The KOOS-PS is a 7-item measure of physical functional derived from the items of the Function, daily living and Function, sports and recreational activity subscales of the KOOS. As with the KOOS it is intended to elicit people’s opinions about the difficulties they experience with activity due to problems with their knee.
Why was a short measure developed?
The need for a short measure was identified as part of an OARSI (Osteoarthritis Research Society International) and OMERACT (Outcome Measures in Clinical Trials) initiative to develop criteria for the failure of medical therapy, more specifically disease modifying drugs for osteoarthritis (DMOADS), such that individuals would be candidates for surgical intervention such as joint replacement. The components of the criteria were determined by the working group to include pain, physical function and structural damage .
Given the overall objective, it was critical that each component be parsimonious and cross-culturally valid to enable use of the measure in multi-centre, cross-country studies and clinical settings. As a result, the decision was made to attempt to shorten the KOOS and to combine the items from the Function, daily living and Function, sports and recreational activity subscales. We chose to use an item response theory approach, specifically the Rasch mathematical model, to develop the short measure. Rasch is a mathematical process that converts ordinal data (such as the responses in the KOOS-PS of none, mild, moderate, severe, extreme) to an interval level data summary score .
Ordinal data have logical order but the distances between scores may be different. In contrast, interval level data has order and the distances be¬tween scores are equal. The advantage of interval level data is that we can accurately quantify physical function to be able to compare individuals and groups and to know how much they have changed. On a measure that has interval level scaling properties, we know that people who change from 10 to 20 points and those who change from 60 to 70 points have all changed exactly ten points and that this ten points is equal along the scale range. On an ordinal scale, although the questions are often summed, it is often assumed but not confirmed that this scenario would represent 10 points that are of equal distance along the scale.
The Rasch model provides a number of advantages over classical test theory methods. Specifically, it provides scores that have the requisite interval level measurement properties for inferential statistical procedures, the estimates place items in a hierarchy of difficulty such that range of difficulty represented by the measure is known; and, the standard error (SE) of an item is independent of the SE of other items such there is improved accuracy and stability of the performance of the items across different samples [3,4]. The references by Tennant , Rasch  and Andiel  provide a description of the Rasch mathematical model and the analytic approach to determining if data fit the Rasch model.
The final items included in the short measure were based on the data from the samples and countries described below meeting the criteria for fit to the Rasch model .
What is the purpose of the KOOS-PS?
The KOOS-PS can be used as part of the criteria, once studies are completed, to determine failure of DMOADS. The KOOS-PS (with only 7 items instead of the 22 of the KOOS Function, daily living and Function, sports and recreational activity subscales) can be used when fewer items are advantageous to limit responder burden and/or cost. For example, the short questionnaire may be preferable when several questionnaires are administered simultaneously, in registries, or in special populations such as the elderly.
For what populations is the KOOS-PS relevant?
The initial development of the short measure included people with a range of knee problems from multiple countries .
Individuals were from community and clinical samples (e.g., people many years post menisectomy, following tibial osteotomy or anterior cruciate repair and just prior to having their knee replaced) and were from Sweden, Canada, France, Estonia, and the Netherlands. These individuals ranged from 26 to 95 years.
What are the measurement properties of the KOOS-PS?
The internal consistency of the KOOS-PS was 0.90 and it represents a unidimensional construct by virtue of the fit of the data to the Rasch model . For patients undergoing a total knee replacement, the internal consistency was 0.89, confirming that the KOOS-PS represents a homogeneous construct. Further, construct validity was supported with a correlation of 0.90 to the PF-subscale of the WOMAC. Finally, KOOS-PS is a responsive measure with a standardized response mean (SRM) of 1.4 .
How do I use and score the measure?
The measure is intended to be self-completed and, as with the full version of the KOOS, can be completed as a mailed survey or in the waiting room. The measure is scored by summing the responses to the 7 items of the KOOS-PS and converting this raw sum to the Rasch-based interval score provided in the KOOS-PS user guide. Since April 2016, the KOOS-PS can be scored in two directions, from no difficulty (0) to extreme difficulty (100), as in the original KOOS-PS publication  and from extreme difficulty (0) to no difficulty (100) in accordance with the KOOS. Please see the KOOS-PS user guide for more information on how to convert the raw summed score to the 0-100 interval scores. To avoid confusion always be explicit about what scoring algorithm you have used!
How do I handle missing items in the KOOS-PS?
Each question must have a response (i.e. no missing data).
What studies are currently using the KOOS-PS?
The measure has been studied in multiple countries: Australia, Canada, the Czech Republic, France, Germany, Italy, the Netherlands, Spain, Sweden, Britain and the United States, to determine the threshold at which individuals are considered candidates for joint replacement. The first phase of this work was cross-cultural validation of the measure through a formal process of forward- and back-translation with reconciliation of language and conceptual meaning differences where no culturally adapted version of the KOOS existed. The second phase included completion of the questionnaire by patients referred to an orthopaedic surgeon for consideration of knee replacement. Data were compared to the surgeon’s decision about the recommendation for surgery. The study concluded that it was not possible to determine cut points for pain and function defining ‘requirement for a total joint replacement’ .
The KOOS-PS is included in the international Consortium for Health Outcomes Measurement (ICHOM) Standard-set for Hip & Knee Osteoarthritis as a measure of functional status .
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7. ICHOM, International Consortium for Health Outcomes Measurement ,Data collection reference guide for Hip & Knee Osteoarthritis. 2015; Cambridge. Retrieved from http://www.ichom.org