Scoring haemophilic arthropathy on X-rays: improving inter- and intra-observer reliability and agreement using a consensus atlas

OBJECTIVES: The radiological Pettersson score (PS) is widely applied for classification of arthropathy to evaluate costly haemophilia treatment. This study aims to assess and improve inter- and intra-observer reliability and agreement of the PS. METHODS: Two series of X-rays (bilateral elbows, knees, and ankles) of 10 haemophilia patients (120 joints) with haemophilic arthropathy were scored by three observers according to the PS (maximum score 13/joint). Subsequently, (dis-)agreement in scoring was discussed until consensus. Example images were collected in an atlas. Thereafter, second series of 120 joints were scored using the atlas. One observer rescored the second series after three months. Reliability was assessed by intraclass correlation coefficients (ICC), agreement by limits of agreement (LoA). RESULTS: Median Pettersson score at joint level (PS(joint)) of affected joints was 6 (interquartile range 3–9). Using the consensus atlas, inter-observer reliability of the PS(joint) improved significantly from 0.94 (95 % confidence interval (CI) 0.91–0.96) to 0.97 (CI 0.96–0.98). LoA improved from ±1.7 to ±1.1 for the PS(joint). Therefore, true differences in arthropathy were differences in the PS(joint) of >2 points. Intra-observer reliability of the PS(joint) was 0.98 (CI 0.97–0.98), intra-observer LoA were ±0.9 points. CONCLUSIONS: Reliability and agreement of the PS improved by using a consensus atlas. KEY POINTS: • Reliability of the Pettersson score significantly improved using the consensus atlas. • The presented consensus atlas improved the agreement among observers. • The consensus atlas could be recommended to obtain a reproducible Pettersson score. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00330-015-4013-8) contains supplementary material, which is available to authorized users.