TDM (Therapeutic Drug Monitoring)

TDM measuring the blood levels of a drug. TDM can be used for most drugs except NRTIs (nukes).

TDM is only used in certain situations to individualise dosing, including:

  • When using combinations where they may be a drug interaction. This is important with new drugs.
  • With pre-existing liver or kidney damage, or haemophilia, and some other medical conditions. For example, drug levels of both amprenavir and abacavir can be too high if your liver is damaged. TDM can find the safest dose.
  • When using not-recommended doses – for example to use atazanavir without ritonavir boosting.
  • If you may not be absorbing drugs properly. For example, if you have severe diarrhoea.
  • For children. Differences in growth rates and the way children process drugs at different ages are not always accounted for. Even when doses are calculated by body weight or body surface area they often need altering.

TDM is recommended in UK guidelines in these and other situations. Poor drug absorption or faster clearance can cause a combination to fail.

Using TDM and resistance tests together produces better results than either test alone.

TDM costs around £70 per drug from Lab21:

Last updated: 1 January 2018.