Diameter, thickness, area, and volume of skin-prick histamine weals. Measurement of skin thickness by 15 MHz A-mode ultrasound.

Twelve healthy persons were prick-tested on the volar forearm with three concentrations of histamine chloride (0.1 mg/ml; 1.0 mg/ml; 10 mg/ml) and an isotonic sodium chloride control. Weal diameters were measured, and skin thickness representing weal thickness was measured by high frequency ultrasound. Relative increases in weal parameters as measured and calculated between weak and strong concentrations of histamine (0.1 and 10 mg/ml) were: mean diameter 139%, weal thickness 46%, tangential area 443%, cross-sectional area 218%, and weal volume 680%. All the parameters of weal size measured and calculated showed significant differences between the three concentrations of histamine. However, a better distinction might be obtained by calculating weal volume. Coefficients of variation ranged from 12-28% for the weal diameter, 11-14% for the weal thickness, 24-53% for the tangential area, 14-39% for the cross-sectional area, and 23-55% for the weal volume. The increase in thickness was correlated to weal diameter up to a diameter of 5 mm (r = 0.64; P less than 0.001), after which weals only expanded tangentially. Weal thickness as measured by ultrasound is a potentially important parameter in experimental studies of inflammatory skin reactions. With this new technique it is possible to compare with normal skin and skin treated with solvent for control. For routine prick-testing of patients with allergy, measurement of weal diameter is still a valid and easy method.