Precise area determination of skin-prick tests: validation of a scanning device and software for a personal computer


Dr L. K. Poulsen, Laboratory of Madical Allergology, Medical Department TTA 7542. National University Hospital. 20 Tagensvej, DK-2200 Copenhagen N. Denmark.


When skin-prick tests (SPTs) are used quantitatively, the circumference of the weal and/ or the flare is outlined using a felt tip pen, and transferred to paper by adhesive tape. The aim of the study was to develop and validate a procedure, objectively and precisely determining these areas after transfer to paper. A system was developed enabling the drawing of the area of weal or flare to be read by a hand-held scanner and calculated on a personal computer. Areas in the 5-500 mm2 range could be determined with day-to-day and interoperator coefficients of variation (CVs) of 3.3% and 1.1%, respectively. Accuracy was determined in two ways: by correlation to cutting/weighing of four times enlarged SPT areas (r2= 0.0, P< 0.0) and by measuring standardized areas (deviations less than intra-assay CV, i.e. 1-2%). For comparison, CVs of alternative methods were also determined: eight different areas (9-76 mm2) were evaluated in quadruplicate using the SPT-scanner (CV=1.1%), by cutting/weighing of paper (CV = 2.2%), by digitizing (CV = 4.4%) or by measuring longest and orthogonal diameters (CV= 13.13%). In conclusion, the scanning device and software provides an objective and reproducible procedure for rapid determination of SPT areas. When areas are determined by scanning, digitizing or cutting/weighing the variations in area determination becomes negligible compared to the variations of the entire skin test procedure.