The Knee Injury and Osteoarthritis Outcome Score, or KOOS, is a 42-item patient-reported outcome measure used to assess knee pain, symptoms, daily function, sport and recreation function, and knee-related quality of life. It can support monitoring and progress tracking for knee injury and osteoarthritis presentations, but it does not diagnose a condition, confirm pathology or clear someone for sport on its own.

Introduction

Knee problems can affect walking, stairs, squatting, running, jumping, sport participation, work demands and confidence with movement.

The Knee Injury and Osteoarthritis Outcome Score, commonly called the KOOS, is a patient-reported outcome measure designed to capture the client’s own view of their knee symptoms, function and quality of life.

It is commonly used for:

• knee injury monitoring
• anterior cruciate ligament injury or reconstruction follow-up
• meniscal injury monitoring
• knee osteoarthritis
• total knee replacement or knee arthroplasty follow-up
• sport and recreation limitations
• long-term progress tracking
• research and clinical outcome reporting

The KOOS is a 42-item questionnaire with five separately scored subscales: Pain, Symptoms, Activities of Daily Living, Sport/Recreation and Knee-Related Quality of Life. Scores range from 0 to 100, where 100 represents no knee problems and 0 represents extreme knee problems.  

Quick Summary

• Outcome measure: Knee Injury and Osteoarthritis Outcome Score
• Abbreviation: KOOS
• Body region: Knee
• Type: Patient-reported outcome measure
• Number of items: 42
• Subscales: Pain, Symptoms, Activities of Daily Living, Sport/Recreation, Quality of Life
• Score range: 0–100 for each subscale
• Higher score means: Better perceived knee status
• Lower score means: More pain, symptoms, limitation or quality-of-life impact
• Related versions: KOOS-12, KOOS-PS, KOOS-Child
• Best used for: Baseline assessment, reassessment, progress tracking and outcome reporting
• Key limitation: KOOS does not diagnose a condition or determine clearance on its own

What Is the Knee Injury and Osteoarthritis Outcome Score?

The KOOS is a knee-specific patient-reported outcome measure.

It was designed to assess short-term and long-term consequences of knee injury and knee osteoarthritis from the client’s perspective.

The five KOOS subscales are:

• Pain
• Other Symptoms
• Activities of Daily Living
• Sport and Recreation Function
• Knee-Related Quality of Life

The KOOS is an extension of the WOMAC Osteoarthritis Index and was developed for use across knee injury and knee osteoarthritis contexts. Official KOOS resources also identify related versions including KOOS-12, KOOS Physical Function Short Form and KOOS-Child.  

Why It Is Used

The KOOS is used because physical testing alone does not always show how a knee problem affects the client’s daily life, sport or confidence.

A client may show improving strength or range of motion but still report:

• pain with stairs
• difficulty squatting
• swelling or stiffness after activity
• poor confidence with running
• reduced sport participation
• fear around pivoting or landing
• poor quality of life because of the knee

The KOOS can help professionals:

• establish a baseline
• identify which domains are most affected
• monitor change over time
• support client education
• guide goal-setting conversations
• compare symptoms and function across reassessments
• combine client-reported outcomes with physical testing
• improve reporting quality in Measurz

The KOOS should support assessment reasoning and progress tracking. It should not be used as a stand-alone diagnostic, treatment or clearance tool.

What It Measures

Pain

The Pain subscale captures knee pain during different activities and situations.

It may provide insight into pain during:

• walking
• stairs
• sitting or lying
• standing
• twisting or pivoting
• knee bending

Symptoms

The Symptoms subscale captures knee-related symptoms such as:

• swelling
• stiffness
• restricted movement
• grinding or clicking
• mechanical symptoms
• general symptom severity

Activities of Daily Living

The ADL subscale captures day-to-day function.

This may include:

• walking
• stairs
• rising from sitting
• bending to the floor
• getting in and out of positions
• routine daily tasks

Sport and Recreation

The Sport/Recreation subscale captures higher-demand function.

