The Lancet Weight Determines Wheal Diameter in Response to Skin Prick Testing with Histamine

Abstract

Background: Skin prick test (SPT) is a common test for diagnosing immunoglobulin E-mediated allergies. In clinical routine, technicalities, human errors or patient-related biases, occasionally results in suboptimal diagnosis of sensitization.

Objective: Although not previously assessed qualitatively, lancet weight is hypothesized to be important when performing SPT to minimize the frequency of false positives, false negatives, and unwanted discomfort.

Methods: Accurate weight-controlled SPT was performed on the volar forearms and backs of 20 healthy subjects. Four predetermined lancet weights were applied (25 g, 85 g, 135 g and 265 g) using two positive control histamine solutions (1 mg/mL and 10 mg/mL) and one negative control (saline). A total of 400 SPTs were conducted. The outcome parameters were: wheal size, neurogenic inflammation (measured by superficial blood perfusion), frequency of bleeding, and the lancet provoked pain response.

Results: The mean wheal diameter increased significantly as higher weights were applied to the SPT lancet, e.g. from 3.2 ± 0.28 mm at 25 g to 5.4 ± 1.7 mm at 265 g (p<0.01). Similarly, the frequency of bleeding, the provoked pain, and the neurogenic inflammatory response increased significantly. At 265 g saline evoked two wheal responses (/160 pricks) below 3 mm.

Conclusion and clinical relevance: The applied weight of the lancet during the SPT-procedure is an important factor. Higher lancet weights precipitate significantly larger wheal reactions with potential diagnostic implications. This warrants additional research of the optimal lancet weight in relation to SPT-guidelines to improve the specificity and sensitivity of the procedure.

Fig 1. The setup of the SPT lancet attached to the SENSEBox Electronic von Frey weight transducer.

Fig 2

A) The mean wheal diameters on the forearms for each lancet weight; 25 g, 85 g, 135 g and 265 g. A higher lancet weight significantly increased the sizes of the wheal reactions. B) The mean wheal diameters on the back for each lancet weight; 25 g, 85 g, 135 g and 265 g. A higher lancet weight significantly increased the sizes of the wheal reactions. A significant difference was also observed when comparing the two histamine concentrations: 1 mg/mL and 10 mg/mL. For each condition; n = 20. * = p < 0.05, ** = p < 0.01. Mean ± SEM.

Fig 3

A) Changes in the mean superficial perfusion on the forearms according to the lancet weights: 25 g, 85 g, 135 g and 265 g. A difference in the mean superficial perfusion was seen from baseline to all the four applied lancet weights. B) On the back a difference in the mean superficial perfusion was seen from baseline to all the four applied lancet weights with both histamine concentrations (1 mg/mL and 10 mg/mL) and from baseline for the lancet weight of 265 g with saline as well. For each condition; n = 20. * = p < 0.05, ** = p < 0.01. Asterisks (*) or (**) placed just above the bars indicates significant difference from baseline superficial skin perfusion. Mean ± SEM.

Fig 4

The percentage distribution of bleeding incidents in response to individual SPTs when using the four lancet weights; 25 g, 85 g, 135 g and 265 g at A) the arm and B) the back, regardless of the applied solution (saline, 1 mg/mL histamine or 10 mg/mL histamine). A higher lancet weight caused increased incidents of bleeding. C) Peak pain intensity in response to the four lancet weights for both arm and back rated on a coVAS. The peak pain intensity increased significantly with increased weight. Notice that pain during SPT, regardless of weight, remains negligible, i.e. < 8 on a 0–100 VAS (N.B.: discrete x-axis is applied for graph C). For each condition; n = 20. * = p < 0.05, ** = p < 0.01. C) Mean ± SEM.

Fig 5

A) Shows individual lancet peak weight measurements from four different clinicians (no. 1–4). The horizontal lines denote the arithmetic mean for each individual. B) Shows intra-reliability and inter-reliability (dotted horizontal line) of applied weight expressed as the coefficient of variation (standard deviation expressed as % of the mean) for each of the four testees and between the four testees. CV% = Coefficient of variation (calculated as the ratio between the standard deviation and the mean in %).

Fig 6. Weighted SPT lancet device.

Depicts a suggested mechanical approach to a locked weight device allowing for SPT within a specific predetermined weight.