Scratch-patch and patch testing in drug allergy--an assessment of specificity

Adverse drug reactions occur frequently in daily practice, but the identification of the eliciting drugs is difficult due to a lack of standardized procedures. The aim of this study was to evaluate and compare scratch-patch and patch tests prospectively with frequently used drugs in persons exposed or not exposed to the drug in order to define a nonirritating concentration.

Altogether, 23 different drugs, mainly antibiotics and analgesics, were evaluated. Any contraindications for patch tests were excluded by a questionnaire. Sixty-one healthy individuals were tested. They were divided into two groups: Group 1 consisted of individuals who hadn't been exposed to the drug; Group 2 was comprised of individuals who had been exposed to the drug, without side effects. Each individual was tested with 10-20 scratch-patch and patch tests. The tests were performed on the upper back. Freshly prepared drug solutions were applied, on one side after scarification of the epidermis, on the other side without. Scratch-patch tests were read at 24 and 48 h, patch tests after 48 and 72 h. In addition, we included 73 individuals with a drug allergy to illustrate that the chosen drug concentrations are able to elicit a positive reaction in truly sensitized individuals.

A positive reaction in the non-exposed and nonallergic individuals was considered to be an irritant reaction (IR). In Group 1 (non-exposed individuals), 341 scratch-patch tests were performed, with 22 showing a reaction at the first reading, 3 at the second. Of the 341 patch tests performed in this group, 21 reactions were observed at the first reading, and 7 at the second. In Group 2 (exposed individuals), 141 scratch-patch tests were performed, with 5 IR at the first reading and 1 at the second. Of the 138 patch tests performed in this group, 6 individuals showed an IR at the first, 2 at the second reading. The majority of IR were four drugs, namely cefuroxim, acetylsalicylic acid, propyphenazone/drofenin-HCl, and celecoxib. Lowering the concentration did not reduce the unspecified irritant skin reaction, while a change of the solvent (petrolatum instead of PBS) did reduce the IR.

Our data show concentrations of 23 drugs suitable for scratch-patch or patch tests, as they do not elicit IR. Fourteen of these test preparations elicited a positive reaction in truly sensitized individuals, demonstrating that the test preparation is suitable to identify sensitized persons. Scratch-patch tests are an interesting alternative to patch tests, as they do not elicit more IR than patch tests but can be evaluated earlier. Certain drugs solved in PBS are irritant over a wide dosage range and need to be applied in petrolatum.