Specimen Collection

5 mL Plain Blood . Pre-dose trough (12 hours since previous dose) recommended.

IMPORTANT: Serum or plasma separator tubes (SST or PST) with gel are not acceptable for the testing of drugs at LabPlus. Significant adsoprtion of clozapine and it metabolites occur in these tubes

URGENT requests:

Urgent requests must be phoned to the chromatography department, ext. 22053. An email, or a writing "urgent" on the request form is not sufficient.

Clozapine analysis is time-consuming and technically difficult. Urgent analysis cannot always be guaranteed. If urgent analysis is agreed to, samples must be in the lab by mid-morning to be resulted same day.

Urgent requests will only be accepted from a psychiatrist or a psychiatry registrar specifically authorised by a psychiatrist consultant

• admission to ACH with clozapine related complications, or
• medical needs that could be compromised if they deteriorate in mental state

A Clozapine level is not required as part of routine monitoring (LabPlus policy):

o " Repeat" request forms will not be accepted. A new form must be completed for each request for a clozapine level.

o The name of the requestor must be clearly written, along with a contact telephone number and signature.

o The clinical information for the test must be clearly written on the request form. "Routine monitoring" or "on clozapine" will not be accepted. If clinical information is not provided, or does not provide sufficient justification for the test, the test may be declined.

If a test is declined, the specimen will be held for a reasonable period (usually 3 weeks but dependant on the stability of the sample). Medical practitioners seeking approval for a declined test should email the on-call Chemical Pathologist ( chemicalpathologist@adhb.govt.nz ) , giving the patient's name and NHI number and the clinical justification for the test. If unable to email, call the on-call Chemical Pathologist via Lablink ( 09-3078995 ) and identify yourself as a doctor.

4 mL Plain Blood (Preferred)

4 mL Heparin Blood
Reference Intervals

Units : nmol/L

Clozapine: nmol/L = ug/L x 3.06

D esmethylclozapine : nmol/L = ug/L x 3.20

Therapeutic range : Clozapine: 1050 to 1800 nmol/L

Therapeutic control can be achieved at levels less than 1000 nmol/L in some patients.

Reference intervals stated are for trough (pre-dose) levels. They are only a guide, and there is significant variability in both clinical response and likelihood of side effects. Some side effects, e.g. cardiac and haematological, are not dose related (see below).

Metabolism: Clozapine is extensively metabolized by cytochrome P450 system in to Clozapine N-oxide and N-desmethylclozapine. The major contributions to this metabolism include CYP1A2, and CYP3A4. Other contributory CYP enzymes include CYP2C19 and CYP2D6.

Both clozapine and desmethylclozapine (the major metabolite) are reported.

Elimination half-life: Plasma level peaks typically after 1-3 hours and half-life is usually 8-16 hours at steady state (1) .

Uncertainty of Measurement : 12%

Turnaround Time: Within 3 days

TURN AROUND TIME IS BASED ON WORKING DAYS (MON -FRI)

Assay Method

Principle : Liquid Chromatography Mass Spectrometry (LCMS)

Diagnostic Use and Interpretation

Clozapine is an atypical antipsychotic indicated for patients with treatment-resistant schizophrenia. Side effects of clozapine include agranulocytosis, myocarditis and cardiomyopathy, constipation, intestinal obstruction, drowsiness and seizures.
Although clozapine levels can vary between individuals and are affected by factors such as gender, age, smoking status and weight (2), routine measurement of clozapine levels is not indicated in every day practice for the following reasons:
1. A significant proportion of patients (approximately 1/5 th ) respond to clozapine treatment even if the serum concentration is below the therapeutic interval quoted above (1) .
2. Following a change in dose, patients with poor initial response at 6 weeks may respond by 4-6 months.
3. There is no agreed toxic threshold for clozapine related CNS toxicity. Seizures can occur at or even below the therapeutic interval in some patients. The risk of toxicity is increased at clozapine concentrations > 3000 nmol/L.
4. Haematological and cardiac side effects (e.g. agranulocytosis, myocarditis and myopathy) are not dose related. A full blood count should be checked on regular basis and if signs of infection are present. Patients should be regularly assessed for symptoms and signs of heart failure. Please review your local DHB guidelines with regards to routine monitoring and frequency of blood tests.

