-containing solvent as well as 0.02 mL polysorbate-free solvent. After 15 min, wheal and erythema were documented and planimetrically quantified.

RESULTS

Unexpectedly 23 of 30 (77%) patients showed false-positive test reactions to the volume of 0.05 mL of the polysorbate-containing solvent whereas the polysorbate-containing solvent with 0.02 mL and the polysorbate-free solvent with 0.02 mL injection volume had no reinforcing effect on skin test reaction.

CONCLUSIONS

Different volumes of polysorbate-containing solutions, all recommended by manufacturers for intradermal tests, may significantly influence test results. The study shows that a polysorbate 80 used at 0.005% in solvents for intradermal test to provide a better bioavailability of allergens produces false-positive reactivity in a surprising and a hitherto unknown 77% if the 0.05 mL volume is injected. It is, therefore, mandatory to strictly adhere to an intradermal test volume of 0.02 mL as only then falsifying effects of the polysorbate additive are avoided, and comparable test results are ensured.

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.