MEASURING CLINICAL STATUS IN CYSTIC-FIBROSIS - INTERNAL VALIDITY AND RELIABILITY OF A MODIFIED NIH SCORE

We examined measurement properties of the NIH Clinical Score for Cysti c Fibrosis (CF) as an index of disease status. This score is being emp loyed as a research tool for defining study populations and as an outc ome measure, yet there are no published data on its reliability or how its items contribute to the overall measure of disease status. Criter ia for scoring some items in the original index lack specificity. In t his study, we used a modified score to have more clearly specified cri teria, while retaining the original weightings and structure. For 200 patients with CF in two centers, we analyzed the total NIH Score and i ts subscores for internal consistency, interrater reliability, and fac tor analysis. Internal consistency indicates how inter-related the ite ms are. The pulmonary subscore and overall score had fairly high inter nal consistency. However, the general subscore had low internal consis tency, suggesting that the items are not measuring a single element of disease status and should not be added. Factor analysis provides addi tional information on the underlying structure and relationships among items. Five factors (groups of items) were identified accounting for 85% of the consistent variance of 14 items. These factors were designa ted by items accounting for most of their variance: general pulmonary, weight, disability, psychosocial, and acute infiltrate. While inter-r ater reliability for the overall index was high, individual items show ed less agreement. The results indicate that most of the variability i n the NIH Score is attributable to pulmonary items in the first factor . The analyses suggest a new scoring structure for the NIH Score; the general subscore items do not contribute to the reliability or account for significant variance. Therefore, they will likely require further refinement or be eliminated. (C) 1994 Wiley-Liss, Inc.