Joint counts are the key outcome measure in rheumatoid arthritis (RA). There is a great variability between different assessors of the same patient; this variability can be reduced by standardized training. The training effect is far less pronounced for the 66/68-joint count compared to the 28-joint count. We evaluated the reason for the higher interrater disagreement in the 66/68 compared to the 28-joint count.
Participants in joint examination seminars evaluated a patient with RA before and after training in the European League Against Rheumatism technique. Joints were rated positive or negative for tenderness and swelling. The number of positive joints and the variability between examiners before and after the training were compared. Concordance was calculated for every single joint using the Fleiss-Kappa test.
In total, 256 health professionals were instructed in the 66/68-joint count and 84 in the 28-joint count. The disagreement between examiners was higher for swelling than for tenderness. After the training, there was a significant reduction of interrater variability, which was more pronounced in the 28 than in the 66/68-joint count. Comparisons between joint counts revealed that the joints of the feet were more likely to be rated negative, yet interrater disagreement was still high.
Standardization of joint examination significantly reduces variability between assessors. The better performance of the 28-joint count is due to the lower number of joints examined, especially the foot joints, which remain difficult to assess reliably even after training.