Dose determination for Clozapine is primarily based upon clinical response/haematological state/metabolic adverse events/cardiac adverse events. Wide inter- and intra-patient variability in plasma Clozapine levels exists and monitoring is useful for assessing compliance or assessing toxic levels.

Whilst some studies have recommended a ‘therapeutic range’ evidence from multiple prospective, randomised, controlled clinical studies on a role for Clozapine TDM in determining dose is lacking. Similarly rigorous evidence of a specific toxic level is not available.

Therefore, the reference range quoted with results (100 to 800 µg/L) is deliberately wider than most proposed ‘therapeutic ranges’ (of between approx 350 to 600 µg/L) and it is based on a range of evidence from different literature reports and on data from our laboratory. Levels > 800 µg/L are flagged as high, < 100 µg/L are flagged as low and levels > 1500 µg/L warrant urgent (telephone) reporting. These ranges have been endorsed by the Director of Mental Health and the Chemical Pathologist at Austin Health. Further reading: Stark A and Scott J 2012, ANZJPsych 46:816-825