Indications and interpretation for serum clozapine levels (3,4):
Monitoring the compliance of clozapine:

• Therapeutic ranges derived from large studies may aid in the interpretation of clozapine levels but these studies are subject to the limitation of being retrospective analysis of laboratory data. (2, 5).
• Serum levels should be interpreted with previous measurements where available.
• Individual variations of clozapine and desmethylclozapine have been derived from our in-house data.
• The within-subject variation of clozapine is about 20% which means that the ratio will vary by up to +/- 41% from the mean on 95% of occasions in a stable patient on a same dose. A change of > 60% is needed between two trough levels to show there has been a significant difference.
• The within-subject variation of desmethylclozapine is about 18% which means that the ratio will vary by up to +/-36% from the mean on 95% of occasions in a stable patient on a same dose. A change of > 53% is needed between two trough levels to show there has been a significant difference.

Signs of toxicity or adverse effects that may be linked to serum levels (e.g. seizures), special caution should be paid to patients in the following categories:

• When increasing dose of clozapine in patients on an existing high dose
• Sudden onset of side effects (or other situations suggestive of irregular compliance)
• Acute inflammatory states may lead to an increase in clozapine levels (6)
• Constipation is a common side effect of clozapine which can affect the degree and rate of clozapine absorption. The GI hypomotility caused by clozapine can result in delayed absorption. Once the bowel moves after a period of constipation, some of the retained but not yet absorbed clozapine in the gut may only get absorbed then. Thus, it is crucial to beware of the cumulative dose of clozapine when the bowel movement pattern changes.

Patients suspected or known to have liver disease

Co-prescription with CYP450 medications:

• Increased clozapine levels: e.g. caffeine, ciprofloxacin, erythromycin, fluoxetine
• Decreased clozapine levels: e.g. carbamazepine, phenobarbitone, phenytoin, rifampicin

Smoking and clozapine:

• Aromatic hydrocarbons in cigarette smoke (not nicotine) are an inducer of CYP1A2, leading to faster clozapine metabolism (e.g. can lower serum clozapine level). Therefore sudden smoking cessation can cause a large increase in serum levels.
• Nicotine replacement therapy (e.g. nicotine patches) use does not affect clozapine levels.

Lack of improvement under recommended doses

Relapse during maintenance

Elderly patients (> 65y)

Patients with renal impairment

Interpretation of desmethylclozapine levels and clozapine/desmethylclozapine ratios:

Desmethylclozapine is the major metabolite of clozapine formed by the enzymatic action of CYP1A2. It has no efficacy as an antipsychotic but it may assist the interpretation of the clozapine level.
There is considerable variation in the clozapine ratio between different people due to genetic variations in metabolism. Based on published articles (4,5,7) and in-house data review, clozapine/ desmethylclozapine ratios in adults are usually 1 - 3 in trough samples taken from compliant patients at steady state. Higher plasma clozapine concentration tends to have a higher clozapine/desmethylclozapine ratio as the major metabolic pathway for clozapine exhibits saturable kinetics.
With regular, stable dosing and lifestyle (e.g. smoking habits), the ratio is expected to remain largely stable for an individual patient. Based on our in-house data, the within-subject variation of the ratio is about 16%. This means that the ratio will vary by up to +/- 33% from the mean on 95% of occasions in a stable patient on a same dose. A change of > 48% is needed between two trough levels to show there has been a significant difference. However, a new baseline ratio will likely occur with a change in long-term lifestyle habit (e.g. starting/stopping smoking).

