Details for subgrouping low back pain. Brian Sweetman, NHS 17 July 2013 Kongsted et al (1) provide useful prognostic information for leg pain related to low back problems. Using the Quebec task force classification for such "sciatica", they classified the leg pain as reaching above or below knee level (along with the other criteria). This cut point is often used in this area of research to distinguish radicular pain from referred pain from the low back. However in a classification cluster analysis we used multiple demarcation levels in the leg and found a more specific cut point at mid calf for these purposes (2,3). Indeed the Quebec system needs to distinguish pain beyond the knee with and without neurological deficit. The mid calf cut point seems better able to segregate the two patterns. Never the less the prognostic implications are comparable between the two studies despite different follow up intervals. If further research is to be done in this field, it is suggested that sufficient information is collected to allow for both classification systems to be analysed. The latter system may allow for more detailed diagnostic discrimination than with the basic Quebec system, and perhaps allow for better targeting of treatment (4). This would seem worthy of further investigation. References 1. Kongsted A, et al. Prognostic implications of the Quebec Task Force classification of back-related leg pain: an analysis of longitudinal routine clinical data. BMC Musculoskeletal Disorders 2013, 14:171. 2. Sweetman BJ. Remote manifestations of low back problems; sciatica and extent of leg involvement ¿ a statistical clinical study. International Musculoskeletal Medicine 2010; 32 (2): 72-7. 3. Sweetman BJ, Sweetman SJ. Midline back pain; A clinical database analysis. International Musculoskeletal Medicine 2013; 35 (1): 20-8. 4. Sweetman BJ, Sweetman SJ. Hypothesis generation for targeted treatment of back pain. International Musculoskeletal Medicine 2013; 35: In press. Competing interests None.