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