A clozapine level test is a blood test that measures the plasma concentration of clozapine to ensure the dose is therapeutic and safe. It's usually taken as a trough sample (just before the next dose) to guide dose adjustments, check adherence, and detect drug interactions or toxicity. Clinicians use results to balance effectiveness (often aiming for ~350–600 ng/mL) against side‑effect risks.

A complete blood count (CBC) is required for clozapine because the drug can cause serious blood disorders, particularly neutropenia and agranulocytosis. Regular CBCs with differential—emphasizing the absolute neutrophil count—detect early drops in white cells, enabling prompt treatment interruption and infection management. Frequent baseline and ongoing monitoring reduces the risk of severe complications and guides safe continuation of therapy.

Clozapine’s therapeutic plasma range is generally considered about 350–600 ng/mL (trough). Levels above ~1,000 ng/mL increase risk of toxicity (seizures, excessive sedation). Individual response varies; factors such as smoking, CYP1A2 inhibitors, age and liver function markedly affect levels. Measure trough concentrations and adjust dose according to clinical response and safety monitoring (white cell counts, side effects).

Monitor full blood count with ANC regularly (weekly then monthly) for agranulocytosis; watch for infection symptoms. Check baseline and early serial ECG, heart rate, CRP and troponin for myocarditis. Monitor weight, BMI, fasting glucose, lipids, blood pressure and temperature. Assess bowel function (constipation/ileus), sedation, hypersalivation, seizure risk, smoking status and interacting medications; adjust dose and labs accordingly.

If clozapine level is high, contact the prescribing psychiatrist or emergency services immediately. Seek urgent review for toxicity signs (severe drowsiness, confusion, seizures, rapid/irregular heartbeat, low blood pressure, high fever). Do not change dose without clinician advice. Clinicians may hold or reduce dose, repeat clozapine level, order ECG and blood tests (including ANC), review interacting drugs and smoking status, and provide supportive care or admission.

Clozapine plasma concentrations above about 1000 ng/mL (≈1 mg/L) are generally considered toxic, with levels above 600 ng/mL linked to increased adverse effects. Toxicity can cause severe sedation, respiratory depression, hypotension, tachycardia, seizures and coma; agranulocytosis is an unpredictable serious risk. If overdose or severe symptoms occur, seek urgent medical attention for monitoring and supportive care.