Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
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.
Monitoring paroxetine therapy
While regular blood level monitoring is not indicated in most patients, the test is useful to identify noncompliance
When used in conjunction with CYP2D6 genotyping, the test can identify states of altered drug metabolism
Steady-state serum concentrations associated with optimal response to paroxetine are in the range of 30 to 90 ng/mL.
The most common toxicities associated with excessive serum concentration are asthenia, anticholinergic effects, anxiety, blurred vision, and changes in sexual function. Toxic range: 400 ng/mL.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
No significant cautionary statements
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.
Clinical References Provides recommendations for further in-depth reading of a clinical nature
1. Physicians’ Desk Reference (PDR). 59th edition. Montvale, NJ, Medical Economics Company, 2005
2. Goodman & Gilman's The Pharmacological Basis of Therapeutics. 10th edition. Edited by JG Hardman, LE Limbird. New York, McGraw-Hill Book Company, 1994, p1994