Quantification of the ratio of clozapine to-N-desmethylclozapine, also known as norclozapine, in blood serum. Clozapine is an antipsychotic drug used to treat schizophrenia and N-desmethylclozapine is a a major active metabolite of clozapine. The trait is serum clozapine-to-N-desmethylclozapine ratio measurement (EFO_0600042, an Experimental Factor Ontology identifier). Also known as: serum clozapine-to-norclozapine ratio measurement.