Factors influencing nortriptyline steady-state kinetics: plasma and saliva levels.

Proportionality between dose and steady-state nortriptyline (NT) plasma levels was found both during initial treatment and after long-term treatment (years) within the NT Plasma level range of 20 to 296 ng/ml. There were day-to-day variations of 10% to 20% (coefficient of variation) but no systematic changes in plasma levels over time. A significant age variation in NT plasma levels was found in 116 patients. Patients over 70 yr of age (n = 23) had higher levels than other age groups (p < 0.001) dose corrected; p < 0.0001 dose and weight corrected). It was found that during episodes of acute inflammatory diseases NT steady-state plasma levels rose after a change in sedimentation rate. Our data show that the use of saliva rather than plasma in therapeutic drug level monitoring of NT cannot be recommended because the saliva/plasma ratio varied both intra- and interindividually by factors of from 2 to 4.