The suggested therapeutic ranges for Quetiapine (100 to 500 mcg/L) and for Norquetiapine (100 to 250 mcg/L) are as suggested by the AGNP consensus guidelines (1). Levels observed in the interquartile ranges (Q1-Q3) of responders were 66-227 mcg/L (2). Carbamazepine will decrease and valproate will increase plasma levels.
Quetiapine is an atypical antipsychotic medication used for the treatment of a number of psychiatric disorders including schizophrenia, bipolar disorder and major depression. It is mainly metabolised through the CYP450 3A4 enzyme and has a short half-life of about seven hours. The major active metabolite of Quetiapine is Norquetiapine, which has mainly noradrenergic effects and a different half-life. The therapeutic ranges for Quetiapine (100 to 500 mcg/L) and for Norquetiapine (100 to 250 mcg/L) follow the recommendation of the AGNP consensus guidelines (1). Therapeutic drug monitoring studies have shown high variation within patients and between patients for Quetiapine levels. Higher concentrations might be associated with sedation rather than with extrapyramidal side-effects.
1. Hiemke C, et al. Consensus Guidelines for Therapeutic Drug Monitoring in Neuropsychopharmacology: Update 2017 [published correction appears in Pharmacopsychiatry. 2018 Jan;51(1-02):e1]. Pharmacopsychiatry. 2018;51(1-02):9–62. doi:10.1055/s-0043-116492
2. Hiemke C. Concentration-Effect Relationships of Psychoactive Drugs and the Problem to Calculate Therapeutic Reference Ranges. Ther Drug Monit. 2019;41(2):174–179. doi:10.1097/FTD.0000000000000582
Performed by LC-MS/MS