Acta Dermato-Venereologica Issue 3, 2017 Volume 97 | Page 20

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Advances in dermatology and venereology Acta Dermato-Venereologica

Assessing Acute Itch Intensity: General Labelled Magnitude Scale is More Reliable than Classic Visual Analogue Scale

Olivia JONES, Igor SCHINDLER and Henning HOLLE * Psychology, School of Life Sciences, University of Hull, Cottingham Road, Hull HU6 7RX, U. K. * E-mail: h. holle @ hull. ac. uk Accepted Nov 17, 2016; Epub ahead of print Nov 21, 2016

The reliable measurement of itch intensity is crucial, both in research as well as clinical contexts. For example, when the reliability of a measurement scale is unknown, it is impossible to determine whether a patient has changed sufficiently to be confident that the change is beyond that which could be attributed to measurement error( 1, 2). One factor that might influence the reliability of measurements is the type of rating scale used to assess itch intensity. Previous research( 3 – 4) has documented the retest reliability of different rating scales for assessing chronic itch intensity. However, a retest reliability analysis of rating scales for acute experimental itch, induced using substances such as histamine or cowhage, is currently lacking.

Here, we compare the test – retest reliability of 3 rating scales commonly used for this purpose.

MATERIAL AND METHODS

First, we considered the visual analogue scale in its classic form( cVAS), where participants indicate itch intensity on a line ranging from 0( no itch) to 100( the most intense itch imaginable). Second, we included a variant of the VAS, where an additional‘ Scratch Threshold’ marker is set at 33 %( tVAS [ 6 ]), defined as itching strong enough to be scratched( 7). Finally, we considered the general Labelled Magnitude Scale( gLMS [ 8 ]), where participants judge the magnitude of itch on a line with quasi-logarithmically placed labels of“ no sensation” at 0,“ barely detectable” at 1,“ weak” at 6,“ moderate” at 17,“ strong” at 35,“ very strong” at 53 and“ strongest imaginable sensation” at 100. Thus, all 3 scales have an identical range, but differ in the type and number of verbal labels provided( Fig. S1 1).

Ninety healthy volunteers took part after giving written informed consent. Twelve participants( gLMS group: n = 7, cVAS group: n = 5) were screened out as non-responders after the familiarization session( i. e., itch intensity ratings did not exceed 15) and one as an outlier( itch response above 3 SD of group mean), resulting in a final sample of 77 participants( 38 females, mean age 24.66 ± 6.5; n = 25 in gLMS group, n = 26 in cVAS and tVAS group). Participants were told the study was investigating the effect of itch on heart rate and were fully debriefed after the final session. The study was approved by the local Ethics Committee at University of Hull. As an experimental itch model, we used the cowhage provocation paradigm( 9). Briefly, 60 – 65 cowhage spicules were placed into a 16 cm 2 area defined by medical tape on the left volar forearm. Spicules were then rubbed into the skin for 45 s. Itch intensity ratings were obtained every 15 s for 10 min using Presentation Version 17.0( www. neurobs. com).

Participants were randomly assigned to a scale group( cVAS, tVAS or gLMS) and took part in 3 experimental sessions

1 https:// www. medicaljournals. se / acta / content / abstract / 10.2340 / 00015555-2584

( mean ± SD 7.04 ± 1.0 days between sessions). Session 1 served as a familiarization session, where participants were trained in the correct application of the rating scale( as recommended by 2) and could experience the novel sensation of cowhage-induced itch. The statistical analyses are described below.

RESULTS

The peak and mean of each time course were used to quantify the overall itch intensity experienced by a participant. Scores did not differ significantly between sessions( Table I). Shapiro-Wilk tests indicated that mean and peak scores were normally distributed( all W > 0.93, all p > 0.09). Scale reliability was estimated by the Intraclass correlation coefficient( ICC) of the respective scores of Sessions 2 and 3, when participants were familiar with the experience of cowhage-induced itch and the scale. For this retest reliability analysis, we used a two-way mixed model, focusing on absolute agreement between sessions( 10).

As shown in Table II, the gLMS had the highest retest reliability. This was the case regardless of which index was used to quantify itch intensity( peak: ICC 0.86; mean: ICC 0.71). The cVAS was the least reliable scale( peak: ICC 0.50; mean: ICC 0.45) and the tVAS had an intermediate reliability( peak: ICC 0.73; mean: ICC 0.64). Associated p-values, obtained using Fisher’ s r-to-Z transformation, indicated that the gLMS was significantly more reliable than the cVAS( p = 0.01, see Table II).

DISCUSSION

The higher retest reliability of the gLMS cannot be explained in terms of response clustering( i. e., the clustering of ratings around the verbal labels, see Appendix S1 1). Instead, our data suggest that retest reliability may be linked to the degree to which scales are open to interpre-

Table I. Descriptive statistics of the two itch indices( mean, peak) for each session and scale group. Columns 5 and 6 provide the t and p value of an independent samples t-test comparing Sessions 2 and 3

Scale group Index

Mean( SD) Session 2

c: classic; t: threshold; g: general.

This is an open access article under the CC BY-NC license. www. medicaljournals. se / acta Journal Compilation © 2017 Acta Dermato-Venereologica. doi: 10.2340 / 00015555-2584 Acta Derm Venereol 2017; 97: 375 – 376