This may include:

• squatting
• running
• jumping
• twisting
• kneeling
• sport participation

Knee-Related Quality of Life

The Quality of Life subscale captures the broader impact of the knee problem.

This may include:

• confidence
• awareness of the knee
• lifestyle impact
• frustration
• participation restriction
• perceived long-term impact

Who It Is Useful For

The KOOS may be useful for:

• exercise professionals
• rehabilitation practitioners
• strength and conditioning coaches
• allied health support teams
• performance professionals
• movement assessment professionals
• students learning outcome measures
• professionals using Measurz or MAT for structured progress tracking

It may be relevant for clients with:

• knee injury
• ACL injury or reconstruction
• meniscal injury
• knee osteoarthritis
• patellofemoral pain or symptoms
• cartilage-related knee symptoms
• post-operative knee recovery
• persistent swelling, stiffness or pain
• sport-related knee limitations

When to Use This Outcome Measure

Use the KOOS when you want to understand how a knee problem affects the client’s pain, symptoms, function and quality of life.

It may be useful at:

• initial assessment
• onboarding
• reassessment
• post-injury monitoring
• post-operative milestones
• return-to-running planning
• return-to-sport planning
• knee osteoarthritis monitoring
• discharge or progress review

The KOOS is most useful when repeated over time using the same version and scoring method.

When Not to Use or When to Be Cautious

Use caution when:

• the client cannot complete the questionnaire independently
• literacy, language or cognitive factors affect responses
• the wrong language version is being used
• the client has multiple body regions contributing to limitation
• the client has not attempted the activities being scored
• many items are missing
• the score is being used as a pass/fail decision
• the result is interpreted without physical assessment context

The KOOS should not be used to:

• diagnose a condition
• confirm injury
• determine tissue healing
• identify the exact cause of symptoms
• clear someone for sport
• replace professional judgement
• replace medical assessment when needed

Equipment or Resources Required

• KOOS questionnaire
• Official KOOS user guide or validated scoring resource
• Measurz recording workflow
• Client-reported symptom and function notes
• Baseline and retest dates
• Optional related physical tests, such as:
  • knee range of motion
  • knee swelling assessment
  • quadriceps strength testing
  • hamstring strength testing
  • single-leg squat
  • hop testing
  • balance testing
  • gait, running or sport assessment

Administration Protocol / Practice

Setup

Explain the purpose of the questionnaire before the client completes it.

Example wording:

“This questionnaire helps us understand how your knee is affecting your pain, symptoms, daily function, sport and quality of life. It does not diagnose the problem on its own, but it helps us monitor change over time.”

Format

The KOOS can be completed:

• on paper
• digitally
• independently
• with assistance
• before a session
• during reassessment
• as part of a Measurz workflow

Client Instructions

Ask the client to:

• answer based on their current knee problem
• choose the response that best matches their experience
• answer every item where possible
• avoid overthinking each question
• ask for clarification if they do not understand the wording
• complete the same version at each retest

Completion Method

Record whether the KOOS was completed:

• independently
• digitally
• on paper
• verbally
• with assistance

This matters because assistance, wording clarification or completion format may influence responses.

Assistance Rules

If help is required:

• explain the instructions without leading the answer
• avoid telling the client which response to choose
• record that assistance was provided
• use the same assistance approach at retest where possible

Missing Item Handling

Do not guess missing responses.

Use the official KOOS scoring guidance for the version being used. If too many items are missing for a subscale, record that subscale as incomplete rather than creating an unreliable score. Official KOOS user guides are available through the KOOS website.  

Scoring Process

Each KOOS item is scored from 0 to 4.

Each subscale is transformed to a 0–100 score.

General scoring direction:

• 100: no knee problems
• 0: extreme knee problems

The five subscales should be interpreted separately rather than combined into one unsupported total score.

