Rational ideation and empiric validation of an innovative digital dermographic tester.

Background: Dermographism is a condition characterized by a weal response to a combination of pressure and traction on skin surface, and its diagnosis is based on medical history, clinical criteria and provocation test. The Dermographic Tester , a pen-sized tool containing a spring-loaded blunt tip, is the most widely used instrument for the provocation test, and it exerts increasing pressures on the skin surface according to an arbitrary units (AU) scale. Analysing the mechanism of function and trying to convert the AUs to SI units (g/mm ), we found that this instrument had some defects and limits that would compromise a true and repeatable quantification of the weal response threshold. Consequently, we decided to develop a new instrument, the Digital Dermographic Tester (DDT), which is engineered with an inside force sensor to implement features lacking in the current tools, in the hope of enhancing the precision of the provocation test.

Aim: To validate the effectiveness and accuracy of the DDT.

Methods: We tested the DDT on 213 participants purposely sampled to obtain three groups, each with a different pattern of reaction to mechanical stimuli. Based on anamnestic, diagnostic and symptomatic criteria, patients were divided into dermographic urticaria (DU), spontaneous urticaria (SU) and healthy control (HC) groups. The DDT was used to apply 12 levels of pressure to the skin surface, and a frequency distribution of positive reactions was displayed for each group.

Results: A force of 36-40 g/mm appropriately differentiated physiological from pathological conditions with high sensitivity and specificity.

Conclusions: The DDT was found to be capable of differentiating patients with DU patients from those with SU and from HCs, and was able to precisely identify the weal elicitation threshold.