The effect of ageing on plasma albumin and plasma protein binding of diazepam, salicylic acid and digitoxin in healthy subjects and patients with renal impairment.

1. Plasma albumin concentration was measured in 118 healthy subjects (aged between 18 and 87 years), in 95 renal patients with creatinine clearances between 15 and 50 ml min-1 (aged between 14 and 79 years) and in 101 uraemic patients maintained on chronic haemodialysis (aged between 27 and 83 years). 2. There was a significant (P less than 0.001) negative correlation between albumin concentration and age in healthy subjects, but no correlation in patients with low creatinine clearance or in uraemic patients. 3. The ex vivo plasma binding of diazepam (1 microM), salicylic acid (2 mM) and digitoxin (37 nM) was studied in groups of age-selected young and aged healthy subjects in patients with low creatinine clearance and in patients with uraemia. The unbound fractions of diazepam and salicylic acid were about double in old compared with young healthy subjects whereas they were similar in young and old patients with lowered creatinine clearance. In uraemic patients, ageing did not affect the binding of salicylic acid whereas the unbound fraction of diazepam was slightly but significantly greater in elderly subjects. The unbound fraction of digitoxin was independent of age in both healthy subjects and in those with renal disease. 4. Decreased plasma binding of diazepam and salicylic acid was partially corrected by extensive dialysis of plasma. The lower plasma binding of diazepam and salicylic acid associated with ageing may be ascribed to the effects of endogenous displacers and to hypoalbuminaemia. The influence of these two factors appears to be drug-dependent.