Clinical use
Therapeutic drug monitoring of voriconazole levels.
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 absorption and metabolism of voriconazole will vary from patient to patient.
- The voriconazole trough concentration should be maintained above 1 mg/L
- Outcomes for bulky or disseminated infections are better when levels are higher than 2 mg/L
- Levels above 6 mg/L are more likely to lead to liver toxicity and
- Levels above 10 mg/L should be avoided
Patient preparation
Obtain sample:
- Oral voriconazole pre-dose and after 3 to 5 days
- IV voriconazole pre-dose and after 3 to 5 days
Specimen requirements
Red top plain clotted blood
Minimum volume
Supply 1 to 2ml serum taken at the appropriate times – and labelled accordingly
Limitations & restrictions
Do not use serum separator tubes with a gel plug for azole drugs.
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.