Reliability of digital algometer–based pressure pain threshold measurement in patients with end-stage knee osteoarthritis: a single-center reliability study

Objective To evaluate the reliability and measurement error of digital algometer–based pressure pain threshold (PPT) assessment inpatients with end-stage knee osteoarthritis (KOA).Methods This prospective observational reliability study included 60 patients with end-stage KOA scheduled for total knee arthroplasty. Pressure pain threshold was assessed by two raters with different levels of experience at baseline and 7 days later at the medial aspect of the affected knee (local site) and the dorsal aspect of the contralateral forearm (remote site), following standardized training and measurement procedures. Three repeated measurements were obtained at each site per session and averaged. Relative reliability was assessed using intraclass correlation coefficients (ICC), with the ICC2,k model used for intra-session reliability based on the mean of three repeated measurements and the ICC2, 1 model used for inter-session and inter-rater reliability. Absolute measurement error was quantified using the standard error of measurement (SEM), relative SEM (SEM%), and the minimal detectable change at the 95% confidence level (MDC95). Inter-rater agreement was examined using Bland – Altman analysis.Results Pressure pain threshold measurements demonstrated excellent reliability across all conditions. Intra-session, inter-session, and inter-rater ICC values all exceeded 0.87. Intra-session ICC2, k values ranged from 0.874 to 0.965, inter-session ICC2, 1 values from 0.870 to 0.984, and inter-rater ICC2, 1 values from 0.921 to 0.953. Relative SEM values remained below 7%, and inter-session MDC95 ranged from 0.33 to 0.72 kg/cm2, representing measurement-error thresholds for detecting change beyond random variability. Bland–Altman analysis showed that most differences lay within the 95% limits of agreement, with no apparent proportional bias. Mean inter-rater differences ranged from 0.09 to 0.19 kg/cm2.Conclusions Under standardized conditions, digital algometer–based PPT assessment showed high reliability with low measurement error in patients with end-stage KOA. Clinical utility requires further validation.