Reduced serum tricyclic levels due to gel separators.

Sixty-three blood samples were obtained from 48 subjects taking tricyclic antidepressants. Samples were simultaneously collected in Becton-Dickinson Vacutainer tubes for measurement of serum and plasma tricyclic antidepressant concentrations. Serum was drawn into red stopper tubes (without gel separator) and into speckled stopper serum separator tubes (with gel separator), while plasma was collected in green stopper tubes (heparinized). Imipramine, desipramine, amitriptyline, and nortriptyline concentrations were all significantly lower in serum separator samples than in either plasma or red stopper serum collection tubes. Serum and plasma concentrations were similar.