• Martijn B A van der Wal, Wim E Tuinebreijer, Åsa Lundgren-Nilsson, Esther Middelkoop, and Paul P M van Zuijlen.
• Association of Dutch Burn Centers, Beverwijk, The Netherlands, mvanderwal@RKZ.nl.
• Qual Life Res. 2014 Sep 1;23(7):2037-45.

PurposeTo investigate whether the Observer Scale of the Patient and Observer Scar Assessment Scale (POSAS) can serve as a generic measure for scar quality across different scar types.MethodsA collection of POSAS scores derived from several clinical trials on burn (n = 404), linear (n = 384), and keloidal scars (n = 282) was analyzed using the partial credit model of the Rasch analysis package RUMM2030.ResultsDifferential item functioning (DIF) was observed for the Observer Scale of the POSAS between the three scar types for the items pliability, thickness, and surface area, which could be solved by item splitting. The items pigmentation and thickness showed disordered thresholds, considerable misfit, and unpredictability.ConclusionUsers of the Observer Scale of the POSAS must be aware that the raw scores obtained from burn, linear, and keloidal scars cannot be compared without the scar-specific DIF adjustment of the items pliability, thickness, and surface area.

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