Test-retest reliability and measurement errors of grip strength test in patients with traumatic injuries in the upper extremity: a cross-sectional study

BACKGROUND: Grip strength (GS) test is an essential aspect of clinical practice with patients with upper extremity injuries. The random error of GS test was hypothesized to be proportional to the level of GS. The purpose of the current study was to estimate a precise range for the measurement error of GS in patients following traumatic injuries in the upper extremity. METHODS: Following traumatic injuries in the upper extremity, 109 participants completed GS tests twice one weekend apart. The Bland-Altman plot analysis was adopted to estimate the precise limits of agreement with 95% confidence interval (CI). RESULTS: The mean of three consecutive trials had a higher intraclass correlation coefficient of 0.974 (95% CI = 0.963, 0.982) than those of one trial and the mean of the first two trials in injured upper extremities. When GS was ≤20 kg, the upper limit of agreement with 95% CI was estimated as (0.41 × average GS + 1.24), while the lower limit was estimated as (− 0.41 × average GS − 0.39). A table of one-to-one matches between averaged GS ≤ 20 kg and transformed ranges of random errors with 95% certainty was created; the standard error of measurement and minimal detectable change with 95% certainty of GS test were 1.8 and 4.9 kg, respectively. When GS was > 20 kg, the width of agreement with 95% CI ranged from − 4.9 to 5.3 kg, and the standard error of measurement and minimal detectable change with 95% certainty were 1.8 and 5.1 kg, respectively. CONCLUSION: The one-to-one match table can be considered as a practical tool to judge a change in GS score is real or due to random errors when it is ≤20 kg.