Why EQ-5D and the Oxford Hip and Knee scores do not measure the same things

The outcome of treatment is always paramount for patients and healthcare professionals. Patient-reported outcome measures have been developed to measure outcomes.Since 2009, all patients in England having hip and knee replacement surgery have been asked to complete the generic EuroQol EQ-5D-3L and EQ visual analogue scale (EQ-VAS) and the condition-specific Oxford Hip Score or Oxford Knee Score for hips and knees, respectively.

EQ-5D-3L has five dimensions with three options each. Each combination has been scaled relative to the best conceivable health state (value 1.0) and the state of dead (value 0) to produce a relative severity score (EQ-Index) with a range from -0.594 to 1.0. This can be used to calculate quality-adjusted life-years. The EQ-VAS is a visual analogue scale from 0 (dead) to 100 (best conceivable health state).The Oxford Hip and Knee scores are similar to each other. They have 12 questions with five options each, scored 0-4. These scores are added, giving a scale with range 0 (no problems) to 48 (extreme problems on all questions).Using over 40 000 records for patients undergoing hip and knee replacements from the National Health Service patient-reported outcome measure database, we compare EQ-5D-3L with the Oxford Hip and Knee scores. To aid comparison, each score was transformed arithmetically to a common 0 (floor) to 100 (ceiling) scale.

EQ-Index, EQ-VAS and the Oxford scores give very different results in terms of change, effect size and correlation.

More research is needed, but some speculative ideas are put forward, which could explain these findings.