Clinical use
Therapeutic drug monitoring of flucytosine.
Background
Therapeutic drug monitoring is the measurement of specific drug concentrations in the blood at timed intervals, in order to maintain a relatively constant concentration of the medication in the circulation. Monitored drugs tend to have a narrow “therapeutic index” – the difference between the toxic and therapeutic doses of medications. For some drugs, maintaining this steady concentration in the blood cannot be achieved by simply giving a standard dose of medication to everyone. Each person will absorb, metabolise, utilise, and eliminate drugs at different rates based upon their age, general state of health, genetic makeup, and the influence of other medications that they are taking. These factors may change over time and vary from day to day or with various disease states.
Therapeutic drug monitoring (TDM) aims to optimise individual dosage regimens, enabling the assessment of safety and efficacy of certain drugs. Current TDM approaches include measuring drug serum concentration together with appropriate medical interpretation.
Candidate drugs for TDM possess one or more of the following: narrow therapeutic range, high pharmacokinetic (PK) variability and a clear relationship between serum concentration and therapeutic response. TDM is also helpful for the drug optimisation in patients with renal, hepatic or gastrointestinal insufficiency.
Reference ranges
- The dose and interval should be adjusted in order to produce peak serum concentrations of about 50 to 100 mg/L; and trough concentrations of 20 to 40 mg/L.
- Levels higher than100 mg/L are toxic.
Patient preparation
Obtain samples:
- Oral flucytosine pre-dose and 2 hours post-dose
- IV flucytosine pre-dose and 30 minutes post-dose
Specimen requirements
- Please supply 1 to 2ml serum taken at the appropriate times – and labelled accordingly.
- Gold top serum separator tube should be used.
Minimum volume
1ml of serum
Turnaround time
<1 day from receipt at the reference laboratory
Analysing laboratory
UKHSA Mycology Reference Laboratory, National Infection Services, UKHSA South West Laboratory, Science, Quarter, Southmead Hospital, Bristol, BS10 5NB
Additional information
Please visit the guidance page for the UKHSA Mycology Reference Laboratory for further information.