Clozapine therapeutic drug monitoring – experience from a large London NHS trust (2024)

Introduction

Clozapine is a high-risk drug that is used widely in Secondary and Tertiary Centres.¹ Therapeutic Drug Monitoring (TDM) advice and recommendation is readily available from distinguished authors.^2,3 The rate of clozapine TDM, appropriate sample collection for TDM, appropriate actions following a clozapine level varies between services and prescribers within our organisation.

Aim

To establish how many current West London NHS Trust (WLT) patients had a clozapine TDM, how many plasma samples collected for TDM were done appropriately, the proportion of patients with an appropriately collected plasma sample result within the largely accepted therapeutic range, and whether there was documentation that the plasma level was reviewed and if any prescription changes were made.

Methods

Approval to undertake the service evaluation was given by the Trust clinical governance and audit committee as ethical review as not required. No patient identifiable details were shared or collected. All patients who had been registered on the Trusts clozapine patient monitoring service for over 8 weeks were enrolled. Anonymised patient demographic data was collected including concomitant medication. Plasma levels were sought for every patient enrolled. The appropriateness of the samples taken were scrutinised. The plasma level result was collected. Electronic patient notes were also scrutinised to assess actions following the plasma level result. Data were collected in a binary yes/no format and results calculated as a percentage. Data was stored and collected in accordance to Trust General Data Protection Regulation (GDPR).

Results

In total 316 patients were included. 97% of these patients had evidence of TDM levels done during the time of the audit. 88% of these patients’ samples were done correctly. Only 45% of patients had levels within the widely acceptable therapeutic range. Of those patients whose levels fell outside the therapeutic range less than half (42%) had documentation that the level was reviewed.

Discussion/Conclusion

It is widely accepted that clozapine TDM when done accurately can be a vital source of information to inform prescribers on the appropriate dose, concordance and toxicity of clozapine therapy. Our Trust showed evidence of routine clozapine TDM. There was some variation in the appropriate sample collection but what was most alarming is the lack of documentation that action was taken when the TDM level was outside the general acceptable range. Standardising the actions following clozapine TDM needs to be a priority for the Trust if clozapine TDM is to continue to be carried out in almost 100% of the patient population.

References

1. Taylor DM, Barnes TRE, Young AH. The Maudsley Prescribing Guidelines in Psychiatry 14th Edition. Wiley Blackwell.

2. Taylor DM et al. The use of clozapine plasma levels in optimising therapy. Psych Bull 1995:19:753-755

3. Perry PJ. Therapeutic drug monitoring of antipsychotics. Psychopharmacology 200; 148:83-89