The KOOS, short for Knee injury and Osteoarthritis Outcome Score, measures the impact of knee injury and osteoarthritis across five subscales, each scored from 0 to 100. Unlike most scales in this field, it has a validated French version. The interpreter is right below.
The minimal detectable change of the KOOS is high: from 14.3 to 19.6 points in younger people, and 20 points or more in older people. The detail is below.
Figures taken from the studies listed at the end of this page, every value carrying its source where it is written. No item is reproduced: the KOOS is under copyright.
What the KOOS measures
The KOOS measures what the knee prevents, in osteoarthritis patients as well as in athletes operated on for a cruciate ligament. It was built from the WOMAC, adding what the WOMAC ignores: sport, recreation and knee-related quality of life.
That broadening is its reason for existing. A thirty-year-old after ligament reconstruction may have no pain when walking and still be unable to return to their sport: the WOMAC does not see it, the KOOS does.
Its reliability has long been established. In 105 patients with a mean age of 71.3 after total knee replacement, seen again at six and twelve months, test-retest reliability exceeds ICC 0.75 for every subscale.
Five subscales, not one score
The KOOS has no global score, and that is deliberate. It yields five subscales each scored from 0 to 100: symptoms, pain, activities of daily living, sport and recreation, and knee-related quality of life. A high score signals a better state, which is the opposite of the DASH and the Oswestry.
Adding the five to make an average destroys the information the structure exists to carry. A knee can improve markedly in pain and stay put in quality of life, and that is precisely what the clinician needs to see.
A rare point in this field: the KOOS has a validated French version, published in 2008 by translation and back-translation then evaluation in a medical and a surgical group, with internal consistency from 0.76 to 0.93. Neither the DASH nor the Berg Balance Scale has an equally established equivalent.
A very high change threshold
This is the figure that should govern every reading of the KOOS, and it is the one least often quoted.
The reference COSMIN review, across 37 articles, places the minimal detectable change of the subscales between 14.3 and 19.6 points in younger people, and at 20 points or more in older people. On a 0 to 100 scale, that means a gain of ten points, which looks considerable, cannot be told apart from measurement noise.
The OPTIKNEE consensus, across 18 studies after cruciate ligament or meniscus injury, points the same way while tightening it: a single threshold is rated of high credibility there, the 18 points for quality of life. Every other one is less established, including those that turn up in reports.
| What is measured | Value | Population |
|---|---|---|
| Minimal detectable change, younger people | 14.3 to 19.6 | COSMIN review, 37 articles |
| Minimal detectable change, older people | 20 or more | COSMIN review, 37 articles |
| Important change, quality of life | 18 | OPTIKNEE consensus, high credibility |
| Important change, sport and recreation | 12.1 | 542 patients after ligament reconstruction |
| Test-retest reliability | ICC > 0.75 | 105 total knee replacements |
| Internal consistency, French version | 0.76 to 0.93 | 67 patients, medical and surgical |
What the score does not tell you
It does not say which subscale moved if you reduce it to one number. That is the most frequent misuse, and it cancels the only advantage the KOOS has over the WOMAC.
Nor does it say the same thing depending on age. The same 16-point change is a real change in a twenty-five-year-old athlete and stays within noise in a seventy-five-year-old patient: the thresholds do not transfer from one population to another.
Finally, the 1998 original publication describes how the instrument was built, not how it performs: its abstract carries no numeric coefficient, and citing it for a reliability figure would be an attribution error.
Why this page does not display the questionnaire
The KOOS is under copyright, and its use requires prior permission from its holders, including for academic work. Neither the items, nor the response options, nor the scoring instructions are reproduced here. The questionnaire and its manual are obtained from the publisher, which makes the validated French version available.
Three administration pitfalls
Turning it into a global score
Averaging the five subscales produces a number that looks like a result and is not one. The KOOS goes into the notes subscale by subscale, with all five values.
Concluding on ten points
Ten points out of a hundred look like a lot and stay below the minimal detectable change in every population studied. Announcing it as an established improvement means presenting noise as a result.
