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. 2016 Feb 23:6:8.
doi: 10.1186/s13601-016-0092-0. eCollection 2015.

Measurement and interpretation of skin prick test results

J P M van der Valk  1 , R Gerth van Wijk  1 , E Hoorn  2 , L Groenendijk  1 , I M Groenendijk  1 , N W de Jong  1
Affiliations

Measurement and interpretation of skin prick test results

J P M van der Valk et al. Clin Transl Allergy. .

Abstract

Background: There are several methods to read skin prick test results in type-I allergy testing. A commonly used method is to characterize the wheal size by its 'average diameter'. A more accurate method is to scan the area of the wheal to calculate the actual size. In both methods, skin prick test (SPT) results can be corrected for histamine-sensitivity of the skin by dividing the results of the allergic reaction by the histamine control. The objectives of this study are to compare different techniques of quantifying SPT results, to determine a cut-off value for a positive SPT for histamine equivalent prick -index (HEP) area, and to study the accuracy of predicting cashew nut reactions in double-blind placebo-controlled food challenge (DBPCFC) tests with the different SPT methods.

Methods: Data of 172 children with cashew nut sensitisation were used for the analysis. All patients underwent a DBPCFC with cashew nut. Per patient, the average diameter and scanned area of the wheal size were recorded. In addition, the same data for the histamine-induced wheal were collected for each patient. The accuracy in predicting the outcome of the DBPCFC using four different SPT readings (i.e. average diameter, area, HEP-index diameter, HEP-index area) were compared in a Receiver-Operating Characteristic (ROC) plot.

Results: Characterizing the wheal size by the average diameter method is inaccurate compared to scanning method. A wheal average diameter of 3 mm is generally considered as a positive SPT cut-off value and an equivalent HEP-index area cut-off value of 0.4 was calculated. The four SPT methods yielded a comparable area under the curve (AUC) of 0.84, 0.85, 0.83 and 0.83, respectively. The four methods showed comparable accuracy in predicting cashew nut reactions in a DBPCFC.

Conclusions: The 'scanned area method' is theoretically more accurate in determining the wheal area than the 'average diameter method' and is recommended in academic research. A HEP-index area of 0.4 is determined as cut-off value for a positive SPT. However, in clinical practice, the 'average diameter method' is also useful, because this method provides similar accuracy in predicting cashew nut allergic reactions in the DBPCFC.

Trial registration: Trial number NTR3572.

Keywords: Allergy; Cut-off value; Histamine equivalent index; Mean wheal diameter; Skin prick test.

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Figures

Fig. 1
Fig. 1
Typical observed wheal forms in SPT’s
Fig. 2
Fig. 2
Definition of D1 and D2
Fig. 3
Fig. 3
Average diameter (method 1) versus scanned area (method 2). A comparison is made between the common-used average diameter method and the scanned area method. Every dot represents one patient. The dotted line shows the trend line of the data. The lower bound value for α is 1 is shown by the red line. The upper bound value for α is 6.67 is shown by the grey line
Fig. 4
Fig. 4
Average diameter (method 1) versus HEP-index area (method 4)
Fig. 5
Fig. 5
Receiver-operating characteristic curves for the 4 SPT methods
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