Modified sharp method: factors influencing reproducibility and variability

In rheumatoid arthritis, joint radiography is still the most frequently used instrument to assess the progression of joint damage. Unfortunately, the poor quality of the radiographic scoring methods available has a negative impact on the power in clinical trials. This study focuses on the influence of the following 4 factors on radiographic scores according to van der Heijde's modification of the Sharp method: intraobserver variation, interobserver variation, follow-up time, and number of measurement occasions within a patient series.

One hundred and seventy-two patients in the early stages of rheumatoid arthritis were followed up. During the first 3 years, radiographs of the hands and feet were taken twice yearly and scored by 3 observers. The scoring process was repeated after an additional 3-year period. Correlation coefficients and differences between observers were calculated to define variability. The influence of the 4 factors on variability was studied.

One observer assigned a significantly higher score than the other 2, who had been trained together. Interobserver variability decreased as follow-up time increased. Interobserver correlation coefficients became higher, with smaller differences between observers for progression scores than for absolute scores. Increasing the number of measurements within a patient series led to higher scores. Intraobserver correlation coefficients were high, and a training effect occurred when the time between measurements was 1 year, resulting in lower scores.

This study demonstrates that, and shows how, the investigated factors influence the variability of the modified Sharp method. It is extremely important to take interobserver variation into account when designing protocols for multicenter clinical trials. A progression scoring method is recommended for studies assessing radiographic damage or clinical trials.