Antidepressant monitoring by LC-MS/MS confirms adherence to SSRIs (citalopram, fluoxetine, paroxetine, sertraline), SNRIs (duloxetine, venlafaxine), and related agents (bupropion, trazodone). Several of these are also common causes of false positives on immunoassay screens, so they matter for interpretation as well as adherence.1
These antidepressants are among the most-prescribed medications, and several are documented immunoassay false-positive triggers: bupropion and trazodone can cause amphetamine false positives, and venlafaxine can cause PCP false positives. A specific LC-MS/MS result resolves both adherence questions and screen discrepancies.3
LC-MS/MS measures the specific antidepressant and, where relevant, its metabolite (e.g., norfluoxetine, desmethylvenlafaxine). Absence in a prescribed patient may reflect non-adherence, PRN use, a drug interaction, or altered CYP metabolism. Because several of these agents cross-react on immunoassays, use definitive testing to distinguish true amphetamine or PCP exposure from an antidepressant-driven false positive.3
Confirms recent use. Detection indicates use within the estimated window; levels do not confirm dose or schedule adherence.
Explains some screen positives. An antidepressant-positive result can account for an amphetamine or PCP immunoassay false positive.3
Metabolite supports ingestion. Where measured (norfluoxetine, desmethylvenlafaxine), the metabolite supports genuine use.
May indicate non-adherence. In a prescribed patient, absence can reflect missed doses, PRN use, or altered metabolism.
Not on routine panels. Antidepressants are not on standard drug screens; only targeted testing detects them.
Timing and cutoff. Absence may reflect timing of last use or levels below the cutoff.
| Antidepressant | Urine analyte(s) | Estimated window |
|---|---|---|
| Fluoxetine | Fluoxetine, norfluoxetine | Parent 5-15 d (acute) to 20-30 d (chronic); norfluoxetine 20-80 d |
| Sertraline | Sertraline | 4-8 days |
| Citalopram / escitalopram | + N-desmethyl-citalopram | 5-8 days |
| Paroxetine | Paroxetine | 4-6 days |
| Venlafaxine | + desmethylvenlafaxine | Parent 1.5-2 d; metabolite 2-3 d |
| Duloxetine | Duloxetine | 2-4 days |
| Bupropion | Hydroxybupropion | Up to 5 days |
| Trazodone | Trazodone | Up to 3 days |
Windows are estimated from half-life (about 5x t-half) and vary widely; fluoxetine and its long-lived metabolite norfluoxetine persist longest.