Plasma hypertonicity: another marker of frailty?

OBJECTIVES: To determine whether plasma hypertonicity might be a marker of early frailty, this study tested the associations between plasma hypertonicity, incident disability, and mortality in nondisabled older adults.

DESIGN: Longitudinal, observational study.

SETTING: Community-based.

PARTICIPANTS: Older adults (> or =70), who reported no disability and gave blood in the 1992 Duke Established Populations for Epidemiologic Studies of the Elderly survey (n=705), were re-interviewed in 1996 for functional status (n=561) and followed for all deaths up to January 1, 2000.

MEASUREMENTS: Plasma tonicity was estimated from plasma glucose, sodium, and potassium measures and used to classify subjects as normo- (285-294 mOsm/L) or hypertonic (> or =300 mOsm/L). Disability was defined as any impairment on the Rosow-Breslau, activity of daily living (ADL), and instrumental activity of daily living (IADL) scales. The relative risk (RR) of any new disability and relative hazard of death associated with hypertonicity were estimated using logistic regression models and Cox proportional hazards models, respectively. All models were controlled for age, sex, race, weight status, current smoking, activity level, plasma blood urea nitrogen and creatinine, cognitive impairment, depression, and chronic disease status. To determine whether observed effects were attributable to plasma glucose alone, all models were repeated on a subsample of nondiabetic, normoglycemic subjects.

RESULTS: Plasma hypertonicity (observed in 15% of subjects) was associated with increased risk of new Rosow-Breslau (RR=2.1, 95% confidence interval (CI)=1.2-3.6), IADL (RR=2.3, 95% CI=1.2-4.3), and ADL (RR=2.7 95% CI=1.3-5.6) disability by 1996 and mortality by 2000 (RR=1.4, 95% CI=1.0-1.9). Results were similar for the normoglycemic subgroup (ADL: RR=2.9, 95% CI=1.0-8.0; IADL: RR=2.5, 95% CI=1.0-6.3; Rosow-Breslau: RR=1.8, 95% CI=0.8-3.9; mortality: RR=1.5, 95% CI=0.9-2.3).

CONCLUSION: Plasma hypertonicity may be a marker of early frailty. It was prevalent in this sample of nondisabled community-dwelling older adults and predicted incident disability and mortality. Further research to identify its determinants and consequences may help inform interventions against frailty.