Transferring a threshold from one population to another
The published thresholds mostly come from young cohorts after ligament injury. Applying them as they stand to a seventy-five-year-old with knee osteoarthritis underestimates measurement noise by at least a quarter.
Frequently asked questions
What does KOOS stand for?
KOOS stands for Knee injury and Osteoarthritis Outcome Score. The questionnaire was published in 1998 by Roos et al., from the WOMAC to which it adds sport, recreation and quality of life.
How is the KOOS scored?
It yields five subscales each scored from 0 to 100, a high score signalling a better state. It has no global score, and the scoring instructions belong to the copyright holders.
How much change counts?
The minimal detectable change runs from 14.3 to 19.6 points in younger people and reaches 20 points or more in older people. The only high-credibility threshold is the 18 points for quality of life.
Is there a validated French version?
Yes, published in 2008 by Ornetti et al., with internal consistency from 0.76 to 0.93. It is one of the few instruments in this field with a genuinely validated French adaptation.
Is the KOOS free to use?
No. The instrument is under copyright and its use requires prior permission, including for academic use. That is why this page reproduces none of its items.
References
6 sources, PMIDs included
- Roos EM, Roos HP, Lohmander LS, Ekdahl C, Beynnon BD. Knee Injury and Osteoarthritis Outcome Score (KOOS): development of a self-administered outcome measure. J Orthop Sports Phys Ther 1998;28(2):88-96. PMID 9699158. The original publication: the questionnaire is built from the WOMAC, a literature review, an expert panel and a pilot study. The abstract carries no numeric coefficient.
- Roos EM, Toksvig-Larsen S. Knee injury and Osteoarthritis Outcome Score (KOOS) - validation and comparison to the WOMAC in total knee replacement. Health Qual Life Outcomes 2003;1:17. PMID 12801417. 105 patients with a mean age of 71.3 after total knee replacement, seen again at 6 and 12 months. Test-retest reliability ICC above 0.75 for every subscale.
- Ornetti P, Parratte S, Gossec L, Tavernier C, Argenson JN, Roos EM, Guillemin F, Maillefert JF. Cross-cultural adaptation and validation of the French version of the Knee injury and Osteoarthritis Outcome Score (KOOS) in knee osteoarthritis patients. Osteoarthritis Cartilage 2008;16(4):423-8. PMID 17905602. The validated French version, by translation and back-translation then evaluation in two groups, medical and surgical. Internal consistency from 0.76 to 0.93. The KOOS is among the few instruments in this field to have one.
- Collins NJ, Prinsen CA, Christensen R, Bartels EM, Terwee CB, Roos EM. Knee Injury and Osteoarthritis Outcome Score (KOOS): systematic review and meta-analysis of measurement properties. Osteoarthritis Cartilage 2016;24(8):1317-29. PMID 27012756. The reference COSMIN review, across 37 articles. It gives the figure that governs every interpretation: the minimal detectable change of the subscales runs from 14.3 to 19.6 points in younger people, and reaches 20 points or more in older people.
- Ingelsrud LH, Terwee CB, Terluin B, Granan LP, Engebretsen L, Mills KAG, Roos EM. Meaningful Change Scores in the Knee Injury and Osteoarthritis Outcome Score in Patients Undergoing Anterior Cruciate Ligament Reconstruction. Am J Sports Med 2018;46(5):1120-8. PMID 29517924. 542 participants after anterior cruciate ligament reconstruction. Minimal important change by predictive modelling: 12.1 points for Sport and Recreation, 18.3 points for knee-related quality of life.
- Macri EM, Young JJ, Ingelsrud LH, Khan KM, Terluin B, Juhl CB, Whittaker JL, Culvenor AG, Crossley KM, Roos EM. Meaningful thresholds for patient-reported outcomes following interventions for anterior cruciate ligament tear or traumatic meniscus tear: a systematic review. PMID 35973755. The OPTIKNEE consensus, across 18 studies with a mean age below 35. A single threshold is rated of high credibility there: 18 points for the quality-of-life subscale. Every other one is less established.
Page written by Anthony Baillon, physiotherapist, co-founder of Physio Learning. The questionnaire itself is by Roos et al., 1998, and it is under copyright: this page documents and interprets it, it reproduces none of its items.