Monitoring blood concentrations of clozapine (Clozaril, Denzapine, Zaponex) for toxicity is now advised in certain clinical situations.

Examples may include when:

• • a patient stops smoking or switches to an e-cigarette
  • concomitant medicines may interact to increase blood clozapine levels
  • a patient has pneumonia or other serious infection
  • poor (reduced) clozapine metabolism is suspected
  • toxicity is suspected
• if blood clozapine level monitoring is carried out, this should be in addition to the required blood tests to manage the risk of agranulocytosis
• for other antipsychotics, where assays and suggested reference values are available (see list in link), blood level monitoring for toxicity may be helpful in certain circumstances, for example in the event of symptoms suggestive of toxicity or when concomitant medicines may interact to increase antipsychotic drug levels
• refer to the full product Summaries of Product Characteristics for other important warnings, interactions, and recommendations for clozapine and other individual antipsychotics

Further information can be found here