Multimorbidity and physical and cognitive function: performance of a new multimorbidity-weighted index.

Background: Multimorbidity is an important health outcome but is difficult to quantify. We recently developed a multimorbidity-weighted index (MWI) and herein assess its performance in an independent nationally-representative cohort.

Methods: Health and Retirement Study (HRS) participants completed an interview on physician-diagnosed chronic conditions and physical functioning. We determined the relationship of chronic conditions on physical functioning and validated these weights with the original, independently-derived MWI. We then determined the association between MWI with physical functioning, grip strength, gait speed, basic and instrumental activities of daily living (ADL/IADL) limitations, and the modified Telephone Interview for Cognitive Status (TICS-m) in adjusted models.

Results: Among 20,509 adults, associations between chronic conditions and physical functioning varied several-fold. MWI values based on weightings in the HRS and original cohorts correlated strongly (Pearson's r=0.92) and had high classification agreement (Kappa statistic=0.80, p<0.0001). Participants in the highest vs. lowest MWI quartiles had weaker grip strength (-2.91 kg, 95%CI: -3.51, -2.30), slower gait speed (-0.29 m/s, 95%CI: -0.35, -0.23), more ADL (0.79, 95%CI: 0.71, 0.87) and IADL (0.49, 95%CI: 0.44, 0.55) limitations, and lower TICS-m (-0.59, 95%CI: -0.77, -0.41) (all P<0.001). We observed monotonic graded relationships for all outcomes with increasing MWI quartiles.

Conclusion: A multimorbidity index weighted to physical functioning performed nearly identically in a nationally-representative cohort as it did in its development cohorts, confirming broad generalizability. MWI was strongly associated with subjective and objective physical and cognitive performance. Thus, MWI serves as a valid patient-centered measure of multimorbidity, an important construct in research and clinical practice.