A radioimmunoassay for nortriptyline (and other tricyclic antidepressants) in plasma.

The radioimmunoassay for nortriptyline described here can detect as little 1 microgram/liter of plasma. Within-day precision and day-to-day precision (CV) were +/- 6 and +/- 11%, respectively, over the concentration range 100-200 microgram/liter. The major metabolite hydroxy-nortriptyline, does not cross react with the antiserum. Results so obtained correlate closely with results by a double-isotope derivative dilution technique. The major advantages of this technique over currently available methods are its sensitivity, convenience (many samples can be processed in one day), simplicity, and cost. Further, prior extraction of plasma samples is not required. Cross-reactivity studies have been carried out with all other available tricyclic antidepressants. The antiserum has the ability to bind these drugs, thus radioimmunoassay for all the tricyclic antidepressant drugs can be set up because concurrent use of more than one of these drugs is rare.