Skin Prick Test Wheal Measurement Protocol

The outcome of the SPT can result in a variety of wheal shapes, as shown in Fig. 1. To determine the average diameter, the mean value of the longest and the midpoint orthogonal diameter (mm) of the wheal were measured (Fig. 2). The area of the wheal was determined by using a flatbed scanner (Hewlett Packard) in combination with software earlier developed by Erasmus MC: Precise Automated Area Measurement of Skin Test (PAAMOST) [6 , 7 (link)]. Mean values of two histamine-induced wheal sizes of the positive control were collected as well. Based on the measured data the HEP-indices were calculated for both the average diameter and area.Fig. 1

Typical observed wheal forms in SPT’s

Fig. 2

Definition of D1 and D2

Consequently the four readings were defined as:

Average diameter (allergen-induced average wheal diameter).

Area (allergen-induced area measured by scanning device).

HEP-index diameter (allergen-induced average diameter divided by histamine-induced average diameter).

HEP-index area (allergen-induced area divided by the histamine-induced average area).