James Twiss and Stephen McKenna

Galen Research Ltd, Manchester, UK

Email: jtwiss@galen-research.com

Rasch analysis of the Dermatology Life Quality Questionnaire (DLQI)

Objectives

• To discuss good scale development methodology in the context of Rasch analysis

• Example - Dermatology Life Quality Index (DLQI)

The DLQI

• 10-item generic dermatology Patient Reported Outcome (PRO) measure

• Scored 0-30. High scores=worse

• Used with 30 different skin conditions

• Available in 55 languages

• Used for treatment selection in the UK

DLQI development

• Uses patient reports of problems

• 49 identified ‘aspects’ condensed into 10 items

• Designed to fit on one side of A4

• Items phrased to include additional aspects

The DLQI - content

Sample DLQI items: Over the last week…

how embarrassed or self conscious have you been because of your skin?

how much has your skin interfered with you going shopping or looking after your home or garden?

how much has your skin made it difficult for you to do any sport?

how much of a problem has the treatment for your skin been, for example by making your home messy, or by taking up time?

The DLQI – response format

• Four point response format

– Very much– A lot– A little– Not at all

• 8 items also have a ‘Not relevant’ option

Psychometric properties

• Classical psychometric properties adequate (Basra et al, 2008)

• One previous Rasch analysis compared 6 language versions (Nijsten and Meads, 2007)

– Overall misfit to the model– Misfit in 3 individual countries– DIF by country for every item

Study aims

• Rasch analysis of DLQI data from two patient groups:

- atopic dermatitis

- psoriasis

• Relate results to development methods used for DLQI

Analyses

Rasch analysis employed RUMM2020

• Overall fit to model• Individual Item fit• Response threshold order• DIF by age and gender• DIF by disease (AD vs psoriasis)• Item-trait coverage

Sample

Psoriasis (n = 146)

Atopic Dermatitis (n = 146)

Gender (%)

Male 73 (50) 73 (50)

Female 73 (50) 73 (50)

Age (Years)

Mean (SD) 44.4 (14.7) 45.5 (16.6)

Range 66 (17-83) 62 (20-82)

Duration (years)

Mean (SD) 20.9 (13.5) 28.2 (17.5)

Range 67 (2-69) 76 (0-76)

DLQI scores

Psoriasis Atopic dermatitis

Mean (SD) 8.8 (6.7) 6.1 (4.6)

Median (IQR) 7 (3.0-12.3) 5 (3-8)

Range 29 (0-29) 26 (0-26)

Overall fit to the Rasch model

Item-trait interaction

PSI

Items Persons

Mean SD Mean SD

0.85 0.98 -0.30 0.81

Initial fit statistics (partial credit model)

0.01 -0.81

Item fit

Item description LocationFit

residual Chi2 p value1. itchy, sore, painful or stinging -1.61 -0.03 3.4 0.50

2. embarrassment/self consciousness -0.52 -1.97 15.0

3. interferes with shopping/looking after home/garden

0.99 -1.18 3.8 0.44

4. influences choice of clothes -0.54 -0.14 4.4 0.35

5. affects social/leisure activities 0.32 12.7

6. affect ability to do sport -0.02 0.29 3.0 0.56

7. prevents working/studying 0.25 0.23 12.6

8. creates problems with partner/close friends/relatives

0.59 -0.89 2.1 0.72

9. causes sexual difficulties 0.89 -1.23 2.2 0.70

10. problems with treatment -0.36 -0.47 3.5 0.48

0.005*

-2.69 0.01

0.01

Problems with items

• 2 - Over the last week, how embarrassed or self conscious have you been because of your skin?

• 5 - Over the past week, how much has your skin affected any social or leisure activities?

• 7 - Over the past week, how much has your skin prevented you working / studying?

DIF analysis

Item description Uniform DIFNon-uniform

DIFitchy, sore, painful or stinging Disease

embarrassment/self consciousness Age group/Gender

interferes with shopping/looking after home/garden Disease Disease

influences choice of clothes Gender*/ Disease

affects social/leisure activities Disease

affect ability to do sport Gender Gender

prevents working/studying Gender/Disease*

creates problems with partner/close friends/relatives

causes sexual difficulties

problems with treatment Age group*

Example of DIF by disease

Prevents working or studying

Person locations (logits)

Exp

ecte

d va

lue

PsoriasisAtopic Dermatitis

Example of DIF by gender

Influences the clothes you can wear

Exp

ecte

d va

lue

Person locations (logits)

FemalesMales

Response thresholds

Response options

• Four point response format

Very much

A lot

A little

Not at all

Source of problem for items 4, 7 and 8

Source of problem for items 6 and 9

Item Map

Clustering of items

Poor coverage of persons

Conclusions

• DLQI – Pre-Rasch PRO

• Rasch analysis highlighted several fundamental problems with DLQI

• Problems probably result from inadequate scale development methodology

• Concern for:

- clinical trials

- treatment decisions

Good PRO Design

Item Reduction

• Cognitive debriefing interviews

• Patient survey & application of Rasch analysis to data

Draft 2

Draft 3

Scale Evaluation

• Scaling properties• Classical psychometrics Draft 4

Item Generation

• Patient Interviews

• Qualitative analysis Draft 1

• Based on sound theoretical model

References

Finlay AY, Khan GK (1994). Dermatology Life Quality Index (DLQI)-a simple practical measure for routine clinical use. Clin Exp Dermatol 19: 210-6.

Basra MKA, French R, Gatt RM, et al(2008). The dermatology Life Quality Index 1994-2007: a comprehensive review of validation and clinical results. Br J Dermatol 159: 997-1035

Nijsten T, Meads DM, de Korte J et al (2007). Cross-Cultural Inequivalence of

Dermatology-Specific Health related Quality of Life Instruments in Psoriasis Patients. Journal of Investigative Dermatology 127: 2315-2322

Nijsten T, Meads DM, McKenna SP (2006). Dimensionality of the dermatology life quality index (DLQI): a commentary. Acta Derm Venereol 86:284-5; author reply 285-6.

Overall fit to the model

Item-Trait interaction

PSI Unidimensionality (CI)

Items Persons

Mean SD Mean SD

0.01 0.85 -0.81 0.98 -0.30 0.81 0.03 (0.01 – 0.06)