Self-assessment using 30-second chair stand test for patients with knee osteoarthritis – an intra- and inter-rater reliability study

We aimed to evaluate the intra-rater reliability in 30-second chair stand test (30CST) as a self-test, and the inter-rater reliability comparing a self-test with a physiotherapist-assessed 30CST for patients with knee osteoarthritis (KOA). The second purpose was to evaluate the classification ability of the 30CST as self-test to distinguish between reduced physical function and normal function. Patients with KOA performed two self-tests of 30CST at home and thereafter a physiotherapist assessed the patient. Absolute agreements were presented with intraclass correlation coefficient (ICC), 95% confidence interval (CI) and standard error of measurement (SEM). Post-hoc analyses were conducted to calculate minimal detectable change (MDC). The classification ability was analysed with receiver operating characteristic (ROC)-curves and area under the ROC-curve (AUC). The significance level was <i>p</i> &lt; 0.05. In total, 114 followed the protocol. Intra-rater and inter-rater reliability were good (ICC 0.97, CI 0.95–0.99, SEM 0.89 and ICC 0.81, CI 0.72–0.87, SEM 2.29 respectively). MDC were 2.5 stands when comparing self-test results individually and 0.23 on a group level. The MDC between self-tests and physiotherapist assessment were 6.4 stands for individual comparison and 0.59 for group comparison. The self-test was 80% likely to detect reduced physical function compared to a physiotherapist assessing physical function with 30CST (AUC = 0.79–0.80). The results indicate that 30CST can be a reliable self-assessment of physical function in lower extremities for patients with KOA. These findings can be useful in areas such as self-screening for reduced physical function, self-assessment in digital physiotherapy, and as measurement in research projects.