Retesting Considerations

Retest at meaningful time points, such as:

• baseline
• after a training block
• after a rehabilitation phase
• pre-return to running
• pre-return to sport
• after a flare-up
• after post-operative milestones
• discharge or progress review

For consistency, record:

• date
• current activity level
• recent flare-ups
• recent training load
• whether the client has returned to the activities being scored
• any major changes in sport, work or daily demands

Safety Notes

The KOOS is a self-report questionnaire, so it does not create physical testing risk.

However, a major worsening in score may support further assessment if the client reports:

• severe deterioration
• major quality-of-life impact
• worsening function
• severe swelling or pain
• unexpected decline
• new mechanical symptoms

Scoring and Interpretation

The KOOS has five separate subscale scores.

Each subscale ranges from 0 to 100.

Higher scores indicate better perceived knee status.

Lower scores indicate more reported problems.

Subscales

• Pain: 0–100
• Symptoms: 0–100
• Activities of Daily Living: 0–100
• Sport/Recreation: 0–100
• Quality of Life: 0–100

What a High Score May Suggest

A higher score may suggest:

• less knee pain
• fewer symptoms
• better daily function
• better sport or recreation function
• better knee-related quality of life
• improved confidence with knee-related activity

What a Low Score May Suggest

A lower score may suggest:

• more pain
• more symptoms
• reduced daily function
• reduced sport or recreation capacity
• reduced confidence
• greater knee-related quality-of-life impact

What the Score Does Not Prove

A KOOS score does not prove:

• the diagnosis
• the tissue source of symptoms
• the severity of structural injury
• readiness to return to sport
• whether imaging is required
• whether a specific intervention caused the change

How to Explain the Result Safely

Example wording:

“Your KOOS results suggest your daily function is improving, but your sport and quality-of-life scores are still more affected. This does not tell us exactly what structure is causing symptoms, but it helps us understand how your knee is affecting the activities that matter to you.”

What the Score May Mean in Different Client Populations

General Fitness Clients

For general fitness clients, KOOS can help show how knee symptoms affect:

• walking
• stairs
• squatting
• gym training
• daily activity
• recreational exercise

Interpretation cautions:

• scores may change with recent activity
• symptoms from other body regions may affect answers
• sport items may be less relevant if the client does not participate in sport

Sport and Performance Clients

For athletes, the Sport/Recreation and Quality of Life subscales are often especially important.

A client may score well on daily activities but still report limitations with:

• running
• jumping
• cutting
• landing
• pivoting
• confidence in competition

Interpretation should include sport-specific testing. A high KOOS score should not be treated as return-to-sport clearance on its own.

Older Adults

For older adults and people with knee osteoarthritis, KOOS may help monitor pain, stiffness, walking, stairs and knee-related quality of life.

Interpretation cautions:

• other health conditions may affect scores
• general strength and balance may influence function
• sport subscales may be less relevant for some clients
• broad reference values should be used cautiously

Youth Clients

Standard KOOS is designed for adults. For children or adolescents, a specific child version, KOOS-Child, is available through official KOOS resources.  

Consider:

• age
• reading level
• comprehension
• parent or guardian assistance
• sport exposure
• whether the questionnaire version is appropriate

Clients With ACL Injury or Reconstruction

KOOS is often used after ACL injury and reconstruction to monitor symptoms, function and knee-related quality of life.

Interpretation should include:

• strength testing
• hop testing
• confidence measures
• sport exposure
• graft or surgical timeline where relevant
• movement quality and workload progression

Meaningful thresholds may differ after ACL injury or meniscal injury, and systematic review evidence suggests thresholds should be selected based on the specific intervention and population.  

Clients With Meniscal Injury

For meniscal presentations, KOOS can help track pain, symptoms, function and sport limitation.

Interpretation should consider:

• mechanical symptoms
• swelling
• range of motion
• squat and kneeling tolerance
• sport-specific pivoting demands
• whether surgery has occurred

Clients With Knee Osteoarthritis

For knee osteoarthritis, KOOS can provide a broader view than pain alone by capturing symptoms, daily function, sport/recreation and quality of life.

