Fluoxetine, Serum or Plasma

Serum or plasma

2 mL

0.6 mL

Red-top tube, lavender-top (EDTA) tube, or green-top (heparin) tube. Do not use a gel-barrier tube. The use of gel-barrier tubes is not recommended due to slow absorption of the drug by the gel. Depending on the specimen volume and storage time, the decrease in drug level due to absorption may be clinically significant.

The efficacy of fluoxetine in the treatment of nondelusional, moderately depressed patients is comparable to that of the tricyclic agents; its efficacy in severely depressed hospitalized patients has not been established. In limited studies, fluoxetine was useful in treating patients with atypical depression, panic disorder, and the depressed component of bipolar disorder. Fluoxetine may be indicated as initial therapy in patients with concurrent obsessive-compulsive disorder. Its efficacy in obesity and bulimia is being explored.

The selection of fluoxetine appears to be most appropriate for patients who are at special risk from sedative, hypotensive, and anticholinergic side effects caused by other antidepressants. In addition, it appears to be relatively safe for use in the elderly, although its very long elimination half-life may pose special problems for these individuals.

This test was developed and its performance characteristics determined by Labcorp. It has not been cleared or approved by the Food and Drug Administration.

Liquid chromatography/tandem mass spectrometry (LC/MS-MS)

Therapeutic: fluoxetine: 91−302 ng/mL, norfluoxetine: 72−258 ng/mL. After dosing at 40 mg/day for 30 days, plasma concentration of fluoxetine in the range of 91−302 ng/mL and norfluoxetine in the range of 72−258 ng/mL were observed. In a well-designed pharmacokinetic study, with patients (N = 38) taking fluoxetine for five weeks of fixed doses between 20 mg and 80 mg per day, the majority of plasma concentrations were between 100−800 ng/mL for fluoxetine and between 100−600 ng/mL for norfluoxetine.

Fluoxetine is well absorbed after oral administration, whether or not food is present, and peak plasma concentrations are attained six to eight hours after a dose. Steady-state plasma concentrations are reached after two to four weeks. Fluoxetine is widely distributed throughout the body, and about 94% of a dose is bound to plasma protein. The drug is metabolized in the liver to norfluoxetine, which also selectively inhibits serotonin reuptake, and other unidentified metabolites. Fluoxetine is excreted in the urine primarily as inactive metabolites. Renal impairment does not appear to alter the pharmacokinetics, although alcohol-induced cirrhosis does prolong the half-life.

The elimination half-life is long: one to four days for fluoxetine and 7 to 10 days for norfluoxetine. Because of the long half-life, the drug can be administered once daily, efficacy is unaffected by an occasional missed dose, and abrupt termination of therapy results in gradual cessation of effects. This drug's pharmacokinetic profile is similar in elderly and younger patients.