KOOS-PS User’s Guide

Updated April 2016

The short form of the Knee injury and Osteoarthritis Outcome Score (KOOS-PS) was developed from the original long version using Rasch analysis. The data analyzed included individuals (n=2145) from 26 to 95 years, ratio of males to females 1:1.4, from Sweden, Canada, France, Estonia, and the Netherlands who had participated in various studies. The sample is diverse with some from a community sample, others many years post-menisectomy or tibial osteotomy, some part of a trial of medial wedge inserts with still others evaluated prior to having a total knee replacement.

The Rasch analysis resulted in a 7 item questionnaire that is cross culturally valid and that provides a true interval level measure such that it can be used to measure change in physical function. The reliability of the 7 items is 0.91 (Cronbach’s alpha). It is a unidimensional construct as demonstrated by the fit to the Rasch model.

Item responses are coded from 0 to 4, none to extreme respectively. The questionnaire is scored by summing the raw response (range 0-28) and then using the nomogram in Table 1a-b, the raw score is converted to a true interval score (0-100). KOOS can be scored in two directions, best to worst and worst to best. See next section for important information on scoring directions.

Important update about scoring direction of KOOS-PS!

KOOS-PS can be scored in two directions, from no difficulty (0) to extreme difficulty (100), as in the original KOOS-PS publication (Perruccio AV et al. 2008, Osteoarthritis Cartilage) (table 1b) and from extreme difficulty (0) to no difficulty (100) (table 1a) in accordance with KOOS. To avoid confusion always be explicit about what scoring algorithm you have used!

KOOS and HOOS were developed in 1999 and 2003 in an orthopedic context where scores traditionally are scored from extreme difficulty (0) to no difficulty (100). This scoring direction is also aligned with some major generic scoring scales like SF-36 and EQ-5D. This scoring direction is achieved when using table 1a to convert the raw summed KOOS-PS score. When using table 1a to score KOOS-PS the score direction of KOOS-PS is aligned with all KOOS subscales. Scoring with table 1a is preferred if you are using KOOS-PS, the Physical function Short scale derived through Rasch-analysis from the two KOOS subscales ADL and Sport/Rec, together with the other KOOS subscales Pain, Symptoms and QOL.

KOOS-PS was originally scored from no difficulty (0) to extreme difficulty (100). This scoring direction is achieved when using table 1b to convert the raw summed scores. KOOS-PS was developed in 2008 as a stand alone short measure of function under the auspices of Osteoarthritis Research Society International (OARSI) and OMERACT. To align with the concurrently developed pain measure (ICOAP) it was decided that both measures should be scored from best (0) to worst (100), as is the tradition in measures developed within rheumatology.

Table 1a: Nomogram for converting raw summed KOOS-PS scores to 0 representing extreme difficulty and 100 representing no difficulty.

Table 1b: Nomogram for converting raw summed KOOS-PS scores to 0 representing no difficulty and 100 representing extreme difficulty as in the original publication of KOOS-PS.