KOOS Score Calculator

What is the KOOS Score?

The Knee Injury and Osteoarthritis Outcome Score (KOOS) is a self-administered patient-reported outcome measure developed by Roos et al. (1998) as an extension of the WOMAC Osteoarthritis Index. KOOS was specifically designed for younger, more active patients where WOMAC lacked sensitivity at higher activity levels. Unlike WOMAC, KOOS adds a Sport & Recreation subscale and a Quality of Life subscale — making it significantly more responsive for patients who run, jump, or pivot.

The five subscales are scored independently on a 0–100 scale (0 = extreme knee problems, 100 = no knee problems). No composite total score is reported — each subscale must be interpreted on its own, as they assess distinct constructs. This is the most important difference from other knee outcome measures.

KOOS Scoring Formula

For each subscale: score each item 0–4, calculate the mean item score, multiply by 25, then subtract from 100. Subscale Score = 100 − (Mean Item Score × 25). If fewer than 20% of subscale items are unanswered, use the subscale mean to impute missing values.

Clinical Applications by Condition

• Knee OA (Conservative PT): Primary PROM for PT interventions. MCID ~10–15 points after 4 weeks. Baseline OA patients typically score 45–65 on Pain and ADL.
• ACL Reconstruction: Primary PROM in Scandinavian ACL registries. Most responsive at 6–24 months post-reconstruction. Sport/Rec subscale most sensitive for return-to-sport decisions.
• Total Knee Replacement: Superior to WOMAC post-TKR for detecting improvements. Pain MCID 13 pts, QOL MCID 16 pts at 6 months. Large effect sizes at 3–12 months across all subscales.
• Meniscus Injury: Sport/Rec and QOL most responsive at 3 months post-partial meniscectomy. ADL subscale shows moderate effect sizes.

KOOS vs KOOS-12 vs KOOS-JR — Which Form to Use?

The full 42-item KOOS is recommended for clinical research and comprehensive outcome assessment when detail is required and patient burden is manageable. KOOS-12 (12 items, 3 subscales: Pain, Function/Daily Living, Function/Sport and Recreation) offers reduced burden with comparable responsiveness for ACL patients and is faster to administer in busy clinical settings. KOOS-JR (7 items, single score 0–28) is validated specifically for TKR monitoring in older patients, reducing questionnaire burden while maintaining adequate responsiveness. Use the full KOOS when complete domain-specific subscale data is clinically indicated.

KOOS vs LEFS vs WOMAC — Choosing the Right Knee Outcome Measure

• KOOS vs LEFS: KOOS is knee-specific with 5 subscales providing granular domain data. LEFS covers all lower extremity joints (hip, knee, ankle) in 20 items producing one composite score — faster and more versatile for mixed caseloads. Use KOOS for knee-specific subscale tracking; use LEFS for any lower extremity condition or when a single score is needed.
• KOOS vs WOMAC: KOOS is preferred for younger, more active patients — the Sport/Rec and QOL subscales are absent from WOMAC. WOMAC (24 items, 3 subscales: Pain, Stiffness, Physical Function) is more commonly used in older patients with advanced OA undergoing TKR. WOMAC data can be extracted from KOOS retrospectively since WOMAC items are embedded within KOOS.
• KOOS vs Berg Balance Scale: For patients with significant balance deficits alongside knee pathology (e.g., post-TKR), consider administering both — KOOS captures knee-specific function while BBS captures balance and fall risk independently.

KOOS Psychometric Properties

The KOOS demonstrates excellent test-retest reliability across all five subscales (ICC = 0.75–0.93). Internal consistency is high (Cronbach's alpha 0.75–0.94 per subscale). Construct validity was confirmed through comparison with SF-36, WOMAC, and Lysholm scores across multiple populations. The scale is validated in over 50 populations across 12 languages. Floor and ceiling effects are minimal for the Pain and ADL subscales in clinical populations, but ceiling effects are common in the Sport/Rec subscale in older or less active patients.

Administering the KOOS

The KOOS is self-administered and takes approximately 10–15 minutes to complete. It can be administered on paper or electronically. Patients should be instructed to answer based on their knee symptoms during the previous week. If a patient is unable to complete more than 20% of items in any subscale, that subscale cannot be scored. For items S4 and S5 ("Can you straighten/bend your knee fully?"), note that the response options are reversed — "Always" scores 0 and "Never" scores 4 — which this calculator handles automatically.