Serum osmolarity and haematocrit do not modify the association between the impedance index (Ht2/Z) and total body water in the very old: The Newcastle 85+ Study

Bioelectrical impedance is a non-invasive technique for the assessment of body composition; however, information on its accuracy in the very old (80+ years) is limited. We investigated whether the association between the impedance index and total body water (TBW) was modified by hydration status as assessed by haematocrit and serum osmolarity. This was a cross-sectional analysis of baseline data from the Newcastle 85+ Cohort Study. Anthropometric measurements [weight, height (Ht)] were taken and body mass index (BMI) calculated. Leg-to-leg bioimpedance was used to measure the impedance value (Z) and to estimate fat mass, fat free mass and TBW. The impedance index (Ht2/Z) was calculated. Blood haematocrit, haemoglobin, glucose, sodium, potassium, urea and creatinine concentrations were measured. Serum osmolarity was calculated using a validated prediction equation. 677 men and women aged 85 years were included. The average BMI of the population was 24.3±4.2kg/m2 and the prevalence of overweight and obesity was 32.6% and 9.5%, respectively. The impedance index was significantly associated with TBW in both men (n=274, r=0.76, p