Timing of sampling
[collapsed][collapse title="When should the samples be taken? " collapsed="collapsed"]
Efavirenz: closest to 10 hours post-dose
For once daily dosing: 12-26 hours post-dose
For twice daily dosing: 6-14 hours post-dose
If malabsorption is suspected, see below “When should samples be taken earlier in the dosing intervals?”. [/collapsed]
[collapsed][collapse title="Why is it recommended to take samples at the end of dosing intervals? " collapsed="collapsed"]
The association between virologic efficacy and plasma concentrations has been most clearly established with the minimum concentration (Cmin). The Cmin, also known as “trough concentration”, is the concentration measured at the end of the dosing interval (i.e. right before a patient’s next dose of the drug being measured). When the Cmin is not available, however, it may be calculated by using the average half-life of the antiretroviral. Nonetheless, this extrapolated Cmin remains an estimation of the true value as the elimination half-life itself remains an estimation based on population data. Therefore, the closer the sample is taken to the end of the dosing interval, the more accurate the estimation will be.
Note that for efavirenz, a sampling closest to 10 hours post-dose should be taken.
Finally, if malabsorption is suspected, samples should be taken earlier in the dosing interval (see below). [/collapsed]
[collapsed][collapse title="When should samples be taken earlier in the dosing interval? " collapsed="collapsed"]
When malabsorption of an antiretroviral is suspected, the sample should be taken at a time when the maximum concentration is expected to be achieved. This time is variable for each antiretroviral but is generally between 1-4 hours after the dose. [/collapsed]
TDM frequency/use
[collapsed][collapse title="Is TDM as a control (for routine follow-up) necessary?" collapsed="collapsed"]
TDM for routine follow-up is generally not recommended with the exception of the paediatric population and pregnant women (see below). Routine TDM may be considered for patients at high risk of virologic failure (ie. multiresistant virus) or patients for whom plasma concentrations may greatly differ from the general population. Routine TDM may also be used for patients who receive nelfinavir and indinavir as use of TDM has been shown to increase the efficacy of these agents. [/collapsed]
[collapsed][collapse title="How often should TDMs be done? " collapsed="collapsed"]
Regular TDM is generally not recommended with the exception of paediatrics or pregnancy (for these populations, please see below). The first TDM should be requested when the patient has one of the indications to do TDM. Subsequent TDMs, if necessary, are recommended with the appropriate delay in the interpretation report and varies according to the result. [/collapsed]
[collapsed][collapse title="When should TDMs be done in pregnancy? " collapsed="collapsed"]
A TDM analysis should be requested prior to pregnancy (if planned) or in the first trimester to have a baseline plasma concentration. Subsequent analyses should be done between the 16th and 20th week of gestation and at the beginning of the 3rd trimester. Another analysis should be completed 2 weeks post-partum. [/collapsed]
[collapsed][collapse title="When should TDMs be done in paediatrics? " collapsed="collapsed"]
It is recommended to systematically follow antiretroviral plasma concentrations every 3 months during childhood and adolescence. [/collapsed]
[collapsed][collapse title="Can TDM be used to assess patient adherence to antiretroviral therapy? " collapsed="collapsed"]
It is difficult to assess patient adherence by the use of TDM although missed doses will be reflected by lower plasma concentrations. [/collapsed]
Blood drawing
[collapsed][collapse title="Which tube should the sample be taken in? " collapsed="collapsed"]
Blood should be drawn into a heparinized tube (nongel). The use of another type of tube may lead to erroneous results. [/collapsed]
[collapsed][collapse title="If TDM is requested for more than one antiretroviral in the same patient, how many tubes of blood are required? " collapsed="collapsed"]
Only one tube is necessary even if several antiretroviral agents require dosing. [/collapsed]
Other
Why is all this clinical information requested?
[collapsed][collapse title="Why is adherence information important? " collapsed="collapsed"]
Doses missed within the last week, particularly those missed within the last 48 hours, may greatly decrease the plasma concentration of the antiretroviral agent. This is taken into account in the interpretation of the result. [/collapsed]
[collapsed][collapse title="Why is TDM not done for nucleos(t)ide reverse transcriptase inhibitors? " collapsed="collapsed"]
TDM is not done for these agents as the relationship between plasma concentrations and efficacy, safety or tolerability has not been clearly established. [/collapsed]
[collapsed][collapse title="What is the usual turnover time to receive the interpretation report? " collapsed="collapsed"]
Reports will generally be sent by mail 2 to 3 weeks after we have received the sample at our laboratory. [/collapsed]
[collapsed][collapse title="What measures are taken to ensure quality of results? " collapsed="collapsed"]
quality control (interne/externe) [/collapsed]