Interpretation should consider:

• flare-ups
• comorbidities
• body mass
• physical activity exposure
• medication changes
• functional goals

Post-Surgical Clients

For post-operative clients, KOOS can help track perceived recovery over time.

Interpretation should consider:

• surgery type
• healing stage
• weight-bearing restrictions
• expected post-operative symptoms
• rehabilitation stage
• medical or surgical guidance where relevant

Meaningful Change, MCID, MDC and Responsiveness

Meaningful change values help interpret whether a score change is likely to matter.

Key terms:

• MCID / MIC: the smallest change that may be meaningful to clients or professionals, depending on the method used
• MDC: the amount of change likely needed to exceed measurement error
• SEM: the estimated measurement error around a score
• Responsiveness: the ability of the measure to detect change over time
• PASS: a score threshold sometimes used to describe a patient acceptable symptom state
• SCB: substantial clinical benefit, used in some surgical outcome studies

KOOS Meaningful Change Evidence

Meaningful change values vary by population, condition, subscale, intervention and method.

For ACL tear and traumatic meniscal injury, the OPTIKNEE systematic review identified meaningful thresholds for patient-reported outcomes after interventions, including KOOS-related thresholds. This supports using population-specific values rather than applying one universal KOOS change score to all clients.  

For knee arthroplasty, post-2000 studies have examined MCID, PASS and substantial clinical benefit thresholds for KOOS-based scores, including KOOS Pain, KOOS-PS and KOOS Joint Replacement. These thresholds are useful only when the client population and outcome version match the study context.  

Practical Interpretation

When interpreting KOOS change:

• compare each subscale to baseline
• use population-specific MCID, MIC, MDC or SEM where available
• avoid applying one universal threshold to every knee presentation
• look for consistent improvement across relevant subscales
• compare score change with client goals
• check whether activity exposure has increased
• consider pain, swelling, strength, range of motion and functional tests

When Values Are Uncertain

Reported meaningful change values may vary by:

• condition
• age
• surgery type
• non-surgical versus surgical care
• subscale
• language version
• follow-up timeframe
• anchor method
• baseline severity

When no matching value exists, interpretation should rely more heavily on:

• baseline comparison
• repeated measurement
• client-reported change
• physical assessment findings
• training or activity exposure
• professional judgement

Normative Data, Reference Values or Comparative Data

Published KOOS reference values are available, but they should be applied carefully.

A 2023 national record-based study established reference values for KOOS and KOOS-12 because the measures were commonly used but lacked national record-based reference values to support interpretation.  

A 2020 Dutch population study developed percentile curves for KOOS in a middle-aged population. The study reported that scores were generally good but were worse in women and with higher BMI, and concluded that percentile curves may be useful as benchmarks in research and clinical practice.  

Reference values provide context, but interpretation should consider whether the reference population matches the client’s:

• age
• sex
• country
• health status
• knee injury history
• osteoarthritis status
• BMI
• activity level
• sport or work demands
• surgical history

Practical guidance:

• Scores closer to 100 generally suggest fewer reported knee problems.
• Lower scores suggest greater pain, symptoms, limitation or quality-of-life impact.
• Sport/Recreation and Quality of Life may remain limited after ADL improves.
• Population reference values provide context, not strict pass/fail thresholds.
• The client’s own baseline is often the most useful comparison.

Reliability and Validity

The KOOS has extensive post-2000 evidence supporting its use across knee injury, osteoarthritis and surgical populations.

A 2003 validation study comparing KOOS with WOMAC in total knee replacement concluded that KOOS was valid, reliable and responsive in that population, and that KOOS may provide improved validity compared with WOMAC while being at least as responsive.  

A 2016 systematic review and meta-analysis synthesised evidence on KOOS measurement properties across knee injury and osteoarthritis populations, identifying 37 eligible studies and using COSMIN methods to assess methodological quality.  

