Therapeutic drug monitoring

Therapeutic drug monitoring is used for drugs with a narrow therapeutic index or window. This is where the concentration necessary for a therapeutic effect is close to that which produces adverse or toxic effects. Drug plasma or saliva levels are monitored to help guide prescribing decisions regarding dosing.

Risk of harm

Harms have been reported when the expected process for therapeutic drug monitoring has not been followed. This has led to patients remaining on doses causing potential toxicity or being sub-optimally treated.

High risk scenarios

The risk of failures in the process increases when:

• staff are not aware of the requirements due to lack of familiarity with the drug or associated processes
• samples are sent to another organisation for analysing
• the organisation responsible for taking the sample is not the one to review and act on the result
• timely action is required outside of an organisation’s standard working hours

Contributory factors

Errors can occur during any part of the therapeutic drug monitoring process. Examples of contributory factors are listed below.

Planning

Lack of awareness that therapeutic drug monitoring is required by all members of the multidisciplinary team.

Lack of policy or guideline to support clinical decision making.

Lack of guidance for expected process outside of standard hours or in contingency situations

Incomplete or ineffective handover regarding clinical plans.

Acting

Samples not taken at the correct time.

Timing not explicitly documented on sample/paperwork.

Incorrect sample bottles used.

Lack of required documentation on sample.

Failure to act on results and make necessary changes to the treatment regimen.

Reviewing

Staff unaware that levels have been reported.

Lack of ownership over responsibility to review and take action.

Mitigation

Opportunities exist to minimise the risk of errors associated with therapeutic drug monitoring.

Guidance

Ensure guidance is available to support staff. Consider how accessible this is and if it is available at the point of care.

Relevant information to consider including is listed below and may vary by drug.

• Specific times that samples need to be taken
• Specific bottle required to collect sample in
• Where samples are sent, particularly if they are analysed off site
• Processes to follow for out of hours sampling if relevant
• Turnaround times expected for results
• How results are reported and process for escalating critical results to clinical teams
• Details on desired therapeutic range
• Action to be taken for abnormal results
• Interacting drugs that may affect levels

Organisations need to consider contingency planning and out of standard hours expectations.

Defined roles and responsibilities

Ensure that all members of the multidisciplinary team involved in the various steps of therapeutic drug monitoring understand what their responsibilities are. This includes ordering, reporting and reviewing results and undertaking actions where required, such as adjusting doses for out of range results.

Consider documenting this within clinical guidelines, policies or standard operating procedures.

Training

Review available training and consider which members of the multidisciplinary team should undertake it.

Consider whether competency based role specific training is appropriate and how the organisation gets assurance that the right staff have completed it.

Consider regular reminders via local safety bulletins or publications.

Digital solutions

Where possible, digital systems should be used to support safe therapeutic drug monitoring.

Potential considerations include:

• linking clinical guidelines to prescribing functionality
• default ordering of blood samples linked to prescribing tasks
• system alerts to remind staff when samples are needed
• sample labels to indicate sample bottle type required
• system reports to identify/highlight patients on TDM drugs to support timely review
• alerts or flags for out of range results