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Test Catalog

Test ID: DOXP    
Doxepin and Nordoxepin, Serum

Useful For Suggests clinical disorders or settings where the test may be helpful

Monitoring therapy


Evaluating potential toxicity


Evaluating patient compliance

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Doxepin is recommended for the treatment of psychoneurotic patients with depression or anxiety, and depression or anxiety associated with alcoholism or organic disease.


Nordoxepin (N-desmethyldoxepin) is the major metabolite and is usually present at concentrations equal to doxepin. Optimal efficacy occurs at combined serum concentrations between 50 and 150 ng/mL.


Like other tricyclic antidepressants, the major toxicity of doxepin is expressed as cardiac dysrhythmias, which occur at concentrations in excess of 300 ng/mL. Other side effects include nausea, hypotension, and dry mouth.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

Therapeutic concentration (doxepin + nordoxepin): 50-150 ng/mL

Toxic concentration (doxepin + nordoxepin): > or =300 ng/mL

Interpretation Provides information to assist in interpretation of the test results

Most individuals display optimal response to doxepin when combined serum levels of doxepin and nordoxepin are between 50 and 150 ng/mL. Some individuals may respond well outside of this range, or may display toxicity within the therapeutic range; thus, interpretation should include clinical evaluation. Risk of toxicity is increased with combined levels > or =300 ng/mL.


Therapeutic ranges are based on specimens drawn at trough (ie, immediately before the next dose).

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Doxepin analytical interferences: sertraline may cause falsely low results, and venlafaxine, diphenhydramine, chlorpromazine, promazine, ethopropazine, or amlodipine may cause false elevations.


Nordoxepin analytical interference: sertraline may cause falsely low results.


This test cannot be performed on whole blood. Serum must be separated from cells within 2 hours of drawing; if serum is not removed within this time, drug levels may be falsely elevated due to drug release from RBCs.


Specimens that are obtained from gel tubes are not acceptable.

Clinical Reference Provides recommendations for further in-depth reading of a clinical nature

1. Wille SM, Cooreman SG, Neels HM, Lambert WE: Relevant issues in the monitoring and the toxicology of antidepressants. Crit Rev Clin Lab Sci 2008;45(1):25-89

2. Thanacoody HK, Thomas SH:  Antidepressant poisoning. Clin Med 2003;3(2):114-118

3. Baumann P, Hiemke C, Ulrich S, et al:  The AGNP-TDM expert group consensus guidelines: therapeutic drug monitoring in psychiatry. Pharmacopsychiatry 2004;37(6):243-265