Strategies for third line HIV therapy

May 2008     View PDF     En español

Checking the blood levels of drugs

Some researchers believe that variations in how a person’s body breaks down (metabolizes) drugs may be largely responsible for treatment failure when a person adheres to their regimen. HIV drugs are generally developed to produce the largest drops in HIV with the fewest side effects in most people. However, some people will accumulate levels of drug that are higher than average, perhaps leading to more side effects. Others won’t maintain enough drug, perhaps leading to resistance. So even though some people may take all their prescribed doses, there may be times when there’s too little or too much drug in their systems.

For this reason, several European nations routinely check the blood levels of HIV therapy after starting a person on a new regimen. When problems are found, they adjust the dose to ensure that a person is getting an adequate dose of drug. This process is commonly called Therapeutic Drug Monitoring (TDM), and it has been used for quite some time in other diseases.

A few studies that have used TDM in treating HIV have shown a better response in people whose levels of drugs are consistently at ideal levels. These and other studies have also shown greater side effects in people whose drug levels were consistently too high.

However, there are two problems with using TDM for HIV. First, the level of drug in a person’s body can vary somewhat from day to day. This can be especially true for women during their periods or pregnancy. Second, most HIV drugs work inside cells, and TDM tests generally measure the amount of drug in blood plasma, or outside cells.

While it’s logical to assume there’s a strong relationship between the amount of drug found outside and inside cells, the limited amount of this research has produced conflicting results. In the US, standard TDM lab kits are not available for HIV drugs. It’s now being studied in the US and may have more of a role in the future. Outside these studies, there’s no way for most people to use TDM to guide their treatment decisions.