A 2024 clinimetric appraisal describes KOOS as a patient-completed questionnaire for young, middle-aged and older adults with knee injuries and/or osteoarthritis, and notes that KOOS is available in many languages and recommended in Australian knee osteoarthritis clinical practice guidance.  

Reliability and validity are stronger when:

• the correct KOOS version is used
• the same scoring method is repeated
• all relevant items are completed
• subscales are interpreted separately
• the correct language version is used
• retesting occurs at meaningful time points
• results are interpreted alongside physical and functional assessment

Interpret cautiously when:

• many items are missing
• the client has not attempted sport or work tasks yet
• only one subscale changes slightly
• the change is smaller than known measurement error or meaningful change
• symptoms are strongly influenced by a recent flare-up
• the score is being used without objective assessment context

Common Errors and Limitations

Common errors include:

• treating KOOS as a diagnosis
• using KOOS as a return-to-sport clearance tool
• combining subscales into one unsupported total score
• ignoring missing items
• using inconsistent scoring methods
• not recording the version used
• not recording the completion date
• comparing scores without considering activity exposure
• over-interpreting small changes
• using reference values as strict pass/fail cut-offs

Limitations include:

• self-report can be influenced by mood, expectations and recent activity
• sport items may be less meaningful if the client has not returned to sport
• scores do not identify the exact physical cause of symptoms
• meaningful change values vary by condition and population
• reference values may not match the client’s age, activity level or condition
• KOOS should be paired with physical tests and client goals

Practical Applications

The KOOS may help professionals:

• document baseline knee status
• identify whether pain, symptoms, sport or quality of life are most affected
• track change over time
• monitor post-injury or post-surgical recovery
• support knee osteoarthritis monitoring
• support goal setting
• improve client education
• communicate progress with a broader team
• strengthen Measurz reports

For athletes, KOOS can help identify whether sport and quality-of-life concerns remain despite improvement in daily function.

For general population clients, KOOS can help track walking, stairs, squatting, swelling, pain and participation.

For knee osteoarthritis, it can help monitor how symptoms affect function and quality of life over time.

For Measurz users, KOOS is most useful when combined with objective measures such as:

• knee range of motion
• knee swelling assessment
• quadriceps strength
• hamstring strength
• single-leg squat
• hop testing
• balance testing
• gait or running assessment

How to Record This in Measurz

Record:

• outcome measure name: Knee Injury and Osteoarthritis Outcome Score / KOOS
• version used: KOOS, KOOS-12, KOOS-PS, KOOS-Child or other
• date completed
• completion method: paper, digital, interview or assisted
• language/version used
• condition or presentation being tracked
• side involved: left, right or bilateral
• Pain subscale score
• Symptoms subscale score
• ADL subscale score
• Sport/Recreation subscale score
• Quality of Life subscale score
• score range: 0–100
• direction of scoring: higher score indicates better status
• missing items, if any
• assistance provided, if any
• current pain score, if relevant
• current symptoms
• current activity or sport exposure
• key functional limitations
• confidence or participation goals
• baseline comparison
• MCID/MDC/MIC/PASS comparison where supported
• related physical assessment findings
• interpretation notes
• retest date
• referral or further assessment notes where appropriate

Record whether the main limitation appears to be:

• pain dominant
• symptom or swelling dominant
• daily function limitation
• sport/recreation limitation
• quality-of-life limitation
• mixed presentation
• unclear due to incomplete responses

This improves:

• repeatability
• communication
• client education
• assessment reasoning
• monitoring over time
• team consistency
• reporting quality

Related Outcome Measures / Internal Links

• Foot and Ankle Outcome Score / FAOS
• Foot & Ankle Disability Index / FADI
• Lysholm Knee Scoring Scale
• Lower Extremity Functional Scale / LEFS
• Copenhagen Hip and Groin Outcome Score / HAGOS
• Hip Disability and Osteoarthritis Outcome Score / HOOS
• Pain Self-Efficacy Questionnaire / PSEQ
• Tampa Scale for Kinesiophobia / TSK
• ACL Quality of Life

FAQs

What does the KOOS measure?

