PURPOSE: To test the reliability and validity of a 3D US erosion score in RA using MRI as the gold standard.

MATERIALS AND METHODS: RA patients were examined with 3D US and 3 T MRI over the 2nd and 3rd metacarpophalangeal joints. 3D blocks were evaluated by two investigators. The erosions were estimated according to a semi-quantitative score (SQS) (0 - 3) and a quantitative score (QS) (mm²). MRI was evaluated according to the RAMRIS score. For the estimation of reliability, intra-class correlation coefficients (ICC) were used. Validity was tested using Spearman's rho (rs). The sensitivity and specificity were also calculated.

RESULTS: 28 patients with RA were included. The ICC for the inter-observer reliability in the QS was 0.41 and 0.13 for the metacarpal bone and phalangeal bone, respectively, and 0.86 and 0.16, respectively, in the SQS.  The ICC for the intra-observer reliability in the QS was 0.75 and 0.48 for the metacarpal bone and phalangeal bone, respectively, and 0.83 and 0.60, respectively, in the SQS.  The correlation with MRI for the metacarpal bone was significant, with values of 0.73 (p = 0.0001) (SQ) and 0.74 (p = 0.0001) (SQS). For the phalangeal bone, bad correlation was found: 0.28 (p = 0.145) (SQ) and 0.26 (p = 0.57) (SQS). The sensitivity and specificity for the metacarpal bone were 86 % and 85 % respectively. For the phalangeal bone they were 60 % and 97 %, respectively.

CONCLUSION: Good inter- and intra-observer reliability and correlation with MRI were seen in the assessment of erosions with 3D US in the metacarpal bone, while the results were low and insignificant for the phalangeal bone, indicating that 3D US still has room for improvement.