The role of generic and disease-specific measures of physical and role functioning in assessing patient outcomes: a longitudinal study

Using longitudinal data from the Veterans Health Study (VHS), we extended our earlier published cross-sectional analysis in comparing the generic SF-36 physical functioning (PF) and role limitations (role-physical [RP]) scales with the disease-specific PF and RP scales using disease attributions. The present study included 569 patients with chronic lung disease (CLD) and 352 patients with chronic low back pain (LBP) who received Veterans Affairs (VA) ambulatory care between June 1993 and March 1996. Consistent with our earlier study, we found that changes in the generic PF and RP scales had higher correlations with the other generic SF-36 scales than those in the disease-specific PF and RP scales over 12 months. On the other hand, disease-specific measures of PF and RP had larger R and t statistic values in discriminating the impacts of symptom-based illness severity as well as clinical services on physical and role functioning. These results suggest that the generic and disease-specific measures of PF and RP behave distinctly different from each other over time. The generic measures of PF and RP tend to assess a broad array of health-related quality of life, whereas disease-specific attributions of PF and RP scales tend to evaluate disease progression and clinical management associated with specific disease conditions. Disease-specific attribution is an important alternative to the development of new disease-specific instruments for assessing illness severity and the impact of clinical services.