The KOOS measures self-reported knee pain, symptoms, daily function, sport and recreation function, and knee-related quality of life.

How many items are in the KOOS?

The full KOOS has 42 items across five subscales.

How is the KOOS scored?

Each subscale is converted to a 0–100 score. A higher score indicates better knee status.

Is there one total KOOS score?

KOOS is usually interpreted using five separate subscale scores rather than one combined total score.

Does KOOS diagnose a knee condition?

No. KOOS does not diagnose a condition. It helps measure the client’s perceived symptoms, function and quality-of-life impact.

What is a meaningful change in KOOS?

Meaningful change depends on the population, condition, subscale and intervention. Use MCID, MIC, MDC, SEM, PASS or SCB values only when they match the client group and KOOS version.

Can KOOS be used for return-to-sport decisions?

KOOS can support return-to-sport reasoning, especially the Sport/Recreation and Quality of Life subscales, but it should not be the only clearance measure.

How often should KOOS be repeated?

It can be repeated at baseline, reassessment, after a training or rehabilitation phase, after surgery milestones and at key return-to-activity points.

Key Takeaways

• KOOS is a 42-item knee-specific patient-reported outcome measure.
• It includes Pain, Symptoms, ADL, Sport/Recreation and Quality of Life subscales.
• Each subscale is scored from 0 to 100.
• Higher scores indicate better perceived knee status.
• KOOS does not diagnose a condition or clear a client for sport.
• Meaningful change values should be matched to the population, subscale and context.
• Published reference values are available, but they should be used as context rather than strict pass/fail standards.
• Measurz should record each subscale, version, completion method, side, baseline comparison, related findings and retest plan.

References

Collins, N. J., & Mills, K. (2024). Clinimetrics: The Knee injury and Osteoarthritis Outcome Score (KOOS). Journal of Physiotherapy. https://doi.org/10.1016/j.jphys.2024.11.011

Macri, E. M., Young, J. J., Ingelsrud, L. H., Khan, K. M., Terluin, B., Juhl, C. B., Whittaker, J. L., Culvenor, A. G., Crossley, K. M., & Roos, E. M. (2022). Meaningful thresholds for patient-reported outcomes following interventions for anterior cruciate ligament tear or traumatic meniscus injury: A systematic review for the OPTIKNEE consensus. British Journal of Sports Medicine, 56(24), 1432–1444.

Mills, K., et al. (2023). Knee injury and Osteoarthritis Outcome Score (KOOS) and KOOS-12: National record-based reference values. The Knee. https://doi.org/10.1016/j.knee.2023.06.001

Roos, E. M., & Toksvig-Larsen, S. (2003). Knee injury and Osteoarthritis Outcome Score (KOOS): Validation and comparison to the WOMAC in total knee replacement. Health and Quality of Life Outcomes, 1, 17. https://doi.org/10.1186/1477-7525-1-17

Roos, E. M. (2023). 30 years with the Knee injury and Osteoarthritis Outcome Score (KOOS). Osteoarthritis and Cartilage. https://doi.org/10.1016/j.joca.2023.09.007

Salavati, M., et al. (2016). Knee Injury and Osteoarthritis Outcome Score (KOOS): Systematic review and meta-analysis of measurement properties. Osteoarthritis and Cartilage, 24(8), 1317–1329. https://doi.org/10.1016/j.joca.2016.03.010

Loef, M., Kroon, F. P. B., Böhringer, S., Roos, E. M., Rosendaal, F. R., & Kloppenburg, M. (2020). Percentile curves for the Knee injury and Osteoarthritis Outcome Score in the middle-aged Dutch population. Osteoarthritis and Cartilage. https://doi.org/10.1016/j.joca.2020.07.013