Do QOL instruments agree? A comparison of the 15D and NHP in hip and knee replacements

Objectives: Several instruments for measuring health-related quality of lif e (HRQOL) have been developed, and others are under construction. The probl em is whether the different HRQOL measures show comparable results. We firs t compared the functional relationship of the Nottingham Health Profile (NH P) and the 15-dimensional measure of HRQOL (15D) in hip and knee replacemen t patients. The hypothesis was that condition or intervention does not affe ct the functional relationship between NHP and 15D changes. Methods: We assessed the agreement of the instruments by comparing observed changes in the 15D and its fitted values derived by regressing the 15D by the NHP dimensions. Patients (n=452) were recruited consecutively from seve n Finnish orthopedic departments during April 1991-May 1992. HRQOL was meas ured prior to surgery and 6, 12, and 24 months postoperatively. Results: There was a different functional relationship between the HRQOL in struments in hip and knee patients; they agreed upon the direction of chang es in HRQOL in 84% and showed opposite signs in 12%. The NHP showed signifi cantly more improvement in quality of life than the 15D. Conclusions: The two instruments were in slight but significant disagreemen t. Estimates of effectiveness can vary according to the instrument used. Co mparisons of effectiveness in healthcare programs measured by differing ins truments need information on the functional discrepancies between the instr uments in the conditions and interventions in which they are applied.