Factors Affecting Reliability of Grip Strength Measurements in Middle Aged and Older Adults

Background: Grip strength is a well-established marker of frailty and a good predictor of mortality that has been measured in a diverse range of samples including many population studies. The reliability of grip strength measurement in longitudinal studies is not well understood. Methods: : Participants (n=130) completed a baseline and repeat health assessment in the Irish Longitudinal Study on Ageing. Grip strength was assessed using dominant and non-dominant hands (two trials on each). Repeat assessments were conducted 1-4 months later and participants were randomised into groups so that 50% changed time (morning or afternoon assessment) and 50% changed assessor between assessments. Intra-class correlation (ICC) and minimum detectable change (MDC 95 ) were calculated and the effects of repeat assessment, time of day and assessor were determined. Results: : Aggregated measures had little variation by repeat assessment or time of day; however, there was a significant effect of assessor (up to 2 kg depending on the measure used). Reliability between assessments was good (ICC>0.9) while MDC 95 ranged from 5.59–7.96 kg. Non-aggregated measures alone, taken on the non-dominant hand were susceptible to repeat assessment, time of day, assessor and repeated measures within-assessment effects whereas the dominant hand was only affected by assessor.   Conclusions: : Mean and maximum grip strength had a higher ICC and lower MDC 95 than measures on the dominant or non-dominant hands alone. The MDC 95 is less than 8 kg regardless of the specific measure reported. However, changing assessor further increases variability, highlighting the need for comprehensive assessor training and avoiding changes within studies where possible.