Background <p>Knee osteoarthritis is becoming increasingly common, and the age-related decline in muscle mass or sarcopenia can exacerbate both conditions in a vicious cycle. Therefore, screening for sarcopenia is crucial for the management of both diseases. The Ishii score as an effective screening tool is currently widely used.</p> Objectives <p>This study aimed to validate the predictive effectiveness of the Ishii test in older adults with knee osteoarthritis patients, establish new cut-off value to assess its accuracy in predicting sarcopenia.</p> Methods <p>A cross-sectional survey was conducted on 404 participants with knee osteoarthritis from China. Patients with sarcopenia were identified according to the 2019 criteria of the Asian Working Group for Sarcopenia. Social demographic variables, body composition, handgrip strength and calf circumference were collected and the Ishii score was calculated. The accuracy of the Ishii score test was determined by using indicators, including specificity, sensitivity, negative and positive predictive values, negative and positive likelihood ratios, Kappa value, Youden index, and receiver operating characteristic curve.</p> Results <p>The Ishii test predicts sarcopenia at an AUC value of 0.85 for males, and at an AUC value of 0.84 for females. The cut-off value is over 108 for males and 119 for females, with the new cut-off value, the sensitivity, specificity, positive and negative predictive values, the kappa value or sarcopenia were 0.95, 0.66, 0.47, 0.98, 0.45 for males, and 0.91, 0.73, 0.47, 0.97,0.48 for females, respectively.</p> Conclusions <p>The study validated that the Ishii test can serve as a practical screening tool in community or clinical settings.</p>