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Clozapine, Serum or Plasma
Evaluate toxicity; monitor therapeutic levels
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)
Patients dosed with 400 mg clozapine daily for four weeks were most likely to exhibit a therapeutic effect when the sum of clozapine and norclozapine concentrations was at least 450 ng/mL.
Clozapine is absorbed rapidly, with peak plasma concentrations occurring within 30 minutes to six hours. Terminal half-life in humans is in the range of 5.5 to 33 hours. Determination of clozapine in plasma is needed to correlate efficacy with plasma concentration. However, pharmacogenomics and coadministration of other agents can complicate the correlation between dose and plasma concentration. Changes in the activity of the cytochrome P450 mixed-function oxidases, especially CYP1A2 and CYP3A4 can affect clozapine levels.1 These oxidases can be induced by drugs such as carbamazepine, with the resulting increase in activity causing 50% decrease in clozapine concentration.2 An additional report3 has indicated some patients may be ultrarapid metabolizers of clozapine due to genetically induced CYP1A2 activity levels. Conversely, clozapine plasma (serum) concentrations may be increased by inhibitors of the P450 oxidases. Amiodarone, cimetidine, ciprofloxacin, fluoxetine, fluvoxamine, indinavir, paroxetine, quinidine, ritonavir, and saquinavir are examples of inhibiting drugs which may cause increases in clozapine levels when coadministered.
A review of adverse metabolic effects4 has indicated clozapine can significantly increase mean levels of glucose and C-peptide, as well as increase in triglyceride levels. Additionally, the Federal Drug Administration and drug provider have issued the following: Patients being treated with clozapine must have a baseline white blood cell (WBC) count and absolute neutrophil count (ANC) before initiation of treatment as well as regular WBC counts and ANCs during treatment and for at least four weeks after discontinuation of treatment.5
Serum or plasma
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.
Transfer separated serum or plasma to a plastic transport tube.
Causes for Rejection
|Order Code||Order Code Name||Order Loinc||Result Code||Result Code Name||UofM||Result LOINC|
|706440||Clozapine (Clozaril), Serum||706441||Clozapine, Serum||ng/mL||6896-5|
|706440||Clozapine (Clozaril), Serum||706442||Norclozapine, Serum||ng/mL||10992-6|
|706440||Clozapine (Clozaril), Serum||706443||Total(Cloz+Norcloz)||ng/mL||12375-2|