Analysis of correlation and causes for discrepancy between quantitative and semi-quantitative Doppler scores in synovitis in rheumatoid arthritis

Objectives. The aim of this study was to evaluate the association between two semi-quantitative Doppler US scoring systems (SQS), and the quantitative scoring (QS) of Doppler pixel count. Methods. Adult patients with RA and inadequate clinical response to anti-rheumatic therapy were examined with musculoskeletal US (MSUS). Dorsal MSUS of the wrists, MCP and MTP 2-5 were performed. MSUS images with sign of synovitis were collected and the QS was measured. Five assessors blinded to the QS evaluated the images independently, according to either SQS method. Association between QS and SQS was studied using correlations and multilevel models taking into account the clustering of ratings at the rater, patient and joint levels. Results. Analysis of the 1190 ratings revealed a strong correlation (ρ = 0.89, P < 0.0001) and significant associations (P < 0.0001) between QS and SQS. Correlations between QS and SQS according to Szkudlarek et al. (ρ = 0.87, P < 0.0001) or Hammer et al. (ρ = 0.91, P < 0.0001) were similar. A total of 239 (20.1%) images were given a SQS grade that did not match that expected based on initial QS, using pre-defined cut-offs. Main explanations for discrepancies were different perceived region of interest (40.7%) and Doppler pixel count near cut-offs between SQS grades (32.3%). Conclusion. We showed that both SQS methods correlated well with QS to assess synovitis, but SQS methods are intrinsically limited when the Doppler pixel count is close to the cut-offs between the SQS grades. Analysis discrepancies between these methods may help further revision of criteria used to assess disease activity with MSUS in RA.