Reliability of commercially available tricyclic antidepressant levels

Recent studies show that there may be a relationship between plasma levels of tricyclic antidepressants and clinical response. A number of commercial laboratories have introduced methods for measuring these drugs in plasma for clinical practice, and each recommends a particular method by which plasma samples should be drawn, processed, and stored. In this pilot study we compare results from 4 laboratories, using identical plasma samples containing amitriptyline and nortriptyline, in order to assess interlaboratory reliability. While there is generally good agreement between our results and those of the commercial facilities, some large interlaboratory discrepancies are evident. Caution is advised when utilizing plasma tricyclic levels to monitor patient progress, and we suggest using a laboratory which is well established in this area.