What is therapeutic drug monitoring?
When patients take anti-HIV drugs, the drugs are absorbed into the body’s cells and the virus in the cells is then affected by the drugs. The amount of drug must be high enough to be absorbed by the body’s cells and therapeutic drug monitoring (TDM) measures whether a patient has too little or too much medicine. Not all drugs can be measured here but many of the most common anti-HIV drugs can be.
Why is therapeutic drug monitoring done?
HIV will not be affected if the drug level is not high enough or is not absorbed well by the body. On the other hand, if levels are too high, the drug may be toxic to the patient or cause unwanted side effects.
Why do drug levels vary?
There is a variety of factors that can influence drug levels. Genetic makeup, kidney or liver problems, body size, and pregnancy are examples. Taking other medications such as anti-tuberculosis or anti-hepatitis drugs can also increase or decrease anti-HIV drug levels.
If pills are not taken according to schedule or are skipped altogether, this will also have an effect on the levels.
When should patients receive therapeutic drug monitoring?
TDM can be helpful if drug treatment is not working and the failure cannot be explained by drug resistance.* TDM might also shed light on unusual or worsening side effects or toxicity. Most people do not need TDM.
What is involved in therapeutic drug monitoring?
There are two types of therapeutic drug monitoring: time and un-timed.
Timed TDM requires an appointment with the TDM nurse. Patients will be asked to follow a specific set of medication instructions. A blood sample will be drawn from the patient immediately before taking medication and at specific times afterwards. This is the best method for seeing precisely how much and how quickly the drug is being absorbed.
The BC Centre for Excellence in HIV/AIDS laboratory can also perform untimed TDM on blood that has been stored by the St. Paul’s Hospital Virology laboratory after routine viral load testing. Untimed TDM does not require drawing extra blood from the patient. Like all tests, we need patient approval bo proceed withis this. This type of test, however, only measures if the drug is present or not.
*See Drug Resistance Testing document on this website for further information or to download the request forms.
