Reliability and construct validity of the compatible MRI scoring system for evaluation of haemophilic knees and ankles of haemophilic children. Expert MRI working group of the international prophylaxis study group A. S. DORIA,* P. S. BABYN,* B. LUNDIN,  R. F. KILCOYNE, à S. MILLER,* G. E. RIVARD,§ R. MOINEDDIN– and H. PETTERSSON  *The Hospital for Sick Children, Diagnostic Imaging, Toronto, ON, Canada;  University Hospital of Lund, Radiology, Lund, Sweden; àUniversity of Colorado Health Sciences Center, Radiology, Denver, CO, USA; §Ho ˆ pital Ste-Justine, Hematology & Oncology, Montreal, QC, Canada; and –University of Toronto, Public Health, Family and Community Medicine, Toronto, ON, Canada Summary. We tested the reliability and construct validity of the Compatible magnetic resonance ima- ging (MRI) scale for the evaluation of haemophilic knees and ankles and compared the diagnostic performance of MRI and plain film radiographs. Sagittal and coronal gradient-echo 1.5-T MR images of knees (n ¼ 22) and ankles (n ¼ 23) were obtained from boys (age range 4–16 years; mean 11 years) in two centres (Toronto, n ¼ 26; Europe, n ¼ 19). The MR images were independently read by four blinded radiologists on two occasions. Number of previous joint bleedings and laboratory level of severity of haemophilia were the reference standards for ima- ging assessment. Both components of the MRI scale demonstrated high inter- and intrareader intraclass correlation coefficients (progressive (P) scale, 0.91 and 0.94; additive (A) scale, 0.81 and 0.92 respect- ively). The correlation between the osteochondral domain of the MRI scale and patient’s age was moderate. Otherwise, correlations between A- and P- scales and clinical laboratory measurements were weak. The areas under the curve (AUCs) used for discrimination of disease severity were similar for the A- and P-scales (AUCs used for mild disease, A-scale, 0.72 ± 0.07; P-scale, 0.69 ± 0.08; P ¼ 0.23; AUCs for severe disease, A-scale, 0.93 ± 0.05; P-scale, 0.87 ± 0.08; P ¼ 0.05). No differences were noted between the AUCs of the MRI and radio- graphic scales used for discrimination of late osteo- articular changes; MRI scales performed better for discrimination of early changes. In conclusion, both MRI scales demonstrated excellent reliability, poor convergent validity, and moderate and excellent validity for discrimination of mild and severe dis- eases respectively. Compared with radiographic scores, the MRI scales performed better for discrim- ination of early osteoarticular changes. Keywords: arthropathy, children, discriminant valid- ity, haemophilia, MRI scoring systems, reliability Introduction Haemophilia has a considerable social and economic impact on the healthcare system [1]. Joints are affected in approximately 90% of people with severe haemophilia [2]. This contributes significantly to the cost and morbidity in the haemophilic population [3]. Given the social and economic challenges facing the care of persons with haemophilia the need for imaging tools that can provide accurate measure- ments of their osteoarticular disease and response to treatment becomes urgent. Such a strategy may improve the diagnosis and follow-up of patients and may help optimize individualized replacement therapy at a time when chronic haemophilic arthro- Correspondence: Andrea S. Doria, MD, MSc, PhD, The Hospital for Sick Children, Department of Diagnostic Imaging, 555 Uni- versity Avenue, Toronto, ON, Canada M5G1X8. Tel.: +1-416-813-6079; fax: +1-416-813-7591; e-mail: andrea.doria@sickkids.ca Accepted after revision 15 May 2006 Haemophilia (2006), 12, 503–513 DOI: 10.1111/j.1365-2516.2006.01310.x Ó 2006 Blackwell Publishing Ltd 503