Possible causes for a rise in clozapine/desmethylclozapine ratio

• Not a trough sample - absorption of clozapine from the last dose has not been completed at the time of sampling
• Enzyme saturation -the clozapine metabolism is saturated with a high clozapine level (usually >3000nmol/L)
• Addition of an enzyme inhibitor e.g. Fluvoxamine
• Stopping smoking

Possible causes for a drop in clozapine/desmethylclozapine ratio

• Poor adherence in preceding days
• Addition of enzyme inducer, e.g. starting smoking

References :

1. Byerly MJ, DeVane CL. Pharmacokinetics of Clozapine and Risperidone: A Review of Recent Literature. Journal of Clinical Psychopharmacology. 1996;16(2):177-87.
2. Rostami-Hodjegan A, Amin AM, Spencer EP, Lennard MS, Tucker GT, Flanagan RJ. Influence of dose, cigarette smoking, age, sex, and metabolic activity on plasma clozapine concentrations: a predictive model and nomograms to aid clozapine dose adjustment and to assess compliance in individual patients. J Clin Psychopharmacol. 2004;24(1):70-8.
3. Schoretsanitis G, Kane JM, Ruan CJ, Spina E, Hiemke C, de Leon J. A comprehensive review of the clinical utility of and a combined analysis of the clozapine/norclozapine ratio in therapeutic drug monitoring for adult patients. Expert Rev Clin Pharmacol. 2019;12(7):603-621.
4. Couchman L, Morgan PE, Spencer EP, Flanagan RJ. Plasma clozapine, norclozapine, and the clozapine:norclozapine ratio in relation to prescribed dose and other factors: data from a therapeutic drug monitoring service, 1993-2007. Therapeutic drug monitoring. 2010;32(4):438-47.
5. Pfuhlmann B, Hiemke C, Unterecker S, Burger R, Schmidtke A, Riederer P, et al. Toxic clozapine serum levels during inflammatory reactions. J Clin Psychopharmacol. 2009;29(4):392-4.
6. Patteet L, Maudens KE, Vermeulen Z, Dockx G, De Doncker M, Morrens , et al. Retrospective evaluation of therapeutic drug monitoring of clozapine and norclozapine in Belgium using a multidrug UHPLC-MS/MS method. Clin Biochem. 2014 ;47(18):336-9.
7. Hiemke C, Bergemann N, Clement HW, Conca A, Deckert J, Domschke K, et al. Consensus Guidelines for Therapeutic Drug Monitoring in Neuropsychopharmacology: Update 2017.
8. Medsafe. 2015. Clozapine -­ Close Monitoring Required. Prescriber Update 36(2): 18 -21. URL: https://www.medsafe.govt.nz/Profs/PUArticles/June2015/June2015Clozapine.htm (accessed 26 July 2019).
9. Bryant, L. (2015). The dangers of smoking cessation? and medicines. Journal of Primary Health Care , 7 (4), 345?347. https://doi.org/10.1071/HC15345

10. McKean, A. (2014). Safer prescribing of high-risk medicines?: Best Practice Journal , (62), 46?49. Retrieved from http://www.bpac.org.nz/BPJ/2014/July/safer-prescribing.aspx

11. Winckel, K., & Siskind, D. (2017). Clozapine in primary care. Australian Prescriber , 40 (6), 231?236. https://doi.org/10.18773/austprescr.2017.067

Contact Information

Emails to chemicalpathologist@adhb.govt.nz will receive priority attention from the on-call chemical pathologist.

If the query concerns a specific patient please include the NHI number in your email.

If email is not a suitable option, please contact the on-call chemical pathologist via Lablink (Auckland City Hospital ext. 22000 or 09-3078995).

Individual chemical pathologists may be contacted but will not be available at all times.

After-hours : contact Lablink (Auckland City Hospital ext. 22000 or 09-3078995) or hospital operator for on duty staff after hours.

Dr Samarina Musaad (Clinical Lead) : SamarinaM@adhb.govt.nz ext. 22402

Dr Cam Kyle: CampbellK@adhb.govt.nz ext 22052

Dr Weldon Chiu: WeldonC@adhb.govt.nz ext. 23427

Dr Campbell Heron: CHeron@adhb.govt.nz ext. 23427

Specimen Transport Instructions for Referring Laboratories

Centrifuge and separate blood as soon as possible after collection.

If sending within Auckland then sample can be sent refrigerated (2-8 degrees) or at ambient temperature (8-24 degrees).

If sending from outside Auckland then send sample refrigerated (2-8 degrees).

Note: If temperature during shipping is likely to exceed 24 degrees then results may not always be reliable.

Last updated at 09:25:30 22/01/2024