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Test Code LAB4207/6709/PAROX Paroxetine, Serum

Important Note

Send Outs - Mayo

Additional Codes

Mayo Code: PARO 


Specimen Required

Collection Container/Tube: Red top(serum gel/SST are not acceptable)

Submission Container/Tube: Plastic vial

Specimen Volume: 0.5 mL

Collection Instructions:

1. Draw blood before next scheduled dose.

2. Centrifuge and aliquot serum into plastic vial within 2 hours of collection.

Method Name

Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS)

Reporting Name

Paroxetine, S

Specimen Type

Serum Red

Reject Due To

Gross hemolysis OK
Gross lipemia OK
Gross icterus OK

Day(s) Performed


Report Available

1 to 4 days

Performing Laboratory

Mayo Clinic Laboratories in Rochester

Test Classification

This test was developed, and its performance characteristics determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the US Food and Drug Administration.

CPT Code Information


LOINC Code Information

Test ID Test Order Name Order LOINC Value
PARO Paroxetine, S 9699-0


Result ID Test Result Name Result LOINC Value
83731 Paroxetine, S 9699-0

Secondary ID


Useful For

Monitoring paroxetine therapy


Identifying noncompliance, although regular blood level monitoring is not indicated in most patients


Identifying states of altered drug metabolism when used in conjunction with CYP2D6 genotyping

Clinical Information

Paroxetine (Paxil and Paxil CR) is approved for treatment of depression. Paroxetine is completely absorbed. Metabolites of paroxetine are inactive. Paroxetine metabolism is carried out by cytochrome P450 (CYP) 2D6. Paroxetine can saturate CYP2D6 resulting in a nonlinear relationship between dose and serum concentration. Paroxetine clearance is significantly affected by reduced hepatic function, but only slightly by reduced renal function.


A typical adult paroxetine dose is 30 mg per day. Paroxetine is 100% bioavailable, 95% protein bound, and the apparent volume of distribution is 17 L/Kg. Time to peak serum concentration is 5 hours for the regular product and 8 hours for the controlled release product. The elimination half-life is 20 hours. Half-life is prolonged in the elderly and with cirrhosis.


Steady-state serum concentrations associated with optimal response to paroxetine are in the range of 30 to 120 ng/mL and in the narrower range of 20 to 65 ng/mL.


The most common toxicities associated with excessive serum concentration are asthenia, anticholinergic effects, anxiety, blurred vision, and changes in sexual function. The toxic range for paroxetine is greater than 120 ng/mL.


Specimens that are obtained from gel tubes are not acceptable as the drug can absorb on the gel and lead to falsely decreased concentrations.

Clinical Reference

1. Hiemke C, Baumann P, Bergemann N, et al: AGNP consensus guidelines for therapeutic drug monitoring in psychiatry: update 2011. Pharmacopsychiatry. 2011;44:195-235

2. Rifai N, Horvath AR, Wittwer CT, eds: Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. Elsevier; 2018

3. Hiemke C, Bergemann N, Clement HW, et al: Consensus guidelines for therapeutic drug monitoring in neuropsychopharmacology: Update 2017. Pharmacopsychiatry. 2018 Jan;51(1-02):9-62

Method Description

Paroxetine, citalopram, venlafaxine, and venlafaxine metabolite are extracted from serum by precipitation with acetonitrile. Dilute methanolic hydrochloric acid is added to form a salt to protect analytes from volatilization during the evaporation of the acetonitrile. High-performance liquid chromatography with detection by tandem mass spectrometer (MS/MS) is used to measure concentration.(Zhu Z, Neirinck L: High-performance liquid chromatography-mass spectrometry method for the determination of paroxetine in human plasma. J Chromatogr B Analyt Technol Biomed Life Sci. 2002;780:295-300)

Specimen Retention Time

2 weeks

Specimen Minimum Volume

0.4 mL

Reference Values

30-120 ng/mL


If not ordering electronically, complete, print, and send a Therapeutics Test Request (T831) with the specimen.

Specimen Stability Information

Specimen Type Temperature Time Special Container
Serum Red Refrigerated (preferred) 28 days
  Ambient  28 days
  Frozen  28 days